Earth's moon, Luna, is about 1/4 the size of Earth. It is roughly 3476 km in diameter. It has only about 1/6 the gravity present on Earth. This means that to calculate your "moon weight", you divide your Earth-weight by six.
The moon rotates on its axis every 29 1/2 Earth-days. It also revolves around the Earth in the same amount of time. Because of this fact, one moon-day is exactly as long as one moon-year. A day on the moon is the same length as a year! Also, it means that the same side of the moon ends up facing the Earth all of the time. In fact, that side of the moon has developed a bulge, due to the constant effect of Earth's gravity. The moon is approximately 4.6 billion years old based on data from the Apollo missions. It certainly appears that the moon is just about the same age as the Earth. This was not always thought to be the case. Arguments have been made in the past in support of the age of the moon being either much greater, or much less than the Earth's age. We now believe that about 4.5 billion years ago, soon after the Earth formed, a large object, about the size of Mars collided with this early Earth. This earliest version of Earth was completely devastated, and enough material was blown out into space by the impact to create the moon out of the "debris". The moon has been with us, each night, ever since.
The moon has several interesting, and highly visible, features.
Highlands: Highlands on the moon are higher, lighter areas on the moon's surface.
Altitudes of the moon's highlands can reach 8000 meters! (That's about 3 miles!)
Lowlands: Lowlands on the moon's landscape are lower, darker areas. Some lowlands are termed "Maria", a Latin word meaning "seas". Long ago, ancient astronomers believed that the moon was covered with liquid water. The Maria were large basins containing the moon's "seas" and "oceans". There are a total of fourteen maria or "seas" on the moon. (Ex: Mare Imbrium-sea of dreams; Mare Tranquillitatis-sea of tranquility; Oceanus Procellarium- Ocean of storms; etc.) It is important to note that there isABSOLUTELY NO EVIDENCE OF WATER ON THE MOON TODAY, OR AT ANY TIME IN THE PAST. Never had it, never will.
Craters: Craters are evidence of the great impacts that have occurred on the surface of the moon throughout its long history. The largest crater on the moon is CRATER COPERNICUS, which is about 91 km in diameter.
Rays: Rays are light colored streaks of dust that extend from the edges of craters in a star-like pattern. Rays are the result of the low gravity on the moon's surface. They are a type of "splash effect" seen only in low gravity situations.
The moon is a desolate place. It has no atmosphere to speak of, and therefore, no weather of any kind. The surface of the moon is not exposed to the forces of erosion or weathering, as are surfaces here on Earth. The lack of erosion means that the surface of the moon doesn't change very much over time. The footprints that the astronauts left in 1969 are still up there in the dust, looking pretty much the way that they did when they were put there over 30 years ago. Average temperatures on the surface of the moon vary greatly from night to day. Daytime temperatures reach +127 degrees F, while nighttime temps dip to near -173 degrees F!
The moon shines by reflected light, producing no light of its own. We on Earth see a different section of the lighted portion of the moon each night, so therefore, we see "phases" of the moon. Every 29 1/2 days the moon goes through all of its phases, as it makes its orbit around the Earth.
Large objects in space, like planets and moons cast different shadows than we do here on Earth. The large shadows cast by celestial bodies have two parts. An umbra is the inner, darker part of the shadow. A penumbra is the larger, outer, lighter portion of the shadow.
When the moon is in the Earth's umbra, we see a Total Lunar Eclipse.
When the moon is in the Earth's penumbra, we see a Partial Lunar Eclipse.
When the Earth is in the moon's umbra, we see a Total Solar Eclipse. A Total Solar Eclipse is a fairly rare phenomenon and worth further study. Follow this link to an image of a total solar eclipse. Once you're there, you may follow other links to interesting sites devoted to this spectacular occurrence.
When the Earth is in the moon's penumbra, we see a Partial Solar Eclipse.
The moon's gravity also affects the Earth, in that it causes a bulge to occur in our oceans. These bulges are called "tides". Every 24 hours on Earth there are two high tides and two low tides in most places. These are called semidiurnal tides. Maine has this sort of tide system. Every six hours, in a given spot, the oceans goes from being at high tide, to low tide. If there's a high tide somewhere at noontime, by six PM, the tide will be a low as it gets. By midnight, the tide would be back to being as high as it gets. In some parts of the world there are only two tides each day, one high and one low, these are called diurnal tides. The high tide on the side of the Earth facing the moon is called a Direct Tide. The high tide on the side of the Earth not facing the moon is called an Opposite Tide. During certain phases of the moon, especially the full moon and new moon, the gravity from the moon works with the gravity from the sun, both sort of pulling in the same direction. This causes a higher than normal high tide called a Spring Tide. At other times during the month, the moon's gravity works against the sun's gravity. The forces sort of cancel out partially, and the high tide is much lower than normal. This is called a Neap Tide, and these tides occur during the first and last quarter phases of the moon.
Our moon also has had a major role in making the Earth the planet that it is today. The Earth-Moon system is really a "twin planet". Most people don't normally think of our moon as a companion planet, but that's really exactly what it is! Our moon is about one quarter the size of this planet, which makes Earth the only planet in our solar system with such a sizable moon in relation to our planet's size. Our moon's gravity stabilizes Earth's tilt on its axis. Without the moon's stabilizing influence, our planet would wobble on it's axis like a top. The axial tilt of the Earth is responsible for our seasons here on Earth. Without our moon, Earth would wobble on it's axis, making for huge changes in our seasons. Farming would be all but impossible because of these seasonal changes. Earth's ability to produce enough food would be devastated. Without a moon we would expect huge climatic shifts, which would also take a terrible toll on life here on our planet. Our moon also stabilized the rotation of the Earth, giving us our familiar 24 hour days. When our Earth was just a young, newly formed planet, our days were only approximately 4 hours long! The moon's steady gravitational influence keeps the Earth spinning at the right speed and angle to provide the sort of stability necessary for the long and successful development of life. The moon is also involved in the biological processes of many organisms on Earth. Many species mate and reproduce in response to the lunar cycles. Though many people think that the moon is just a dusty, dead object, Earth Scientists realize that our Earth owes its very existence to this lovely little satellite. The moon allows Earth to be the wonderful world that it is, in more ways than most people can imagine. Now that YOU know, perhaps you'll think a little bit more about the moon the next time that you see it up there in the sky.
Nothing lasts forever though, and our moon is slowly escaping its Earth orbit. The moon is slowly slipping away from Earth's grasp at a rate of about 1.5 inches per year. In another two billion years it will have receded so far that it won't be able to keep us steady and we'll have to come up with some other solution. In the meantime, remember to think of the moon as more than just another attractive feature of the night sky. It means everything to us!