Page 8 - Curriculum Visions Dynamic Book

Closely coupled
The Earth and the Moon are bound to one another by gravity, just as they are both bound to the SUn.
The orbit of the Earth around the Sun is strongly influenced by the Moon. The Earth and the Moon actually orbit the Sun around their combined center of gravity, a point inside the Earth about 4,700 kilometres from its center. As a result, this unequal dumbbell-like pair each traces a very uneven path through space.
Sometimes the Moon is closer to the Earth (its perigee), when it orbits at just under 360,000 km altitude. At other times it is at its greatest distance
from the Earth (its apogee), when it may be almost 407,000 km from the Earth. This is due to the combined effects of the Earth, the Sun, and the other planets.
Plane of Earth’s orbit
The Earth’s polar axis tilts by as much as 23.5° to its orbit.
Plane of Moon’s orbit
The Moon’s polar axis only tilts by 1.5° relative to its orbit.
The combined center of gravity of the Earth and Moon is 4,700 km from the center of the Earth.
Similarly, the orbit of the Earth is influenced not just by the Sun but also by the positions of the other planets. As a result, the orbit of the Earth around the Sun is a wavy track, while the orbit of the Moon is like an open spiral.
The close coupling of the Earth and the Moon has many effects. One of the most significant is the braking effect that the Earth has had on the Moon. As a result, the Moon is in synchronoUs rotation and always faces the same way to the Earth; we never see the reverse side.
Although the Earth’s spin rate is slowing down, the Earth and Moon together keep the same momentUm. The only way for this to happen is for the Moon to move slowly away from the Earth. The result of this is that the Earth’s day and month get longer.
Sidereal and synodic time
It takes 27.322 days (27 days, 7 hours, 43 minutes, and
12 seconds)—called the sidereal month—for the
Earth and Moon to return to the same place against the background of the stars. But the Earth and Moon are also orbiting the Sun, and that has an important effect too.
 The Earth and Moon each spin on an axis tilted to the plAne containing their orbit. This diagram shows the orbit of the Moon around the Earth.
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