Moon Struck


The Moon dominates the night sky. It is said to influence things on earth. We know it influences the tides. It has also been said that more babies are born during a full moon and that more crimes are committed during a full moon. It influences painters, poets, and composers. It is even said that it influences mental health. Where do you think the word lunatic comes from? Is the Moon one of your favorite things in the night sky? Do you enjoy watching it go through its phases? Here are a few facts that will make observing the Moon all the more enjoyable.


Moon Data Box

Diameter: 3476 km

Mass: 0.0123 x Earth

Mean density: 3.34 x water

Volume: 0.02 x Earth

Escape velocity: 2.37 km/sec

Mean sidereal rotation period: 27.32 days

Apparent magnitude (full): -12.7

Mean distance from Earth: 384,400 km


"Luna, or as it is better known, the Moon is our closest neighbor in space. The plane of the Moon's orbit is inclined about 5 degrees with respect to the plane of the Earth's orbit. When that is projected onto the Celestial sphere, the Moon's path closely follows the ecliptic, never straying more than 5 degrees north or south. The Moon can travel as far north as +2835 degrees declination or as far south as -28.5 degrees." Your Guide to the Sky by Rick Shaffer page 19.

Because the Moon approximately follows the ecliptic and thus the full Moon must be opposite the Sun, the full Moon in the winter is high in the sky, and in the summer, low.

We always see the same face on the Moon, even as it goes through its phases. This makes us think that the Moon is not rotating. This isn't what is really happening. because the Moon rotates on its own axis in exactly the same time that it takes to make an orbit around the Earth, it always presents the same face to us. If we were traveling in space, we would see this easily, but being on the Earth and at the center of the motion we do not notice it. More careful observation shows that the Moon has a small wobble in its rotation, so that we see very slightly more than half of its surface, 59 percent. The other 41 percent is permanently hidden to Earth. This slight wobble in the moons rotation is called libration.

Moon's Orbit

The Moon's orbit is elliptical so its distance from the earth ranges from 226,000 at its closest distance to the earth and 252,00 at its farthest distance from the earth. The terms used to describe the closest and farthest points of any satellite from Earth are perigee and apogee.

If you remember from the page on celestial coordinates, the ecliptic is the apparent path the sun, Moon, and planets appear to follow through the stars. The constellations of the zodiac lie along the ecliptic. It takes the moon 27.32 days to travel its path along the ecliptic.

We are more aware of the Moon's synodic period. That is the period between successive conjunctions with the sun.


"A conjunction occurs when two objects are lined up with our line of sight on Earth. When the Moon is lined up with the Sun, we say it is "in conjunction with the Sun." That is the technical term, but it's much more commonly known as the "new moon." The period between new moons, or the Moon's synodic period is 29.53 days, which is often called the lunar month. It is also called a lunation." Your Guide to the Sky by Rick Shaffer page 20.

The Phases of the Moon

During the lunar month or lunation the Moon goes through all of its phases. It all depends upon angle of the Moon and the Sun.


When the Moon is lined up with the sun from our view on earth, we have a new moon The sun is illuminating the far side of the Moon and the side facing the earth is dark. Because of that we don't see any Moon at all. The bright sun blocks it from our view. Besides the new Moon rises at approximately 6:00 AM and sets at approximately 6:00 PM. The Moon is not exactly lined up with the sun. When that happens we have a solar eclipse.


As the Moon moves eastward along the ecliptic it grows fatter and fatter. or "waxes fat." The Moon is moving away from the Sun in the sky and we see a little more of its sunlit side every night. This is the waxing crescent phase. This phase of the moon is a great time to see Earthshine. The Earth's water and clouds reflect sunlight which illuminates the dark side of the moon. We see this when the Moon is a crescent That is Earthshine. It is beautiful especially through binoculars.


The moon has waxed until half of the moon's disc is illuminated. It is at the first-quarter phase. because it is one fourth of way through its synodic period or lunation. It is one fourth of the way along its journey and 900 degrees east of the sun along the ecliptic. We on the earth are looking at the sunlit side of the Moon from off to the side. That is why the Moon is half light and half dark. Because it is 90 degrees from the sun, the Moon rises at around midnight and sits around noon.


The Moon continues to grow or to wax as it travels along the ecliptic. This is called a gibbous (bulging) Moon. This is the waxing gibbous phase.


The full Moon is half way around the ecliptic and 1800 from the Sun. We see the entire sunlit side of the Moon. This is the full moon. During this phase the Moon rises as the Sun sits and sits as the Sun rises.


Now the Moon is waning, or growing slimmer as it begins the second half of its journey along the ecliptic. This is the waning gibbous phase. If you watch carefully, you will notice that the moon rises and sets a little later every night now.


Now the moon is three quarters of the way along its journey and once again it is half light and half dark. Notice that the side that is lit during this phase is the side that was in darkness during the first quarter phase. The first quarter phase rose at noon and sit at midnight. This phase rises at midnight and sits at noon. This is the third quarter phase.


From now on the Moon is going to continue to wan until it is a crescent. This phase of the Moon is a waning crescent, Like the first and third quarter, the part of the Moon that is lit is the opposite of the waxing crescent. The Moon will continue to wan until we can't see it at all and it is a new moon again. Now it will start all over again. This is the Moon's synodic period or a lunation.

"Because its orbit moves the Moon eastward among the stars, the Moon rises later each night than the night before. If you timed the rising of the Moon each day for a month, you'd find it rises on the average fifty minutes later each night than the night before, but the variation can be as much as pause or minus fifteen minutes."

Your Guide to the Sky by Rick Shaffer page 20.

It would be a good family project to watch for each of these phases and note on a calendar or in a viewing log the dates your family observed them. It would also be fun for a family to make a note of when they see the Moon rising and sitting. Many people like to make a game our of seeing who will be first to see the waxing crescent. In order to see the slimmest crescent right after the new moon, you will need to be out looking for it as the sun sets for it will set very soon afterwards.

There is one more fun fact about the phases of the Moon that sparks the imagination. The phases of the Moon and the phases of the Earth and the Moon compliment each other. That means when we see a full Moon on the Earth , the astronauts on the Moon see a new Earth. when we see a waxing crescent Moon the astronauts see a waning crescent Earth. Fun, huh?

In his book Secrets of the Night Sky, The Most Amazing Things in the Universe You Can See with the Naked Eye, Bob Berman writes about another fun Moon fact. Have you noticed that the crescent lies on its back like a boat in February through April and the rest of the year it is on its side like a bow? I didn't know this until I read Mr. Berman's book. He also writes that the crescent is always on its back in the tropics. He said that a smiling crescent is the logo of the tropics. The folks in the polar areas only see a sideways crescent.


Observing the Moon

Get out your binoculars and your moon map. There are many wonderful things to be seen on the surface of the moon. First of all you will need a few definitions commonly used to describe where you are on the moon.

"A limb is just the edge of an object's disc. At full moon, you can see the entire limb of the Moon. At all other times, part of the Moon is in darkness. The edge of the Moon you see is then either the sunrise line for the period between new moon and full moon, or the sunset line for the period between full and new moon. Both these lines make the termination of the sunlit portion of the disc, so the are called the terminator.

Your Guide to the Sky by Rick Shaffer page 74

Most of the features of the Moon are seen best when they are near the terminator. This is true for all of the features except the maria and rays. These are best seen near the full Moon.


What can you see?


Valleys, rilles and domes can been seen with a telescope. You can see some of the other features with binoculars.

The members of the Astronuts, my astronomy club, are working on our Lunar Certificate through the Astronomical League. The Astronomical League sponsors many different observing clubs designed to encourage amateur astronomers in the sharpening of their observing skills. This is an excellent step for anyone who enjoys astronomy.

Moon Links

Touring the Moon with Binoculars From Sky and Telescope

Sky and Telescope' s Moon Map

I still get really excited about things that I see in the night sky. That is why I started this mailing list. Those who join this list will get email letters about things like meteor showers, planets that are up and all kinds of other wonderful events. I hope to someday include short articles about constellations. I just want everyone else to have as much fun as my astronomy club and I are having. If you are interested, sign up.




Updated December 4, 2002

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