The first type of eclipse on our list, a lunar eclipse, occurs when the earth's shadow gets between the sun and moon, blocking sunlight from reflecting off the moon's surface as it normally does. It can only occur during a full moon, but, because the earth's orbit around the sun and the moon's orbit around the earth aren't perfectly aligned, it usually doesn't happen even then. Up to three times per year, however, the three bodies are aligned to produce a dark shadow that falls across the moon.
In a total lunar eclipse, the earth's shadow completely covers the moon and we get an event known as a "blood moon," for the red color the moon takes on. More often, the earth's shadow will cover only part of the moon in a partial lunar eclipse. In a partial lunar eclipse — and before and after the peak of a total lunar eclipse — a chunk of the moon is dark while the rest remains visible.
Lunar eclipses are easy to watch, and don't require any special glasses or telescopes to view. Of course, as with most sky events, being in an area with less light pollution makes it easier to see the changing face of the moon. Aside from the consideration, though, lunar eclipses — when they occur — are visible in some stage from approximately half the earth.
Partial Solar Eclipse
In a partial solar eclipse, the moon crosses in front of the earth to partially block our view of the sun. Because the sun emits so much light, we on earth generally won't notice that our day has become any darker — but, if we look towards the sun, a chunk will be missing from the circle, as if something took a bite out of it.
Unlike with lunar eclipses, where nearly all of the hemisphere that's experiencing nighttime can see it, a partial solar eclipse isn't visible everywhere bathed in daylight. However, it can nonetheless often be seen by a relatively large swath of the earth's population, and its appearance may range from looking like there is only the tiniest sliver of the sun missing, to a sight approaching a total solar eclipse.
The number of partial solar eclipses is variable, but they appear on average twice per year. They are more likely to occur near the poles, but remember: even during a partial eclipse, it's dangerous to look directly at the sun. If you're preparing to watch an eclipse, be sure to have effective eclipse glasses on hand.
Annular Solar Eclipse
An annular solar eclipse is less common than a partial solar eclipse. It refers to an event when the earth, moon, and sun are perfectly aligned, but the moon is far enough from the earth in its orbit that it doesn't completely block out the sun. This is due to the fact that the moon's orbit is not a perfect circle centered around the earth, which leaves it further from — and closer to — us at various points during the lunar month.
During an annular solar eclipse, you'll see a thin ring of light surrounding a silhouette of the moon where the sun is in the sky. This ring is all that's visible of the sun, peeking around the sides of the moon, and it leads to these events also being known as "ring of fire" eclipses. Because so much of the sun is blocked, certain areas of earth will experience relative darkness for a few minutes during an annular eclipse. Outside this peak area, a wider band of the earth will experience a partial eclipse.
Even though much of the sun is blocked during an annular eclipse, it's still dangerous to look directly at it. Use eclipse glasses if you want to look directly at the flaming ring, and, if you're hoping to catch one soon, check out NASA's page on solar eclipses for information on when and where the events will be visible in the coming years.
Total Solar Eclipse
This is the big one, the top dog, the MVP of eclipses: the total solar eclipse. When this happens, the earth, moon, and sun are in perfect alignment, and the moon is close enough to earth — while the earth is far enough from the sun — for the moon to completely obscure the face of the sun. Sound impossible? A total solar eclipse is the rarest of eclipses and only happens about once every year or two. Even when it happens, however, it is still only a relatively narrow band of the earth — the zone of totality — that will experience it as a total eclipse, while a larger population will experience a partial one.
In a total solar eclipse, only the solar corona is visible. The solar corona looks like a glowing circle around a dark sun, which can appear to be a hole in the sky. As the sun is blocked out in the zone of totality, darkness falls as if it were nighttime, and temperatures often drop quickly. The longest a total solar eclipse can last is seven and a half minutes, though most are much shorter, and viewers will experience longer partial eclipses both before and after.
A total solar eclipse is the only time it is safe to look directly at the sun. If you find yourself preparing to view a total eclipse, though, you'll likely want to have eclipse glasses with you anyway, in order to view it safely in its partial stages as the moon slowly eats up the sun. So when is the next total solar eclipse? December 14, 2020 — in South America. Pack your bags, we're headed to Argentina!