Friday, July 15, 2011

Movements of the Moon - Part II

So, in yesterday's post, we discussed how the Moon goes through a cycle of phases - New, Waxing Crescent, 1st Quarter, Waxing Gibbous, Full, Waning Gibbous, 3rd Quarter, Waning Crescent, and New again - in 29.53 days. Today we're going to see why it does this each synodic month.

Let's look at the diagram below - the phases of the Moon are entirely due to the geometrical relationship between the Earth, Moon, and Sun. By the way, contrary to popular misconception, the phases of the Moon ARE NOT due to any shadow from the Earth on the Moon (lunar eclipses are, but that's a different story).

In this diagram, the Sun is off to the right.  Let's imagine we're looking down from above the North Pole of the Earth (the Earth really isn't pictured that way in the image, but ignore that!).  Also ignore the fact that the Earth and Moon sizes are a bit off (the Moon's diameter is about 1/4 the diameter of the Earth) and that their relative distances here are way off (if the Moon and Earth were scaled to the size of a tennis ball and a basketball respectively, they'd be 24 feet away from each other!  Read this for more information).  In other words, the diagram is NOT to scale.

Also in the diagram above, note that the side of the Earth facing the Sun is lit and the side facing away from the Sun is not lit.  Daytime and nighttime on planet Earth.  Now imagine the Earth rotating counterclockwise (from above the North Pole) on its axis once every 24 hours so someone standing on the Earth will travel from night past the terminator (the line between dark and light) into daytime (they'll see the Sun rise in the eastern sky).  The Sun will then rise higher in the sky until it's over their head and then sink into the west and the person travels on the rotating Earth back toward the opposite terminator.  The Sun will set on the western horizon as the person rotates back into the dark half of the Earth and will be back where they started after the 24-hour day. 

For now, we'll ignore the fact that the Earth's axis is tilted and that's why there are more than 12 hours of daylight for us here in the mid-latitudes during the summer and less than 12 hours of daylight in the winter.  We will come back to this later because the Moon's orbit around the Earth is a bit tilted too!

Back to the diagram of the Moon orbiting the Earth.

Similar to the Earth, the side of the Moon facing the Sun is also lit and the side facing away is unlit.  Day and night on the Moon occurs for the same reason it does on the Earth.  There is no permanently "dark side of the Moon" despite Pink Floyd album names.  The only different is that a "day" on the Moon is much longer than a day on the Earth because the Moon rotates on its axis much more slowly.  More on this later.

Anyway, as you can see from the diagram above, when the Moon lies between the Earth and the Sun, the side facing the Sun is lit and the side facing the Earth is unlit.  Also note that such a Moon is high in the sky around noon for the daytime Earth (the side of the Earth that's lit).  Therefore we simply don't see the Moon in the sky.  It's the New Moon phase.

Conversely, when the Moon is on the opposite side of the Earth from the Sun, the side facing the Earth is completely lit and it's high in the night sky around midnight.  Therefore we see it as a Full Moon.

The Moon orbits the Earth in a counterclockwise fashion (as seen from above the North Pole) just as the Earth rotates counterclockwise.  So you can see the 1st Quarter Moon is high in the sky around sunset and from the Earth, you would see the right-hand side of the Moon as lit.  Similarly, the 3rd or Last Quarter Moon would be seen around sunrise and you would see the left-hand side lit (unless you're upside-down in the Southern Hemisphere).

Hopefully, you can also visualize why you see waxing or waning crescent or gibbous moons.  From the perspective of being on the Earth, you will see either less than half the Moon lit (crescent phase) or more than half the Moon lit (gibbous phase).   Note that waxing crescents would be seen in the afternoon and waning crescents in the morning (before noon).  Waxing gibbous is after sunset and waning gibbous before sunrise.  In other words, don't bother looking for a crescent moon high in the sky in the middle of the night.  It will either be setting in the evening after sunset (with the right side lit) or rising in the early morning hours before sunrise (left side lit).

Make sense?  Next we'll talk about how long it takes the Moon to orbit around the Earth.

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