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Lunar Eclipse

Tomorrow night, which is Sunday, September 27th, 2015, there will be a lunar eclipse that starts around 8 PM Eastern Daylight Time. I am sure that most people know what a lunar eclipse is, but I thought that it might be interesting to discuss the phases of the moon in relationship to the lunar eclipse.  Also, it is a good excuse to talk about the dark side of the moon.

Let’s first discuss the phases of the moon. The illustration below shows four phases of the moon, which are spaced roughly one week apart for a month. The figure is obviously not to scale, but the relative positions of the Earth, the sun and the moon are roughly accurate.  When the moon is furthest away from the sun, so that the order of the bodies from left to right, go moon, Earth, sun, then the moon is completely full.  This is because the sunlit side of the moon is completely facing the Earth, allowing us to see the whole half of the moon.

An illustration of the phases of the moon, showing the moon in four different positions (roughly every week for a month)
An illustration of the phases of the moon, showing the moon in four different positions (roughly every week for a month)

When the moon is between the Earth and the sun we can’t see the moon at all, and it is termed a new moon. Half of the moon is still illuminated by the sun, but we just can’t see that side, since it is facing away from us.

When the moon is off to the side of the Earth, we can see a quarter of the moon, since we see roughly half of the part that the sun is shining on.

Interestingly, you can only see the moon during certain times of the day, depending on the phase.  For example, during a full moon, you can only see the moon when it is dark out (like the person standing on the dark side of the Earth in the illustration – they can see the full moon). That is because, if you are on the dayside of the Earth, then you can’t see the moon, since the Earth is between you and the moon.  If you are at dawn or dusk, the full moon will be on the horizon (opposite to the sun).

The exact opposite is true for a new moon (not that you can actually see the new moon, but when there is a tiny sliver of the moon you can see) – you can only see a new moon during the day – never at night. If you are on the night side of the Earth, the Earth would be between you and the new moon.  If you are at dawn or dusk, then you could see the new moon just on the horizon (close to the sun).

Now, the “dark side of the moon” is a complete misnomer. As you can see from the illustration, the dark side of the moon changes all of the time.  Really, the “dark side of the moon” is simply the side of the moon that we can’t actually see. Ever.

It is theorized that the Moon was once part of the Earth. A long time ago a gigantic asteroid slammed into the Earth, and ended up ejecting a chunk, which ended up being the moon.  As the moon was cooling down and orbiting around the Earth, a heavy part formed. The heavier side of the moon faces the Earth all of the time, so that the moon is considered “phase-locked” with the Earth, meaning that the same side of the moon faces us all of the time. This is illustrated below, with a little circle showing the heavy part of the moon, which constantly faces the Earth.

An illustration of the phases of the moon, with the same side of the moon facing the Earth highlighted.
An illustration of the phases of the moon, with the same side of the moon facing the Earth highlighted.

The side that is facing us is dark sometimes (new moon) and in sunlight sometimes (full moon).  The “dark side of the moon” is simply the side of the moon that we can’t see, since we only see one side of the moon ever.  It is dark about half the time and in sunlight about half the time, just like the side of the moon that is facing us. The first time that humans ever observed the side of the moon that we can not see is when the USSR’s Luna-3 orbited the moon in 1959 and sent pictures back to Earth.

Mercury is phase-locked with the sun, so that the same side of Mercury faces the sun all of the time.  This makes one side of Mercury super-hot, and the other side of Mercury super-cold.  It is theorized that if Earth were phase-locked with the sun, then no life on Earth would have formed, since one side would be extremely hot, and the other side would be extremely cold, making it an inhospitable hell-hole. Since the Earth doesn’t have any really heavy bits to tug one side towards the sun, we lucked out and have a nice 24 hour day. Perfect for humans!

A lunar eclipse is when the moon passes into the shadow of the Earth, as illustrated below.

A lunar eclipse!
A lunar eclipse!

One might imagine that a lunar eclipse would happen every single time that there is a full moon.  It would if the Earth and the moon were always in the same orbital plane, but they are not. The moon’s orbital plane is tipped with respect to the Earth’s orbital plane, so that when there is a full moon, it is almost always either a bit above or below the Earth’s shadow.  There are only a few hours of each month in which the moon is in the orbital plane of the Earth; and it happens that this month those few hours are when it is a full moon, and it will pass into the shadow of the Earth. A relatively rare treat.

Hopefully it won’t be cloudy where you are. We are supposed to be partly cloudy, which seems like the best you can ask for in Michigan.

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