Eclipse 101


As we admire the rarity of a total solar eclipse, many questions come to mind that not only occur to us now, but have puzzled eclipse watchers for thousands of years. Here are a few basic questions and their answers, with more on the way as we get closer to the August 21, 2017 event.

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The rods and cones in the human retina are very sensitive to light. Even a thin sliver of the sun’s disk covers thousands of these light-sensitive cells. Normally during daylight conditions, the iris contracts so that only a small amount of light passes through the lens and then reaches the retina. This level of indirect sunlight is perfectly OK and the eye has evolved over millions of years to safely see the daylight world under most circumstances. The problem is that the sun’s surface is so bright that if you stare at any portion of it, no matter how small, it produces enough light to damage individual retinal cells. It takes a few seconds for this to happen, but afterwards you will see a spot as big as the solar surface you glimpsed when you look away from the sun at some other scenery. Depending on how long you gazed at the sun and how badly the retinal cells were damaged, this spot will either fade away in time or remain permanent. You should never assume that you can look away quickly enough to avoid eye damage because every person is different in terms of their retinal sensitivity, and you do not want to risk being the one who damages their eyes just to try to look at the sun. If you want to see what the sun looks like, use a properly-equipped telescope. Or why not just go online and view thousands of pictures taken of the sun by telescopes and NASA spacecraft!
There is a misunderstanding being circulated that during a total solar eclipse when the moon has fully blocked the light from the sun, that there are still harmful ‘rays’ that can injure your eyes. This is completely false. When the bright photosphere of the sun is completely covered, only the faint light from the corona is visible, and this radiation is too weak to have any harmful effects on the human retina.
The misunderstanding comes about because of using the general term ‘solar eclipse’ to describe both the total phase when the sun disk is completely blocked, and the minutes before and after totality when there is still some of the sun’s disk visible. It is harmful to view even a sliver of the sun disk because of its intensity, and so to simply say that you should not view a solar eclipse is rather inaccurate.
There is no evidence that eclipses have any physical effect on humans. However, eclipses have always been capable of producing profound psychological effects. For millennia, solar eclipses have been interpreted as portents of doom by virtually every known civilization. These have stimulated responses that run the gamut from human sacrifices to feelings of awe and bewilderment. Although there are no direct physical effects involving known forces, the consequences of the induced human psychological states have indeed led to physical effects.
Solar retinopathy is a result of too much ultraviolet light flooding the retina. In extreme cases this can cause blindness, but is so painful that it is rare for someone to be able to stare at the sun for that long. Typically, eye damage from staring at the sun results in blurred vision, dark or yellow spots, pain in bright light or loss of vision in the center of the eye (the fovea). Permanent damage to the retina has been shown to occur in ~100 seconds, but the exact time before damage occurs will vary with the intensity of the sun on a particular day and with how much the viewer's pupil is dilated from decongestants and other drugs they may be taking. Even when 99% of the Sun's surface (the photosphere) is obscured during the partial phases of a solar eclipse, the remaining crescent Sun is still intense enough to cause a retinal burn. Note, there are no pain receptors in the retina so your retina can be damaged even before you realize it, and by then it is too late to save your vision!
Many people will obtain eclipse viewing glasses. To date three manufacturers have certified that their eclipse glasses and hand-held solar viewers meet the ISO 12312-2 international standard for such products: Rainbow Symphony, American Paper Optics, and Thousand Oaks Optical. These companies may be found online and the glasses ordered, but you really need to order your glasses many months in advance because of the anticipated huge audience that could number in the hundreds of millions. Also, NASA is partnering with GOOGLE and making arrangements to distribute viewing glasses to many institutions and groups along the path of totality. If you are a photographer or amateur astronomer, you will want professional-grade solar filters to cover your binoculars, telescope or camera. Companies like Thousand Oaks Optical and others you can find by using the keyword ‘Solar filters’ have these filters for sale, but again due to the large number of likely customers along the path of totality, you need to order your filter many months in advance. Do not wait until June, 2017 to get your filter. You will also need some time to learn how to use the filter with your optical system, and if you are photographing the eclipse, take lots of test shots to get the right solar disk size and sharpness.
Actually, although filters and glasses do safely block the intense sunlight that is known to damage retinas, the infrared ‘heat’ from the sun can also make viewing uncomfortable as it literally warms the eye. This is why staring at the sun for minutes at a time even with proper filters can still over-heat the tissues and fluids in the eye, and the consequences of this heating can be dangerous as well. To avoid this problem before totality takes place, try not to use your filters without frequently looking-away to cool your eyes. During totality, there is no adverse heating of the eyeball since the solar disk is not visible.
Absolutely not! Everyone needs to be reminded that eyes never evolved on Earth to look at the sun without suffering severe damage. We have many built-in reflexes to prevent this. There are no higher organisms on this planet that can do so and not run an enormous risk of being blinded. Most of the time, astronomers are anxious for you to look at the sky and deeply enjoy the sights that you see. There is never a disclaimer that this is dangerous. The ONLY exception is in viewing solar eclipses. It is an inherently dangerous activity that you have to do very carefully in order not to suffer eye damage. There are specific steps you can take, based on the experience of thousands of professionals, not only in astronomy but in medicine. So, bottom line: read the safety warnings and make sure you understand how to view the eclipse before August 21, 2017 so that the only lasting impression you have is a wonderful memory of the event, not a damaged retina!

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Eclipses occur due to the special coincidence of the moon and the Sun being the same angular size. The Sun is 400 times wider than the moon, but it is also 400 times farther away, so they coincidentally appear to be the same size in our sky. This is what allows us the phenomenal beauty of the total solar eclipse. (Note: You can give the audience the experience of the change in apparent size of an object close by and the same object farther away. They can use their hands to measure angular size.)
Because the geometry required for a total solar eclipse has nothing to do with local noon. It has to do with when the lunar shadow sweeps across your location during the time when the Sun is above the horizon. Even so, it is certainly possible for the Sun to be in full eclipse before it rises at your particular location.
Eclipses only occur if the Moon is located within 0.5 degrees of the plane of the ecliptic, on a line that passes through the center of the Sun and the Earth. The Moon travels along an orbit that is inclined by 5 degrees to the ecliptic plane, so there are only two opportunities each month when it passes through the plane of the ecliptic. These points are called the ascending and descending nodes. Eclipses of the Sun only occur if new moon occurs when the Moon is near of one of these nodes. A similar argument explains why lunar eclipses do not occur every full moon at the node opposite the Sun from the Earth.
King Henry I of England, the son of William the Conqueror, died in 1133 CE. This event coincided with a total solar eclipse that lasted over four minutes on August 2 . Historian William of Malmesbury recounts this "hideous darkness agitated the hearts of men”. After King Henry’s death, a struggle for the throne threw the kingdom into chaos and civil war.
Historians and astronomers believe that the legendary eclipse that two Chinese astrologers Hsi and Ho failed to forecast occurred on October 22, 2134 B.C.E, making it the oldest solar eclipse ever recorded in human history. The Babylonian eclipse on May 3, 1375 BCE is the oldest successfully predicted and recorded in the western world, and there is evidence that the Babylonians knew about the Saros Cycle (18 years 11 days) and could use it to predict the approximate years of eclipses.
Other than watch it with your family and friends, you may want to create a time capsule, which you would open on April 8, 2024 when the next total solar eclipse occurs over the continental United States. You might want to write a letter to your older self and describe what you think you might be doing in 2024, or include some of your favorite items, or a copy of your daily newspaper.
Nearly all public schools will not be in session, so there will be few formal education events involving your students. However, before the current 2016-2017 school year ends, you should check with your science teachers to see if they might have some ideas for summer projects involving the eclipse. For educators, it is highly recommended that you alert your students to this event when school closes in May-June, and equip your students with an Eclipse Project along with your planned summer work activities. This eclipse will occur in the week before school begins in 2017-2018 and will be an excellent talking point to start your school year in many subjects such as art, English and of course science and math.
The only requirement for a total solar eclipse is that the angular size of the sun has to match the angular size of some other object that passes in front of it. When the disk of the object is smaller than the sun, this is called a transit. It is also called an eclipse when the disk of the object is much larger than the sun, but in general this would not allow the corona to be viewed, which is how we define total solar eclipses viewed from Earth. When humans were rooted to the surface of Earth, this was only the case for the moon as the eclipsing object. But there are many known moons and asteroids across our solar system, and from a suitable vantage point near any of them, we can find a distance where again the angular size of the object matches that of the sun to form a total solar eclipse. There are so many different vantage points to choose from that each case has to be specified. For example, eclipses need not be observed from the surfaces of a planet. In fact, Venus and the outer planets have inaccessible surfaces. Instead, we might consider standing on the surface of a planetary moon and waiting for another moon to pass in front of the sun. Given the myriad of planetary moon orbits, finding those instances where the angular sizes match is a significant computational challenge. Jupiter frequently passes across the sun as viewed from its moons, but its diameter is huge compared to the sun. There are 5 satellites capable of completely occulting the Sun: Amalthea, Io, Europa, Ganymede and Callisto. All of the others are too small or too distant to be able to completely occult the Sun, so can only transit the Sun.
UT or "Universal Time" is a single time standard that applies to all locations on Earth as opposed to your local time which will vary depending on your location (longitude). For all practical purposes, UT is equivalent to GMT (Greenwich Mean Time) which establishes a line of longitude that passes through Greenwich England (called the Prime Meridian) as the standard from which all other times are measured and UTC (Coordinated Universal Time) which is based on International Atomic Time (time measured very precisely as vibrations of a cesium atom).

The following chart may be used to convert UT to your US time zone during the eclipse.
UT table
Example: 18:23 UT = 12:23pm MDT

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The totality only lasts a few very brief minutes and it may be the only such event you ever get to see in your life, so please make sure that you are not so distracted with projects that you miss the event! That said, many people try to photograph the eclipse, which requires lots of pre-planning before the event and sometimes involves specialized equipment. Other simple projects can involve measuring temperature, daylight and animal behavior changes in your immediate area. These supplementary activities are simple to perform and require a minimum of distraction from your personal enjoyment of the event!
You will need to purchase a solar filter that will reduce the brightness of the sun so that the light intensity does not destroy your camera. If you ONLY take a photo at the moment of totality, you will not need this filter, and will be rewarded by being able to photograph the faint corona, which will not be visible if you have the filter in place. Most digital cameras with telephoto lenses of 100 mm or larger will show a disk for the eclipse that will show some detail. As a trial, photograph the full moon at night. It will be the same diameter as the total eclipse, so you can practice on the moon first to get the right telephoto lens combination. There are many places on the internet where you can get detailed information such as Mr. Eclipse
It has been reported during many eclipses that many different animals are startled by totality and change their behavior thinking that twilight has arrived. You can explore this yourself with your own pets, or by watching local wildlife, especially birds.
These are among the most ephemeral phenomena that observers see during the few minutes before and after a total solar eclipse. They appear as a multitude of faint rapidly moving bands that can be seen by placing a white sheet of paper several feet square on the ground. They look like ripples of sunshine at the bottom of a swimming pool, and their visibility varies from eclipse to eclipse. 19th century observers interpreted them as interference fringes caused by some kind of diffraction phenomenon. The Sun, however, is hardly a "point source" and the patterns are more random than you might expect from diffraction effects.
The simplest explanation is that they arise from atmospheric turbulence. When light rays pass through eddies in the atmosphere, they are refracted. Unresolved distant sources simply "twinkle," but for nearby large objects, the incoming light can be split into interfering bundles that recombine on the ground to give mottled patterns of light and dark bands, or portions of bands. Near totality, the image of the Sun is only a thin crescent a few arc seconds wide, which is about the same size as the atmospheric eddies as seen from the ground. Bands are produced because the Sun's image is longer in one direction than another. The bands move, not at the rate you would expect for the eclipse, but at a speed determined by the motion of the atmospheric eddies.
It would probably be equal to the typical daytime minus nighttime temperature difference at that time of year and location on the Earth. It would be modified a bit by the fact that it only lasts a few minutes, which means the environment would not have had much time to thermally respond to its lowest temperature, so it would probably only be 3/4 or 1/2 the maximum day-night temperature difference. Because the patch of the shadow travels faster than the speed of sound, weather systems will only be affected very locally directly under the instantaneous footprint of the eclipse. The main effect is in the "radiant heating" component which goes away suddenly at the moment of eclipse and produces a very fast temperature decrease. If the wind is blowing, your body probably exaggerates, by evaporative cooling, how large the actual temperature swing actually is.
The short answer is a definite ‘yes!’, but of course you have to be careful that you minimize glimpsing the bright sun with your eyes without the benefit of a proper filter. As for your camera, there is no valid reason why you would want to point your smartphone camera at the brilliant, un-eclipsed sun without putting a filter over the lens. During totality, you do not need the filter, of course!
Unless you have a telephoto lens for your smartphone, you will only be able to take unmagnified images of the eclipse in your sky. These photos can be very exciting because the field-of-view is large enough that you can compose the shot with your friends and local scenery in the shot, at the same time a recognizable, eclipsed sun during totality hangs dramatically in the darkened sky. You will easily be able to capture with most smartphone cameras the darkened disk of the moon surrounded by a clearly recognizable bright solar corona. Many examples of these kinds of wide-angle shots can be found on the Internet. Of course, if you use the camera’s digital zoom, you will see a pixelized, enlarged image that will not show much actual detail in the corona. To get around this, you need a telephoto lens for your smartphone.
There are many styles of telephoto lenses for smartphones. Avoid the ‘clip on’ lenses because they constantly slip and have to be precisely lined up on the camera lens to work. They are often of low optical quality. The best lenses are rated as 12x and above, and come with their own smartphone mounting bracket. At these magnifications, a tripod is essential because of camera jitter. A decent 12x lens and tripod adapter will cost you about $30.00, but you can also use this system for great ‘close up’ shots in sport and nature settings too! The telephoto lens will give you enough magnification that you will clearly see some of the details in the bright corona. You should test your system by taking night-time photos of the moon so you understand how large and detailed the moon will appear in your shot. The sun/mon during eclipse are equal-sized so this is a good way to compose your eclipse shots too. Also experiment with the settings on your camera using a downloadable app like Camera+ or NightCap Pro, which allow you more flexibility in setting up the exposure, f/stop and other factors. For more information on eclipse photography with smartphones, read the project details found at our Citizen Explorers page.
Above all, don’t forget to put your smartphone down and enjoy the eclipse with your own eyes!
Well…no. As you get close to totality, you should be able to notice a power drop in the output of your panels, which will reach a minimum when the sun is in full eclipse, and then your power levels will recover as the moon moves away from the sun. In fact, this may be a fun science project if you can get in touch with many people in other cities that also have solar panels they can monitor. Just like some folks will be watching for temperature changes and the dimming of sunlight during the eclipse, you can measure the drop in solar power reaching Earth’s surface as a companion observation! Go for it!!

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The next total solar eclipse visible from the United States occurs on August 21, 2017. The track goes from Oregon at its start, and exits on the east coast near South Carolina. After that, the next one visible from the Lower-48 will be on April 8, 2024 which will track northeast from Texas to Maine and cross the path of the 2017 eclipse near Carbondale, Illinois. Since 1503, there have been 15 total solar eclipse paths that have crossed the path of the August 2017 eclipse. Calculations show that it will take about 1000 years for every geographic location in the Lower-48 to be able to view a total solar eclipse.
The last total solar eclipse viewed from contiguous United States was on Feb. 26, 1979 whose path passed through the northwestern U.S. states of Washington, Oregon, Idaho, Montana, North Dakota, and Canadian provinces of Saskatchewan, Manitoba, Ontario and Quebec. After the August 2017 total solar eclipse, the next annular solar eclipse that can be seen in the continental United States will be on October 14, 2023 which will be visible from Northern California to Florida. Following this, we will have a total solar eclipse on April 8, 2024 visible from Texas to Maine.
The Babylonians knew how to predict lunar eclipses with some accuracy, but solar eclipses are far more difficult because the 'footprint' on the Earth is only a few tens of miles across and requires arc minute positional accuracy and forecasting for any specific locale. Apparently Thales, c. 610 B.C. E, is credited with predicting a solar eclipse from knowledge of a previous eclipse and using the Saros cycle. He predicted the year, but not the month and the day. It wasn't until Ptolemy's time that solar eclipse forecasting became more accurate.
Here are some of the major towns and cities:
Corvallis, Albany and Lebanon, Oregon
Idaho Falls, Idaho
Casper, Wyoming
Grand Island, Lincoln Nebraska
St Joseph, Missouri
Kansas City, Kansas
St Louis, Missouri
Bowling Green, Kentucky
Nashville, Tennessee
Greenville, South Carolina
Columbia, South Carolina
Here are the local mid-eclipse times for some of the major towns and cities along the path of totality:
10:17 am
Corvallis, Albany and Lebanon, Oregon

11:34 am
Idaho Falls, Idaho

11:44 am
Casper, Wyoming

1:00 pm
Grand Island, Lincoln Nebraska

1:08 pm
St Joseph, Missouri

1:09 pm
Kansas City, Missouri

1:14 pm
Columbia, Jefferson City, Missouri

1:18 pm
St Louis, Missouri

1:28 pm
Bowling Green, Kentucky

1:28 pm
Nashville, Tennessee

2:39 pm
Greenville, South Carolina

2:43 pm
Columbia, South Carolina

Although many of the areas covered by the path of totality are privately-held, many parks span the path and are open for visits, although you can expect that parking may be a serious problem!
  • Appalachian Trail (TN, SC, GA)
  • Blue Ridge Parkway (TN, NC)
  • Craters of the Moon (ID)
  • John Day Fossil Beds (OR)
  • Grand Teton National Park (WY)
  • Fort Laramie National Historic Site (WY)
  • Scott’s Bluff NM (NE)
  • Agate Fossil Beds National Monument (NE)
  • Homestead National Monument of America (NE)
  • Harry S Truman National Historic Site (MO)
  • Ulysses S. Grant National Historic Site (MO)
  • Stones River (TN)
  • Obed WSR (TN)
  • Great Smoky Mountains National Park (TN)
  • Ninety Six National Historic Site (SC)
  • Congaree National Park (SC)
  • Charles Pinckney National Historic Site (SC)
  • Fort Sumter National Monument (SC)
  • Mammoth Cave National Park (KY)
  • Lewis & Clark National Historic Trail (Omaha)
  • Fort Donelson National Battlefield (TN)
  • Manhattan Project National Historic Park (TN)
Including Alaska, Hawaii and the Lower-48, here are the total solar eclipses visible during this period and what was going on at those times:
July 11, 1991 – (Hawaii) Most popular song: ‘I Do it for You’ by Bryan Adams was the top song. Popular movie.’Terminator 2 ‘. Price of gasoline: $1.14. Stock Market: 3000. President in office: George H.W. Bush. Major current event: Nigeria Airways DC-8 crashes at Jeddah, Saudi Arabia, killing all 261 passengers and crew on board. Major scientific event this year: First website goes online at CERN(August); Dinosaur extinction attributed to asteroid impact in Yucatan.
July 10, 1972 – (Alaska) Most popular song: “Lean on Me” by Bill Withers was the top song. Popular movie. ‘Deliverance’. Price of gasoline: $1.59. Stock Market: 930. President in office: Richard Nixon. Major current event: Chess match between world champion Boris Spassky of the Soviet Union, and United States champion Bobby Fischer, in Iceland. Major scientific event this year: The Soviet space probe Venera 8 landed on Venus in July. Mariner 9 sends pictures from Mars in February. Apollo 16 launched in April. First scientific hand-held calculator on sale.
February 26, 1979 – (Lower-48) Most popular song: Le Freak by Chic was the top song. Popular movie The Deer Hunter. Price of gasoline: $0.86. Stock Market: 821. President in office: Jimmy Carter. Major current event: Ayatolla Khomaini seizes power in Iran. Major scientific event this year: WHO certifies global eradication of smallpox.
March 7, 1970 – (Lower-48) Most popular song: Bridge over Troubled Water by Simon and Garfunkel. Popular movie – Patton. Price of gasoline: $0.36. Stock Market: 784. President in office: Richard Nixon. Major current event: First Boeing 747 flies. US lowers voting age from 21 to 18. Major scientific event this year: Venera 7 becomes first spacecraft to reach surface of Venus.
July 20, 1963 – (Alaska, Maine) Most popular song: Surf City by Jan and Dean. Popular movie – Cleopatra. Price of gasoline: $0.30. Stock Market: 694. President in office: John Kennedy. Major current event: ZIP codes introduced in the US. Major scientific event this year: British geophysicists publish proof of seafloor spreading on the Atlantic Ocean floor.
According to a map in which the tracks of all total solar eclipses from 1000 to 2000 BCE have been overlain, there are two such areas; one in north eastern Colorado centered on Fort Morgan, and one in Nebraska centered near Lewellen, that have not seen a total solar eclipse in over 1000 years. Each area is about 50 km across.
There will literally be thousands of different venues to choose from across the continental United States. Some will offer lectures about eclipses, safe locations for viewing, proper viewing glasses, and many may host a variety of podcasts and other social media opportunities. Some of these will be featured on the NASA Eclipse 2017 website and/or on our Eclipse Megacast, which will be live-streaming webcast. Because most locations will not have schools in session, your local museum will likely have a schedule of planned events to choose from. Also check out your local amateur astronomer club, which will be especially active and will likely offer safe telescopic observing of the event.
Total solar eclipses have, indeed, been a popular theme. We have a short list of these examples on our webpage at The most famous example is Carly Simon’s ‘You’re so Vain ‘ where she sings ‘. . . you flew your Lear jet up to Nova Scotia To see a total eclipse of the sun’. Her lyric refers to the March 7, 1970 total solar eclipse, and this is the only known recorded song that mentions a specific eclipse. This will make future historians very happy if they try to date when the song was written if no other records exist. This is kind of like what archaeologists do with Babylonian cuneiform eclipse records today. By the way, there was a later ‘Nova Scotia’ eclipse on July 10, 1972, but Simon’s song came out in 1971. Time travel had not been invented yet.
This is easy…just show up! Find a location in your area where there are likely to me many people, or contact your local museum and get on their list of ‘eclipse helpers’. If you have a telescope with the right filters, bring it with you and set it up so that people can view the eclipse before totality starts. Also, if you have one of those special ‘Hydrogen-alpha’ filters you can use it to show people what the surface of the sun looks like .There may still be a few sunspots to view this late in the current sunspot cycle. Be prepared to answer all kinds of unusual questions, so make sure you have done your homework on the basic details about the sun, its corona and other features such as its size relative to Earth and other similar things.
The only reason that total solar eclipses only last a few minutes for a ground-based observer is that that is about how much time it takes the lunar shadow, traveling at over 2,000 km/hr to pass you’re your geographic location. But what would happen if you could fly along the path of totality at the same speed as the lunar shadow? Well, you would see a continuous eclipse for as long as the shadow touches Earth, which can be several hours from start to finish. Astronomers figured this out long ago, but had to wait for the invention and deployment of non-military supersonic jets to carry out such a chase. The scientific benefit is enormous because now you can study the sun’s corona for much longer than the 2-5 minutes usually allowed. On June 30, 1973, Concorde 001 intercepted the path of a solar eclipse over North Africa. Flying at Mach 2.05 the aircraft provided seven observers from France, Britain and the United States with 74 min of totality bounded by extended second (7 min) and third (12 min) contacts. The former permitted searches for time variations of much longer period than previously possible and the latter provided an opportunity for chromospheric observations of improved height resolution. The altitude, which varied between 16,200 and 17,700 m, freed the observations from the usual weather problems, and greatly reduced atmospheric absorption and sky noise in regions of the infrared.
The solar eclipse of March 20, 2015, was the first eclipse for which a significant impact on the power system occurred. The continental Europe and Great Britain generated about 90 Gigawatts of solar power and production was estimated to have decreased by up to 34 GW compared to a clear sky day. Also for wind farms there is a potentially smaller effect because as temperatures decrease during the eclipse, winds also slacken a bit, and this could cause wind turbines to rotate less vigorously.
Of course if you are on the path of totality, you will lose nearly 100% of your normal daylight illumination when totality happens. It will seem like late-twilight with a narrow ring of daylight encircling your horizon. If you are only able to watch the partial eclipse, your experience will be quite different. Unless the sun is at least 75% covered by the moon, you may not even notice much illumination change at all unless you are aware the eclipse is happening. It is only by the time the eclipse reaches about 90% coverage that you will start to notice a landscape dimming, and by about 95% you will definitely be aware of something happening to the sunlight even if you did not know there was an eclipse at that moment. The details, of course, will change depending on how aware you are of the exact timing of the event, and even your emotional state and its impact on your sensory acuity. One thing you may notice once the eclipse reaches about 90% is that the shadows on the ground will start to seem less sharp as the light source (the illuminated sun) becomes smaller. Also look for crescent-shaped shadows cast by light passing through gaps in the leaves on trees.

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There are two orbit locations where eclipses can occur. These are the points in the lunar orbit that intersect the ecliptic plane where the Sun moves in the sky. Called the ascending node and the descending node, eclipses can occur at either node. The Moon must be in the full moon phase as it passes the node in order for a lunar eclipse to occur. Similarly, solar eclipses only occur during new moon when this phase occurs at either node.
Because the Moon moves to the east in its orbit at about 3,400 km/hour. Earth rotates to the east at 1,670 km/hr at the equator, so the lunar shadow moves to the east at 3,400 – 1,670 = 1,730 km/hr near the equator. You cannot keep up with the shadow of the eclipse unless you traveled at Mach 1.5.
Astronomers first have to work out the geometry and mechanics of how the Earth and Moon orbit the Sun under the influences of the gravitational fields of these three bodies. From Newton's laws of motion, they mathematically work out the motions of these bodies in three-dimensional space, taking into account the fact that these bodies have finite size and are not perfect spheres, and that the Earth and Moon are not homogeneous bodies. From careful observation, they then feed into these complex equations the current positions and speeds of the Earth and Moon, and then program the computer to "integrate" these equations forward or backward in time to construct ephemerides of the relative positions of the Moon and Sun as seen from the vantage point of the Earth. Eclipses are specific configurations of these bodies that can be identified by the computer. Current eclipse forecasts are accurate to less than a minute in time over a span of hundreds of years.
The orbit of the moon is not stable. Because of tidal friction, the orbit of the Moon is steadily growing larger, so that the angular size of the moon from the Earth is shrinking. The moon's orbit is increasing by about 3.8 cm (1.5 inches) per year. When the moon's mean distance from Earth has increased an additional 14,600 miles, it will be too far away to completely cover the sun. This is true even at perigee when its disk will be smaller than the sun's disk even when the sun is farthest from Earth at aphelion. At the current rate that the moon's orbit is increasing, it will take over 600 million years for the last total solar eclipse to occur. A complicating factor is that the size of the sun itself will grow slightly during this time as it evolves as a star, which will act to make the time of "no more total eclipses" a bit sooner than 600 million years.
Solar eclipses are fairly numerous, about 2 to 4 per year, but the area on the ground covered by totality is only about 50 miles wide. In any given location on Earth, a total eclipse happens only once every hundred years or so, though for selected locations they can occur as little as a few years apart. An example is the August 21, 2017 and April 8, 2024 eclipses, which will be viewed at the same spot near Carbondale, Illinois. Eclipses of the Moon by the Earth's shadow are actually less numerous than solar eclipses; however, each lunar eclipse is visible from over half the Earth. At any given location, you can have up to three lunar eclipses per year, but some years there may be none. In any one calendar year, the maximum number of eclipses is four solar and three lunar.
The positions of the Sun and Moon are known to better than 1 arc second accuracy. This means that on the Earth, the location of the track of totality is probably known to about (1.0/206265.0) x 2 x pi x 6400 km = 0.19 kilometers or a few hundred meters at the Earth's equator.
  • "Atlas of Historical Eclipse Maps for East Asia 1500BC to 1900 AD" by F.R. Stephenson and M.A. Houlden, (Cambridge University Press) 1986.
  • "Canon of Eclipses" by Theodor Oppolzer, translated by Owen Gingerich in 1962. (Dover Books, New York).
  • "Canon of Solar Eclipses" by Jean Meeus and Hermann Mucke, (Astronomiches Buro, 1983) Vienna Austria, second edition.
  • We all know that the Ancient Chinese were so fearful of the sun being ‘eaten by a dragon’ that they clanged pots and other noisy things to scare off the dragon and bring back the sun. This tradition apparently goes back a very long time, and may have been started several thousand years ago. We know that Ancient Chinese astrologers were carefully searching for eclipses as far back as 2100 BCE. But what of other civilizations such as ancient Egypt and those such as the Incas, Mayas and Aztecs? Amazingly, there are no recorded documents or hieroglyphs that suggest the sudden and unpredicted absence of the sun disks associated with Quetzalcoatl (Inca) or Ra (Egypt) was noteworthy in the archeological record. Part of this, in the case of Egypt, may be due to the fact that most of the eclipse tracks for the period from 2,000 BDC to 1000 BCE, for example, passed over extremely low population density areas in Egypt where there would be very few people to notice the 2-5 minute dimming of the sun. However, the total solar eclipses of 1883, 1532 and 1337 BCE passed over Cairo, and the eclipses of 1949, 1257 and 1123 BCE passed close to Luxor. Perhaps there are records somewhere yet to be translated, discovered, or critically analyzed, that mention such unusual solar events, no doubt witnessed by thousands of people each time in these high-population areas.
    During the last century, the precise timing and track of totality could be used to make ultra-precise measurements of the lunar orbit and improve the mathematical model for the gravitational interactions between earth and the moon. In 1919, a total solar eclipse was used to test Einstein’s Theory of General Relativity. Studies of the solar corona during totality were also used to examine its structure and changes in time, and to relate the features seen with details on the solar surface. Currently, there have been attempts to detect interplanetary dust falling into the sun by searching for its faint infrared light beyond the corona. There are also studies of the solar transition region being performed by the glimpses of it provided during totality. Recently, lunar profile data from the NASA LRO mission have been used to predict the exact timing and brilliance of Bailey’s Beads shortly before totality. So new scientific uses for this spectacular phenomenon are found nearly every year!
    This is about 7.5 minutes. The longest total solar eclipse from 4000 BCE to 8000 CE, a span of 12,000 years, will occur on July 16, 2186 and will last 7 minutes 29 seconds. Its path sweeps across Colombia, Venezuela and Guyana. The August 21, 2017 total solar eclipse, by comparison, will last a maximum of 2 minutes 43 seconds. About 70% of eclipses last longer than this.
    During the 5,000-year period from -1999 to +3000 (2000 BCE to 3000 CE), Earth will experience 11,898 eclipses of the Sun. The statistical distribution of eclipse types for this interval is as follows: 4,200 partial eclipses, 3,956 annular eclipses, 3,173 total eclipses and 569 hybrid eclipses. That means that, every 1000 years you have 840 partial eclipses, 791 annular eclipses, 635 total eclipses and 114 hybrid eclipses. That works out to 2-3 eclipses of all kinds each year, and about 2 total solar eclipses every 3 years.
    Well…my birthday is November 23. The last total solar eclipse on my birthday was in 2003. The next one is in the year 2337, followed by the years 2356 and 2728, so the intervals are 334 years, 19 years and 372 years. So depending on which part of the cycle you are on, you may either wait about 20 years or about 350 years for the next occurrence! Check out the Five Millennium Canon of Eclipses to find the one nearest your birthday. It is located at
    On July 28, 1851 the Royal Prussian Observatory at Königsberg (now Kaliningrad, in Russia) commissioned one of the city's most skilled daguerreotypists, Johann Julius Friedrich Berkowski, to record a still image of the event.
    A Saros Cycle is approximately 6585.3211 days, or 18 years, 11 days, 8 hours in length. One saros period after an eclipse, the Sun, Earth, and Moon return to approximately the same relative geometry, a near straight line, and a nearly identical eclipse will occur. The Moon will have the same phase and be at the same node and the same distance from the Earth. In addition, because the saros is close to 18 years in length (about 11 days longer), Earth will be nearly the same distance from the sun, and tilted to it in nearly the same orientation (same season). Given the date of an eclipse, one saros later a nearly identical eclipse can be predicted. Each total solar eclipse track looks similar to the previous one, but is shifted by 120 degrees westward. The August 21, 2017 total solar eclipse is part of the Saros 145 series. The previous total solar eclipse in this series occurred on August 11, 1999. The next one will be on September 2, 2035. The first cycle in this series occurred on January 4, 1639, and the last one will be on April 17, 3009.
    Of course the most spectacular use has been to study the faint corona of the sun, which can be observed by spacecraft such as the Solar and Heliospheric Observatory (SOHO) by making artificial eclipses, but ground-based telescopes and photography have also made many historical contributions to understanding the shape, structure and extent of the corona. Also, total solar eclipses have been invaluable in improving our understanding of the lunar orbit. Whether a total solar eclipse occurs at a specific location and time on the surface of Earth depends on the lunar orbit, the motion of the moon along the orbit, the earth-moon distance and other factors. Sophisticated physics-based computer models have been used for over a century to make accurate predictions of each eclipse to the second, and to the nearest mile on Earth. The best way to do this is to look at historical sightings of total solar eclipses from centuries or even millennia in the past. These sightings are often made by observers at specific geographic locations and who indicate the time of the eclipse from that location. These distant-in-time observations can be calculated by the modern eclipse models and compared with the historical sighting, then the models can be adjusted by improving the parameters of the physics calculation until agreement is reached. This process sometimes results in new ‘science’ related to the shape of the moon, or gravitational perturbations in the lunar orbit that can take centuries to build up to measurable effects. For example, in 1989 an astronomical event recorded on a clay tablet found in 1948 among the ruins of the ancient city of Ugarit, Syria, was identified as a description of a total solar eclipse that occurred on 3 May 1375 BCE. The information was used to provide a reference point to establish the long-term evolution of angular momentum in the Earth-Moon system
    Apart from being a wonderful word to use in the game of Scrabble, this astronomical term is an event in which one astronomical object is lined-up with another. This leads to the pithy aphorism: all eclipses are syzygys but not all syzygys are eclipses. For example, Full moon and New Moon are syzygys involving the lining up of the Sun, Earth and Moon, therefore, lunar and solar eclipses are syzygys. When a planetary moon passes across the face of another body but does not eclipse it, this is called a transit. From Earth, the small disks of Venus and Mercury can be seen passing across the face of the sun during transits of Venus and Mercury. These also involve the straight-line alignment of the Sun, Earth and each planet. On June 3, 2014, the Curiosity rover on Mars observed the planet Mercury transiting the sun, marking the first time a planetary transit has been observed from a celestial body besides Earth. Previously, the Curiosity rover has captured images of the Martian moons Phobos and Deimos transiting the sun.
    The sun will be well-way from its maximum sunspot numbers during this cycle (Number 24) which peaked in 2013 with about 130 spots during the peak month. By August, 2017 the average number should be about 30 per month. Sunspot minimum will occur sometime in ca 2020 if the current trends continue. What this means is that there may be fewer large sunspots, and the ones you see before eclipse will be concentrated near the equatorial zone of the sun. As we get closer to the time of the eclipse, make sure you check with the NASA Solar Dynamics Observatory (SDO) page at to see what the sun looks like a day or so before the eclipse. That way you can identify any large ‘active regions’ before you try to search for them at your telescope!
    Yes. Totality currently can never last more than 7 min 32 s. This value changes over the millennia and is currently decreasing. By the 8th millennium, the longest theoretically possible total eclipse will be less than 7 min 2 s.
    The last transits of Venus occurred on June 8, 2004 and June 5, 2012, with the next pair predicted for December 10, 2117 and December 8, 2125. Transits of Mercury are much more common, with the most recent one occurring on May 9, 2016 and the next one on November 11, 2019. It is not unreasonable to ask when we might expect such transits to occur during the time of a total solar eclipse. The next anticipated simultaneous occurrence of a solar eclipse and a transit of Mercury will be on July 5, 6757, and a solar eclipse and a transit of Venus is expected on April 5, 15232.
    No. If the corona of the sun were not so bright, you would see the moon very faintly illuminated by earthshine. Our Earth is fully illuminated during the eclipse and it reflects quite a bit of light into space. Some of this lands on the lunar surface and provides a secondary source of illumination. But because the sun’s corona is so bright, your eyes will not see this earthshine effect.
    Because 223 synodic months is not identical to 239 anomalistic months or 242 draconic months, The 18-year saros periods do not endlessly repeat. Each series begins with the Moon's shadow crossing Earth near the north or south pole, and subsequent events progress toward the other pole until the Moon's shadow misses Earth and the series ends. A full series from start to finish lasts about 1,300 years. The August 21, 2017 eclipse is part of Saros Series 145 that includes 77 eclipses of which the August eclipse is number 22. All eclipses in this series occur at the ascending node of the lunar orbit. The series began with the partial solar eclipse of January 4, 1639 visible at the North Pole. The series will end with the partial solar eclipse of April 17, 3009 visible from the South Pole. The length of this series is 1,370 years. By the way, the total solar eclipse of August 21 is preceded two weeks earlier by a partial lunar eclipse on August 7, 2017, which occurs during the same eclipse season when the sun is nearest this node.
    Amazingly we may actually have a plausible answer to this question! For an eclipse to occur, you first need a star, and then a planet with a moon for which the moon will provide the eclipse. At 1 billion years after the Big Bang, the oldest known planet PSR B1620-26 b had already formed. Located in the globular cluster Messier-4 about 12,400 light-years from Earth, it bears the unofficial nicknames "Methuselah" and "The Genesis Planet" because of its extreme age: about 12.8 billion years. The planet is in orbit around the two very old stars: A dense white dwarf star and a neutron star. The planet has a mass of 2.5 times that of Jupiter, and orbits at a distance a little greater than the distance between Uranus and our own Sun. Each orbit of the planet takes about 100 years. Like the large planets in our solar system, it is not unreasonable to assume that Methuselah may also have one or more moons, and that one may provide an eclipse of the white dwarf star from the surface of Methuselah, or that from the vantage point of one of its moons, another moon may provide such an eclipse or transit!
    Well…not exactly! An eclipse requires that the sun or star be fully covered by the disk of a planet as the planet passes between the star and an observer on Earth. From basic geometry, the amount of starlight dimming depends on the ratio of the circular area of the planet to the circular area of the star. For an eclipse, 100% of the star’s light is dimmed. This requires that the planet have the same diameter as the star, which is physically impossible. However, when the planet’s diameter is much smaller than the star, astronomers call this a transit, and this is one of many methods that are actually used to detect planets orbiting distant stars. For example, a Jupiter-sized planet has a diameter of 143,000 km while a sun-like star has a diameter of 1.4 million km, so the ratio of their areas is 1/100. As this exoplanet transits the disk of its star as viewed from Earth, the brightness of the star will dim by 1%. Since the 1990’s, astronomers have detected over 3,600 exoplanets orbiting 2,700 stars. Of these, NASA’s Kepler observatory has detected over 2,300 of the confirmed exoplanets using the transit method.
    Another exciting aspect of exoplanet transit detections is that the star’s light passes through the atmosphere of the exoplanet on its way to Earth. By using an instrument called a spectroscope, astronomers can examine the way the atmosphere absorbs the star’s light to detect the composition of the atmosphere. Dozens of exoplanets have been studied in this way so far. Most reveal signs of carbon dioxide, water vapor and methane. If oxygen is ever discovered, this will be an important sign that the planet harbors a biosphere of some kind!
    The moon’s limb is not perfectly smooth because of the mountain ranges and canyons that pepper the moon’s circumference as viewed from Earth. Shortly before the moon fully blocks the disk of the sun during a total solar eclipse, flashes of light can often be seen around the circumference of the moon’s blackened disk. These are caused by sunlight passing through the canyons around the limb of the moon.
    The namesake for these ‘diamond ring’ flashes is Francis Baily; a prominent English astronomer and four-time president of the Royal Astronomical Society. His vivid description of the phenomenon (following an eclipse on May 15, 1836) caused it to be associated with his name in 1836, but he was not the first historically-named person to discover this phenomenon. More than a century earlier, the famous English astronomer Sir Edmond Halley (discoverer of Halley’s Comet) described this spectacular phenomenon and also gave a correct explanation for it during an eclipse in 1715: "About two Minutes before the Total Immersion, the remaining part of the Sun was reduced to a very fine Horn, whose Extremeties seemed to lose their Acuteness, and to become round like Stars ... which Appearance could proceed from no other Cause but the Inequalities of the Moon's Surface, there being some elevated parts thereof near the Moon's Southern Pole, by whose Interposition part of that exceedingly fine Filament of Light was intercepted."
    Thanks to the results from the NASA Lunar Reconnaissance Orbiter, which measured details across the entire moon to 2-meter accuracy, we can now predict exactly when and where these brilliant flashes of light will appear as a total solar eclipse takes place, because now we know where and how deep the lunar limb canyons will be. Still, despite our abilities to predict it, this lovely effect and its diamond ring-like character will continue to mesmerize observers for all times to come!
    The first thing to realize is that the moon is in its ‘new’ phase, so whatever gravitational effects you might expect during a total solar eclipse also happen any time there is a New Moon, which happens every 28 days.
    Starting as an observer on the ground, you are under the gravitational influence of Earth, the moon and the sun. At the time of the August 21, 2017 eclipse, Earth will be 151.4 million kilometers from the sun, and the moon will be located 365,649 km from the surface of Earth. Using Newton’s Law of Gravity, we can calculate the force of the sun, moon and Earth on an 80 kg person. Earth accounts for 784.1 Newtons of force (176.42 pounds), the moon provides 0.0029 Newtons (0.01 ounces) and the sun provides 0.4633 Newtons (1.6 ounces). But because our Earth rotates, this also provides an ‘anti-gravity’ centrifugal force we can also calculate. So if we add the forces with their correct directions we get a total gravitational force of 784.1 – 0.0029 – 0.4633 = 783.634 Newtons or 176.317 pounds. So, you will be about 1.7 ounces lighter!
    The gravitational effect of the sun and moon being on the same side of Earth during New Moon is actually far more dramatic when you look at what happens to our entire planet. First, the gravitational ‘tidal’ force of the moon and sun cause a body tide in the solid rock of Earth. If you are on the same line defined by the centers of Earth, the sun and moon, Earth’s crust actually bulges upwards by about 40 millimeters across a thousand-kilometer area on Earth’s surface. So as you watch the total solar eclipse, feel free to imagine that you are standing on the ground 40 millimeters closer to the sun than it would be several hours later!
    National Aeronautics and Space Administration