The Moon is an astronomical body that orbits the Earth, being Earth's only permanent natural satellite.
It is the fifth-largest satellite in the Solar System, and the largest among planetary satellites relative to
the size of the planet that it orbits. The Moon is thought to have formed about 4.5 billion years ago, not
long after Earth. The most widely accepted explanation is that the Moon formed from the debris left
over after a Mars-sized body collided with Earth.
The Moon is in synchronous rotation with Earth, always showing the same face with its near side
marked by dark volcanic maria that fill the spaces between the bright ancient crustal highlands and the
prominent impact craters. It is the second-densest satellite among those whose densities are known.
The Moon is the brightest object in the sky after the Sun, and it is visible from Earth in its entirety. The
Moon's gravitational influence produces the ocean tides, body tides, and the slight lengthening of the
day. The Moon's current orbital distance is about thirty times the diameter of Earth, causing 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.
The Moon's apparent size in the sky is almost the same as that of the Sun, since the star is about 400
times the lunar distance and diameter. Therefore, the Moon covers the Sun nearly precisely during a
total solar eclipse, allowing the relatively safe direct observation of the photosphere of the Sun. This is
because the distance between the Moon and Earth is about 400 times the diameter of the Moon, and
the diameter of the Sun is about 400 times the diameter of the Moon.
