What’s Up? Transits, Occultations, Conjunctions, and Eclipses

Chance line of site encounters in the heavens have historically been of great interest to civilizations dating back thousands of years. Solar and lunar eclipses have been viewed with great anticipation and fear. Close associations or “conjunctions” between planets or between a planet and the moon acquired magical properties from mystical practices such as astrology. When we look out into the day or night sky, we notice that the sun, moon, and all the planets (four are naked eye visible) appear to follow a line across the sky. This line is called the ecliptic and is the representation of the Earth’s orbital plane in the sky. The sun follows this line precisely. The planets and our moon follow this line approximately and so only occasionally appear very close together. Occultations occur when a big object like say, the moon, moves in front of a smaller appearing object, like a planet. Transits occur when an apparent small thing like a planet moves in front of a big thing, like the sun. Transits of the inner planets (Venus and Mercury) across the face of the sun have been opportunities for astronomers to use some clever methods to measure the dimensions of the solar system. Rare stellar occultations, where a planet moves in front of a distant star, can provide important information about the planet’s atmosphere by observing the way the star light is diminished. Similarly, most planets orbiting other stars have been detected by the transit detection method, again, watching the diminished light curve as the planet moves in front of the star.

Check it out for yourself!
National Aeronautics and Space Administration