Lunar Eclipse and Lunar Phases

Few weeks ago a close friend of mine (Dev) asked me what the difference between a lunar phase and lunar eclipse is. I thought I knew the answer. But as I embarked upon an explanation, it dawned on me that I didn’t quite understand the lunar motions very well. Alas, all those years I have been looking at the moon… without realizing (and hence, marveling at) how it moved in space.

What I did know was: as the Moon orbits around the Earth, it’s always the same side of the Moon that faces the Earth. This is because the time it takes for Moon to rotate once is identical to the time it takes to complete an orbit around the Earth — 27.3 days. Hence, the Moon has a ‘near side’ and a ‘far side’ with respect to the Earth. We never observe the ‘far side’ from the Earth.

Now, at any given time, half of the Moon is illuminated by the Sun and the other half is dark due to Moon’s own shadow. When the Sun and the Moon are on the opposite sides of the Earth, the entire ‘near side’ is illuminated and thus we can see the entire ‘near side’: this is called full moon (poonam). When the Moon is between the Earth and the Sun, the ‘far side’ is illuminated and the ‘near side’ is dark, so we can’t see the Moon at all: this is called new moon (amavasya). And, the rest of the time (between full moon and dark moon) we’re able to see only a portion of the ‘near side’ — the portion that’s illuminated by the Sun.

Had the Moon revolved around the Earth in the same plane as the plane of Earth’s revolution around the Sun, every new moon would have resulted in a solar eclipse (i.e. the Moon covers the Sun and its shadow falls on the Earth) and every full moon would have caused a lunar eclipse (i.e. the Earth’s shadow falls on the Moon). But that doesn’t happen every time, because the plane of Moon’s orbit and the plane of Earth’s orbit are not the same (tilted by 5 degrees).

Every once in while, the Earth, Moon and Sun get aligned. If that happens during full moon, the Earth’s shadow on the Moon creates a lunar eclipse; and if it happens during new moon, a solar eclipse occurs.

So, that’s the difference between the lunar eclipse and lunar phases. The former is caused by Earth’s shadow on the Moon, while the latter by Moon’s illumination by the Sun (and how much of it we can see from the Earth).

Here’s a 3-minutes video explaining the phases of the Moon.

Related posts: One in a Blue Moon, Happy Leap Day!

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