Monday, May 16, 2022

The difference between a lunar eclipse and a new moon

Trust NASA to explain things happening in the night sky. I've been reading about a total lunar eclipse we're supposed to be able to see in our neighbourhood. (Actually, I just checked and, guess what, it's cloudy - so much for that!) Apparently this is the first of two this year, the next being on 7th November, but the next such event after that is not scheduled until March 2025 (typically, they occur every two or three years).

But, hold on, I thought, how can there be an eclipse of the moon? Isn't that just what happens every 28 days? The earth gets in the way of the reflection of the sun's light from the moon every month, what we normally just call a "new moon". So, what's the big deal? Well, ScienceAlert (with the help of a NASA video) explains what I have been missing, although it's still not that easy to understand, even if it's not (quite) rocket science. 

A lunar eclipse occurs when the moon passes through the Earth's shadow. But the moon's orbit around the earth is slightly tilted relative the earth's orbit around the sun by about 5°, so that the earth and the moon's shadow casts rarely shade each other completely. This relative tilt, though, changes slightly as time goes on, and every now and then this puts the moon in just the right position to pass through the earth's shadow, causing a lunar eclipse for a period of an hour or so. The moon does not completely disappear from us during the eclipse, though, due to sunlight scatttered and refracted through the earth's atmosphere, and it appears a deep dramatic red or copper colour. Before and after, the moon would appear as a normal full moon. The NASA video makes it clearer than my words do.

So, the difference between a lunar eclipse md a new moon is that a lunar eclipse is when the earth comes between the sun and the moon, blocking the sun from illuminating the moon from earth's perspective, while a new moon is when the moon comes between the sun and the earth. A lunar eclipse only occurs on a full moon night, but not EVERY full moon because of the relative tilt of the moon's orbit around the earth (a partial lunar eclipse occurs about twice a year, but a full lunar eclipse is a rarer event). Here is another explanation of the difference between the two (also with a short video), if you are not fully satisfied.

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