What are Baily’s Beads, and who discovered them?
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!