The Moon is Earth's only known natural satellite, and the fifth largest satellite in the Solar System. It is the largest natural satellite of a planet in the Solar System relative to the size of its primary, having a quarter the diameter of Earth and 1⁄81 its mass. The Moon is the second densest satellite after Io(moon), a satellite of Jupiter. It is in synchronous rotation with Earth, always showing the same face; the near side is marked with dark volcanic maria(lunar mare) among the bright ancient crustal highlands and prominent impact craters. It is the brightest object in the sky after the Sun, although its surface is actually very dark, with a similar reflectance to coal. Its prominence in the sky and its regular cycle of phases have since ancient times made the Moon an important cultural influence on language, calendars, art and mythology. The Moon's gravitational influence produces the ocean tides and the minute lengthening of the day. The Moon's current orbital distance, about thirty times the diameter of the Earth, causes it to appear almost the same size in the sky as the Sun, allowing it to cover the Sun nearly precisely in total solar eclipses.
|Moon just past full, in the darkness |
of the night sky.
The Moon, located 238,000 miles from Earth, has a temperature of 225° F during the day and drops down to –243° at night. With a diameter of 2,159 miles (3,475 kilometers), the Moon is just one-quarter the size of Earth. The Moon spins on its axis once every 27.3 Earth days. It takes the Moon 27.3 Earth days to revolve around our planet one time.
The Moon's surface is covered with craters, mountain ranges, rilles (long narrow channels), and lava plains. The vast, dark regions we see on the Moon's surface are called maria, or seas. They are actually very large, smooth lava beds. The bright, light areas on the Moon's surface are called highlands. The Moon is covered with a solid, rocky crust about 500 miles (800 km) thick. Underneath the crust, scientists think there is a partially molten zone that leads to a small core of iron. Craters on the Moon come in a wide variety of sizes. The largest crater measures 1,600 miles (2,575 km) across, while the smallest is the size of a pinprick.
To escape the Moon's gravity, you need to travel 5,200 miles (8,400 km) per hour, compared to 25,000 miles (40,200 km) per hour necessary to escape Earth's gravity. Earth's gravity is six times greater than the Moon's.
|Apollo 11-17 First man on the Moon - Apollo Moon Landings|
On July 20 ( July 21 GMT ) 1969 the first man stepped on the moon. During the next 3 years 6 missions to the moon was made and a total of 12 astronauts walked on the moon. These missions returned over 380 kg of lunar rocks, which have been used to develop a detailed geological understanding of the Moon's origins (it is thought to have formed some 4.5 billion years ago in a giant impact event involving Earth), the formation of its internal structure, and its subsequent history. After the Apollo 17 mission in 1972, the Moon has been visited only by unmanned spacecraft, notably by the final Soviet Lunokhod rover.
|The monthly changes of angle between the direction of illumination by the Sun |
and viewing from Earth, and the phases of the Moon that result
Throughout each month the moon will change from a crescent shape to a full moon. The stages in between are called phases. After seven days the moon will change from a crescent to a half moon. For the next seven days the moon will change from being a half moon to a full moon. The reason why it has full and half moons depends on how the sunlight reflects off of the surface of the moon and how much of it we can see. The shadow on the moon always goes from east to west. The line that divides the part we can see from the dark part is called the terminator.
The moon rises and sets every day, appearing on the horizon just like the sun. The time depends on the phase of the moon. It rises about 30 to 70 minutes later each day than the previous day, so the moon is out during daytime as often is it's out at night. At the time of the new moon, the moon rises at about the same time the sun rises, and it sets at about the same time the sun sets. As the days go by (as it waxes to become a crescent moon, a half moon, and a gibbous moon, on the way to a full moon), the moon rises during daytime (after the sun rises), rising later each day, and it sets at nighttime, setting later and later each night. At the full moon, the times of moonrise and moonset have advanced so that the moon rises about the same time the sun sets, and the moon sets at about the same time the sun rises. As the moon wanes (becoming a half moon and a crescent moon, on the way to a new moon), the moon rises during the night, after sunset, rising later each night. It then sets in the daytime, after the sun rises. Eventually, the moon rises so late at night that it's actually rising around sunrise, and it's setting around sunset. That's when it's a new moon once again.