Universe

Earth And Moon

The Earth's Moon is the fifth largest in the whole solar system, and is bigger than the planet Pluto.


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Earth & Moon Animation

Earth & Moon

The Moon's Orbit and Rotation

The Earth's Moon is the fifth largest in the whole solar system, and is bigger than the planet Pluto. The Moon has a nearly circular orbit (e=0.05) which is tilted about 5° to the plane of the Earth's orbit. Its average distance from the Earth is 384,400 km. The combination of the Moon's size and its distance from the Earth causes the Moon to appear the same size in the sky as the Sun, which is one reason we can have total solar eclipses.

It takes the Moon 27.322 days to go around the Earth once. Because of this motion, the Moon appears to move about 13° against the stars each day, or about one-half degree per hour. If you watch the Moon over the course of several hours one night, you will notice that its position among the stars will change by a few degrees. The changing position of the Moon with respect to the Sun leads to lunar phases.

It takes the Moon 27.322 days to go around the Earth once. Because of this motion, the Moon appears to move about 13° against the stars each day, or about one-half degree per hour. If you watch the Moon over the course of several hours one night, you will notice that its position among the stars will change by a few degrees. The changing position of the Moon with respect to the Sun leads to lunar phases.

Moon's Orbit and Rotation

Earth and Moon

Have you ever heard the term the 'far-side' of the Moon? Because of the effect on the Moon of tidal forces due to the Earth, the same side of the moon always faces the Earth. In other words, it takes the Moon the same amount of time to rotate around once as it does for the Moon to go around the Earth once. Therefore, Earth-bound observers can never see the 'far-side' of the Moon. Tidal forces cause many of the moons of our solar system to have this type of orbit.

If you could fly to the Moon at a constant speed of 1000 kilometers per hour, which is the speed of a fast passenger jet, it would take sixteen days to get there. Apollo astronauts reached the Moon in less than four days even though they coasted "uphill" almost the entire distance. They got a fast start.

Lunar-eclipse The Sun happens to be 400 times the Moon's diameter, and 400 times as far away. That coincidence means the Sun and Moon appear to be the same size when viewed from Earth. A total solar eclipse, in which the Moon is between the Earth and Sun, blocks the bright light from the Sun's photosphere, allowing us to see the faint glow from the corona, the Sun's outer atmosphere.
When the Moon is at apogee, it is 11% farther from Earth than it is at perigee. This is far enough that it cannot entirely block the bright light, so eclipses which occur near apogee are not total.
Perigee 363,300 km
Mean 384,400 km
Apogee 405,500 km
eclipses.png

Gravitational interaction (tides on the Earth caused by the Moon) transfers kinetic energy from Earth to the Moon, slowing Earth's rotation and raising the Moon's orbit, currently at a rate of 3.8 centimeters per year.

Orbital characteristics - Moon

Moons orbit
Perigee 363,104 km (0.002 4 AU)
Apogee 405,696 km (0.002 7 AU)
Semi-major axis 384,399 km (0.002 57 AU)
Eccentricity 0.054 9
Orbital period 27.321 582 d
(27 d 7 h 43.1 min)
Synodic period 29.530 588 d
(29 d 12 h 44.0 min)
Average orbital speed 1.022 km/s
Inclination 5.1450 to the ecliptic
(between 18.290 and 28.580
to Earth's equator)
Longitude of ascending node regressing by one revolution in 18.6 years
Argument of perigee progressing by one revolution in 8.85 years
Satellite of Earth

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