Where is the Moon?
Published on Aug 9, 2006 at 12:01 pm.
1 Comment.
Filed under moon, skywatching.
Go out and look for the Moon. If you go out tonight (August 9), you’ll probably find it (unless buildings, trees, or clouds are in the way!). But, If you wait for a week, when you go out to look for the Moon, you’ll have to wait until after midnight for it to rise. Well, this makes sense, if you think about it, because the Moon is going around the Earth. That is actually why it goes through phases. Sun is always shining on the Moon, but as it goes around the Earth, our perspective changes The side towards the Sun is always lit, so when the Moon is between the Earth and the Sun, the dark side is facing Earth, so we don’t see the Moon. At Full Moon, the Moon is positioned opposite the Earth from the Sun, so from Earth we see the whole thing lit up. At First and Last Quarter phases, then the Sun, Earth, and Moon make a sort of right triangle (actually it is a tiny bit off, but let’s now worry about that just now). So, from out persepective, we only see half of the Moon lit up.
The orientation of the Moon also dictates when it is up. New Moon is lined up with the Sun, as seen from Earth, so it rises at sunrise and sets at sunset. Full Moon is opposite the Sun, so it rises at sunset and sets at sunrise. Firt Quarter rises at noon, is highest in the sky at sunset, and sets at midnight. Last Quarter is the opposite: it rises at midnight, sets at noon, and is highest in the sky at sunrise.
The Full Moon is opposite the Sun in the sky. That means that it is highest at midnight. (Note: this is midnight Local Mean Time, which probably isn’t the same as what your clock is set for, but it should be within an hour or so of midnight on the clock.) But being opposite the Sun has an interesting effect on where you need to look in the sky to find the Moon.
As the Earth goes around the Sun, it is tilted about 23.5 degrees on its axis. This means that the Sun sometimes is shining more direcly on the Northern Hemisphere, and sometimes more directly on the Southern Hemisphere. So, if you are in the Northern Hemisphere, the Sun will appear higher in the sky when it is shining more directly on that Hemisphere (as it has been for the last few months). However, if you are in the Southern Hemisphere, then the Sun has been lower in the sky for the last few months because it was shining more directly on the Northern Hemisphere. That will soon change, though. From late September until late March, the Sun will be shining more directly on the Southern Hemisphere (most directly so in December and January), and so the Sun will be higher in the sky down South, but lower in the sky here in the Northern Hemisphere. This is the reason for the seasons, by the way.
But this does have an effect on where you look for the Moon. Today is Full Moon. So, the Moon is opposite the Sun. But, the Sun is still north of the equator, so it is rather high in the sky than average at noon in the Northern Hemisphere, but lower in the sky than average in the Southern Hemisphere. The Moon is opposite. So the Full Moon won’t appear terribly high in the sky tonight here in the Northern Hemisphere (in fact, it will be quite low in the sky for observers from very far north). But, for Southern Hemisphere observers, it will appear rather high in the sky.
The extremes of this effect can be observed near the solstices. For a Full Moon in late June or early July, then the Sun is the farthest north that it can get, the Full Moon will be very low in the sky for observers in the Northern Hemisphere. For most observers in the temporate zones, it would appear no higher than a few fists above the southern horizon! In fact, for observers north of the Arctic Circle, the Full Moon would not be visible at all!
It is the other way around in late December or early January. At that time, the Sun shines more directly on the Southern Hemisphere, so it is low in the sky for us here in the north, but that makes the Full Moon, which is opposite the Sun, appear the highest that it can be in our skies. At the same time, a Full Moon near that time of the year would be the lowest that it could be for observers in the Southern Hemisphere.
So, go out tonight, look at the Full Moon, and note where you see it. Then keep going out over the next few months at Full Moon, and note where you see the Moon. For parents and teachers, this is a great exercise for the kids to do to learn the motions of the Moon in the sky!
-Astroprof
Sue Hawkins on February 28, 2007 at 1:32 pm: 1
Thank you so much. My 18 month old son loves to look at the moon, but I never seem to know where to look for it. Now I do. This explanation was very easy to follow, and (hopefully) easy to remember as my son grows older and wants to learn about the moon himself.