The international space station (ISS) orbits earth at an altitude of approximately 400km. It remains in orbit and does not crash into earth because the outward centrifugal force due to its motion is as big as the inward gravitational pull from earth. 

However, the presence of a thin atmosphere at its altitude creates drag, gradually slowing down the station and reducing its centrifugal force, causing earths gravitational pull to slowly pulling it toward earth. 

To counteract this, the ISS must be periodically boosted back into its orbit.

Properties of the Moon

In contrast, the moon does not experience atmospheric drag due to its location of averagely 384 0000km away from earth. It orbits earth, stable and governed by gravitational forces, around a common center of gravity between it and earth. 

This center is located approximately 4 700km from earths center (inside earth itself). The moon orbits us in 27 days while we always observe the same side because the moons rotational period matches its orbital period around earth.

Giant-impact theory

The most widely accepted explanation for the moon's formation is the giant-impact theory. According to this hypothesis, a Mars-sized object collided with the early earth, leaving a cloud of debris in space.

This cloud formed under the influence of its own gravity the moon. This explains why the moons composition is similar to earths mantle.

Tidal forces

The gravitational pull of the moon interacts with earths rotation, creating tidal forces: on side of earth facing the moon, the attractive force of the moon predominates and, on the side opposite to the moon, the centrifugal force predominates. They are the reason that earth is not a perfect sphere, it is slightly elliptical and stretch earth along the axis and cause ocean tides.

Tidal forces also cause earths rotational speed to slow down over time. This phenomenon results in the gradual lengthening of days. Currently, the length of a day increases by approximately 1,6 seconds every 100 000 years. Accordingly, 400 million years ago, a day lasted only about 22 hours and 2 billion years ago, a day was as short as 4 hours.