Distance and Rotation

By measuring the time it takes for a laser beam to reach reflectors placed on the Moon during the Apollo missions and return to Earth, scientists have found that the Moon is slowly moving away from Earth, at a rate of about 3.8 centimeters per year. At the same time, the rate of Earth’s rotation  is slowly decreasing over time. Scientists know how much the rate of Earth's spin is decreasing based on studies of historical eclipse observations, fossils, and sediment deposits.

The total energy of the Earth–Moon system is conserved, meaning it will be constant over time. As Earth loses energy and slows down, the Moon gains energy, which increases its orbital period and distance from Earth. Scientists can calculate the total energy of the Earth–Moon system using the masses of the Earth and the Moon, how fast they spin on their axes, and the distance between them. Since they know the rate that Earth’s spin is decreasing and the rate that the Moon is moving away, scientists can calculate what these values were long ago.

For example, the diagram shows that about 900 million years ago, the Moon had a distance of about 54 Earth-radii, compared to ~60 Earth-radii today. With the Moon at that distance, Earth’s day was about 19 hours long; there were 23 to 24 days in a month and about 464 days in a year.

Take Notes

Explain whether information about changes in Earth's spin and the Moon's distance from Earth over time support the giant impact hypothesis. How might the angle of the collision, as shown in the simulation on page 2, have affected Earth’s spin?