All the planets except Venus and Mercury have moons. The Earth's Moon
is the fifth largest in the whole solar system, and is bigger than the
planet Pluto. The Moon's orbit is almost a perfect circle, so that
the Moon is about 384,400 km away all the time. The Moon's size and
it's distance from the Earth cause the Moon to appear the same size in
the sky as the Sun, which is one reason we can have total solar
eclipses.
Like all moons, our Moon is going around it's planet, the Earth. It takes the Moon about 27 days to go around the Earth once. If you check on the Moon several times during one night, you will notice that it moves relative to the stars around it. As the Moon goes around the Earth, different portions of it are lit up by the Sun, causing lunar phases.
Have you ever heard the term the 'far-side' of the Moon? The Earth's
gravity produces tidal
forces on the Moon. This causes the same side of the moon to
always face the Earth. People living on the Earth can never see the
'far-side' of the Moon, unless they go there! Tidal forces cause many
of the moons of our solar system to always face their planets.
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. It's average distance from the Earth is
384,400 km. The combination of the Moon's size and it's 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 it's 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.
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 one time as it does for the Moon to go around the
Earth one time. 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.
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. It's average distance from the Earth is
384,400 km. The combination of the Moon's size and it's 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.
The Moon's orbital period is 27.322 days. Because of this motion, the Moon appears to move about 13° against the stars each day, or about half of a degree per hour. If you watch the Moon over the course of several hours one night, you will notice that it's 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.
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. The rotation period and the orbital period of the Moon are
the same. 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.
The source of this material is Windows to the Universe, at http://www.windows.ucar.edu/ at the University Corporation for Atmospheric Research (UCAR). ©1995-1999, 2000 The Regents of the University of Michigan; ©2000-01 University Corporation for Atmospheric Research. All Rights Reserved. Site policies and disclaimer