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Vocabulary of the eclipse

A partial lunar eclipse is pictured beside a building in Berlin, Germany, August 7, 2017. REUTERS/Hannibal Hanschke

Solar eclipse: When the moon blocks out some or all of the sun. A total solar eclipse is only possible on Earth because of a cosmic quirk of geometry: The sun's diameter is 400 times wider than the moon's, but it is also 400 times farther away.

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First contact (C1): The moment when the Moon takes its first tiny nibble out of the solar disk — the beginning of the partial phase of an eclipse.

Second contact (C2): The instant when the total or annular phase of an eclipse begins. For a total eclipse, this is synonymous with the disappearance of the first diamond ring.

Fourth (last) contact (C4): The instant when the Moon no longer covers any part of the solar disk. This signals the conclusion of the partial phase of an eclipse.

Baily's Beads: Caused by shafts of sunlight shining through deep valleys on the lunar limb (edge), they look like a series of brilliant beads popping on and off. They appear just prior to second contact and just after third. They're named after the English astronomer Francis Baily, who first described them during the annular eclipse of May 15, 1836.

Corona: The Sun's upper atmosphere, visible as a pearly glow around the eclipsed Sun during totality. Its shape (sometimes elongated, sometimes round) is determined by the Sun's magnetic field and is linked to the sunspot cycle.

Diamond ring: A single Baily's Bead, shining like a brilliant diamond set into a pale ring created by the pearly white corona. It's the signal that totality is about to start (second contact) or has ended (third contact).

New Moon: The lunar phase when the Moon is located in the same direction in the sky as the Sun. New Moon is the only lunar phase during which an eclipse of the Sun can occur.

Partial eclipse: A solar eclipse where the Moon covers only a portion of the Sun. A partial eclipse precedes and follows totality or annularity, but a partial can also occur by itself. A partial solar eclipse is visible over a wider swath of Earth than is totality or annularity.

Penumbra: The portion of the Moon's shadow in which only part of the Sun is covered. An observer standing in the penumbra sees only a partial solar eclipse.

Total eclipse: A solar eclipse where the apparent diameter of the Moon is large enough to completely cover the Sun's photosphere (even if only momentarily) and reveal the faint solar corona.

Totality: The maximum phase of a total solar eclipse, during which the Moon's disk completely covers the Sun. Totality occurs between second and third contact. It can last from a fraction of a second to a maximum of 7 minutes 31 seconds.

Umbra: The darkest part of the Moon's shadow. Within the umbra, the Moon appears larger than the Sun. An observer standing in the umbra sees a total solar eclipse.

Umbraphile A solar-eclipse aficionado; a person who will do almost anything, and travel almost anywhere, to see totality. Another term for an umbraphile is "eclipse chaser."

Source: American Astronomical Society. Selected excerpts from "A Solar Eclipse Glossary" on the website