The photos above show assorted phases of the Moon and various stages of solar and lunar eclipses. Can you identify what's shown in each image? If you're not sure you can, you're in good company. We regularly see images of solar eclipses with captions describing them as lunar eclipses (and vice versa) and photos of different phases of the Moon described as showing stages of a lunar eclipse (and vice versa). We hope this page will clear up the confusion.
Eclipses of the Sun and Moon happen only a few times each year, and each eclipse is visible to only a fraction of Earth's population. But the Moon goes through its cycle of phases every month, and everyone on the planet gets to watch, so people are more familiar with lunar phases than with how the Moon looks during a lunar eclipse. And not everyone realizes that during some stages of a solar eclipse, the Sun can take on the same shape as the Moon at some phases of its monthly cycle.
Phases of the Moon
The cycle of lunar phases lasts about 29½ days, measured from any particular phase to the next occurrence of the same phase. Here's what the progression of phases looks like and the corresponding "age" of the Moon, which astronomers measure from one new Moon to the next:
Do you know why the Moon shows different phases at different times? The Moon circles Earth once a month as Earth circles the Sun once a year. When the Moon is opposite the Sun in the sky, we see its near side fully illuminated. We call this phase "full Moon." When the Moon is in the same direction as the Sun, its illuminated side (the far side) faces away from us, and the near side is dark, so we don't see it. We call this phase "new Moon." (During a solar eclipse, you can see the new Moon — in silhouette against the Sun!) Between new Moon and full Moon more and more of the near side comes into daylight. Between full Moon and new Moon more and more of the near side falls into darkness. So the Moon shows different phases because different parts of the lunar surface are in sunlight at different times as the Moon goes around Earth.
This website focuses exclusively on solar eclipses, but we're going to mention lunar eclipses here so you can see how the shape of the Moon during a lunar eclipse differs from what you see as the Moon goes through its monthly cycle of phases.
A lunar eclipse occurs when the Moon passes through Earth's shadow. This can happen only at full Moon, that is, when the Moon is opposite the Sun. In contrast, a solar eclipse occurs when the Moon passes between the Sun and Earth, casting its shadow on our planet. This can happen only at new Moon.
At the Moon's distance, Earth's shadow (which is circular, because Earth is a sphere) spans about 2⅔ lunar diameters, so it takes a while for the Moon to enter, cross, and exit that shadow. Exactly how long depends on the Moon's path through the shadow; the closer to the center it goes, the more time it spends in eclipse. The next illustration shows the progress of the total lunar eclipse of September 2015, when the Moon passed below the center of Earth's shadow:
The Moon travels east against the starry background by about its own diameter every hour, so it takes about that long for the Moon to enter Earth's shadow and again to exit; these are the times of partial lunar eclipse. Totality, when the Moon is entirely within the shadow, can last up to a few hours, depending how close to a "bull's eye" the Moon makes with the shadow. The total phase of the eclipse pictured above lasted about 1¼ hours, which is fairly typical.
Note that during the partial phases of a lunar eclipse the line between darkness and daylight is always concave. When deep in partial eclipse, the Moon takes on roughly the same shape as during its waxing (growing) or waning (shrinking) crescent phase. (The difference is subtle but noticeable if you look closely: The "horns" of the crescent reach farther around during a crescent Moon than during a lunar eclipse.) But when close to the beginning of the partial eclipse before totality and close to the end of the partial eclipse after totality, the Moon doesn't look at all like it does during its waxing or waning gibbous phase, when the line between darkness and daylight appears convex rather than concave.
Finally we come to solar eclipses:
During a solar eclipse, the Sun can appear as a crescent just like the Moon! And, as during a lunar eclipse, the line between sunlight and darkness is always concave — but now it's sunlight on one side of the line and the silhouette of the Moon on the other side. Note that during totality, you're seeing the exact moment of new Moon, something that's otherwise invisible. That's just one more remarkable aspect of total solar eclipses, which are already remarkable in so many ways! (See "The Solar Eclipse Experience" to understand why.)
OK, so now you should be able to identify all the photos at the top of this page. All were shot from the Northern Hemisphere. From left to right, they show a partial lunar eclipse after totality, a partial solar eclipse before totality, a waning crescent Moon, the uneclipsed Sun, a total solar eclipse, a full Moon, a partial solar eclipse after totality, a partial lunar eclipse before totality, and a waxing crescent Moon. How did you do?!