A solar eclipse glossary

Total solar eclipse seen from Varanasi, India; Wikimedia Commons

In a week’s time, on August 21, a total eclipse of the sun will dim American skies; it will be the first such eclipse to be seen in the continental United States in 38 years, making it the cosmic episode of the decade. In 1925, the New York Times described a solar eclipse as “the most magnificent free show nature presents to man.” Glossophilia takes a rocket-ship ride through some of the light-fantastic lingo of solar eclipses (definitions courtesy of the OED and NASA). We’ll also ask an important and relevant spelling question: should we capitalize “Sun”, “Moon” and/or “Earth” when we’re writing about this heavenly happening?

Solar eclipse: An eclipse in which the sun is obscured by the moon.

Corona: The rarefied gaseous envelope of the sun and other stars. The sun’s corona is normally visible only during a total solar eclipse, when it is seen as an irregularly shaped pearly glow surrounding the darkened disc of the moon.

Umbra: The fully shaded inner region of a shadow cast by an opaque object (such as the Moon), especially the area on the earth or moon experiencing the total phase of an eclipse.

Antumbra: The part of the Moon’s shadow that extends beyond the umbra. Similar to the penumbra in that the sun is only partially blocked by the moon.

Penumbra: The partially shaded outer region of the shadow cast by an opaque object, such as the moon.

Total eclipse – A solar eclipse in which the moon’s umbral shadow traverses Earth (Moon is close enough to Earth to completely cover the sun). During the maximum phase of a total eclipse, the sun’s disk is completely blocked by the moon. The sun’s faint corona is then safely revealed to the naked eye.

Partial eclipse: A solar eclipse in which the moon’s penumbral shadow traverses Earth (umbral and antumbral shadows completely miss Earth). During a partial eclipse, the Moon appears to block part (but not all) of the sun’s disk.

Annular eclipse: A “ring of fire” solar eclipse that happens when the moon’s size (as seen from Earth) is not quite big enough to cover the entire sun. This happens when the moon is at apogee, or the closest approach to Earth in its orbit. During the maximum phase of an annular eclipse, the sun appears as a blindingly bright ring surrounding the moon.

Hybrid eclipse: A solar eclipse in which the moon’s umbral and antumbral shadows traverse Earth (the eclipse appears annular and total along different sections of its path)

Eclipse magnitude: the fraction of the sun’s diameter occulted by the Moon. It is strictly a ratio of diameters and should not be confused with eclipse obscuration.

Eclipse obscuration: a measure of the sun’s surface area occulted by the moon

Greatest eclipse: (for solar eclipses): the instant when the axis of the moon’s shadow cone passes closest to Earth’s center

Saros: the periodicity and recurrence of solar (and lunar) eclipses is governed by the Saros cycle, a period of approximately 6,585.3 days (18 years, 11 days, 8 hours).

To capitalize or not to capitalize

We’re very galaxy- and planet-centric when it comes to talking about the names of our heavenly bodies. Because we Earth-dwellers have only one moon, when referring to “the moon” we all know which moon we’re talking about: hey, it’s our moon, not Saturn’s or Jupiter’s, and we definitely know which moon that definite article refers to. The moon is a generic noun, because it doesn’t really have a name. If it didn’t have the definite article in front of it — i.e. if we just called it “Moon” — we would probably capitalize it, because that would be its name. Ditto for the sun: even though we know there are plenty of suns out there, “the sun” is our sun, and it doesn’t have a proper name, so even though it’s big, it’s still just a generic noun — albeit with a definite article always in front of it.

Our earthly dwelling does have a name — Earth; it’s not just known as “the planet” (cf. “the moon” or “the sun”). And our neighbors all have names too: Venus, Mars, Jupiter, etc. So we obviously capitalize the proper name of our heavenly home. And we don’t need a definite article in front of it, ‘cos that’s its name. You don’t put a “the” in front of your name, do you?

FYI, NASA seems to capitalize the Moon and the Sun. Could that be because NASA knows there are other moons and suns out there (it doesn’t just believe it, the way we mortals do), and they want to distinguish our moon and our sun and to give them the credit they’re due by bestowing proper names upon them?

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