The moon has been a constant feature in the sky above us. We see it change through the phases, and it constantly reminds us of the world where we live.
The moon is also constant in the fact that it always has the same side facing the Earth. This is due to the fact that it rotates on its axis in the same amount of time that it revolves around our planet.
In terms of distance, it is only moving about 3.8 cm from the earth every year. Considering how far the moon is from the earth, that is a relatively small distance, but it is significant over time.
NASA was able to determine this because a reflector array was placed on the moon in the Sea of Tranquility when astronauts visited in 1969. Laser beams are sent from the earth and bounce off the reflector.
Since the speed of light is constant, they are able to determine exactly how far the moon is from the earth because of the time it takes for the light beam to travel in two directions.
Dr. Jean Dickey explains this in an article, saying: “Lunar ranging involves sending a laser beam through an optical telescope. The beam enters the telescope where the eye piece would be, and the transmitted beam is expanded to become the diameter of the main mirror, then bounced off the surface toward the reflector on the moon.”
Although they can measure the 3.8 cm drift from the earth, they are sure it has happened since the moon was likely formed billions of years ago.
For one thing, scientists look at the layers of rock and eroded settlement on the moon to determine how close it was to the earth billions of years ago. They look to Australia and the eroded settlement layers to get an indication of how it went through the natural cycles.
One particular cycle they were interested in is the Milankovitch cycle. This cycle has changed over time to affect the earth’s orbit, the tilt of the axis, and the reception of sunlight.
Another cycle that helps to make these determinations is the climatic precision cycle. Tereza Pularova, a science writer, explains this cycle in a Space.com article, saying: “As Earth spins, its axis wobbles in a circle. This effect is called axial precession. As a result, in our era, the axis points in the North toward the star Polaris, which is known as the North Star.”
By linking the sediment deposits to that cycle, scientists had estimated that the moon was a lot closer to the earth when the rocks formed some 2.5 billion years ago. Their estimations are that it was about 37,282 miles closer to the Earth, and a day was only 17 hours long.
One thing is for certain. The moon isn’t going anywhere soon. It may be moving, but it has been moving for millions of years and it will still be there millions of years from now.