1. How Mirrors Left on the Moon Measure Its Distance
In July 1969, astronauts on Apollo 11 placed a special device on the Moon’s surface. It was a laser ranging retroreflector, which is a mirror made of small glass prisms arranged to reflect light back in the exact direction it came from [3]. Scientists on Earth shoot a short burst of laser light at this mirror. By measuring the time it takes for the light to bounce back, they can calculate the distance to the Moon because the speed of light is known. NASA explains that this method allows them to measure the distance with a precision of just a few millimeters [1]. By repeating this measurement over many decades, scientists noticed that the distance to the Moon was slowly changing.
2. The Moon Moves Away About 3.8 cm Each Year, Like Growing Fingernails
The Moon is moving away from Earth by about 3.8 cm (1.5 inches) each year [1][2]. One explanation compares this speed to the rate at which an average person’s fingernails grow [7]. The average distance between the centers of Earth and the Moon is about 385,000 km (239,000 miles) [2]. While 3.8 cm is a tiny amount compared to that huge distance, the measurements are precise enough to clearly see the change. This raises an important question: why is the Moon moving away from us?
3. How Ocean Tides Pull the Moon Forward
The Moon’s gravity pulls on Earth’s oceans, creating a bulge of water toward the Moon. However, because Earth rotates, the water is dragged along the seabed by friction. This causes the tidal bulge to shift slightly ahead of the Moon’s position, rather than staying directly underneath it. Because this bulge contains a massive amount of water, its gravity pulls on the Moon, tugging it forward in its orbit [1][4]. This is similar to pushing a swing from behind to make it go higher. It is not that the water physically pushes the Moon, but rather that gravity creates a forward pull. Since the Moon moves in a nearly circular path, being pulled forward causes its orbit to expand, moving it farther away. During this process, some of Earth’s rotational energy and angular momentum are transferred to the Moon’s orbit. However, most of this energy is turned into heat by tidal friction [5]. The Moon is not moving away on its own; it is gaining momentum from Earth’s rotation.
4. Why Earth's Day Gets 0.0016 Seconds Longer Every Century
Earth’s rotation slows as energy is transferred to the orbit, with most becoming heat. This means that a day on Earth gets slightly longer. Astronomy Notes explains that a day lengthens by about 0.0016 seconds (1.6 milliseconds) every 100 years [4]. Another estimate, based on historical observation records, suggests it lengthens by about 1.72 milliseconds every 100 years [5]. This change happens over a century, not a single year. It is a time span much shorter than a blink of an eye, so humans cannot notice it in their lifetime. However, if this process continues for hundreds of millions of years, the length of a day in the past must have been very different. We can find clues to verify this in fossils.
5. What Coral Fossils Show About Shorter Days 400 Million Years Ago
Coral fossils preserve daily growth lines and yearly layers. By counting the daily lines within a yearly layer, scientists can determine how many days were in a year at that time. In 1963, paleontologist John Wells studied corals from about 400 million years ago, during the Middle Devonian period. He reported that a year then contained more than 400 days [6]. If the length of a year was the same as it is today, this means each day was shorter than the current 24 hours. Calculations suggest those days were approximately 22 hours long. Other fossil records estimate that a day was about 23.5 hours long roughly 70 million years ago [5]. These estimates point to shorter days further in the past. However, the uncertainty in these estimates increases the further back we look.
6. Why We Cannot Multiply 3.8 cm by Millions of Years to Find the Past
The rate of 3.8 cm per year is a recent measurement. If we assume this speed has been constant in the past and calculate backward, we would find that Earth and the Moon were touching about 1.5 billion years ago. This contradicts evidence that the Moon has existed for at least 4.46 billion years, so the speed of retreat must have been slower in the past [7]. Explanations suggest that the retreat rate has not been constant; there were likely periods of rapid change and periods of little change mixed together. Why the current speed is higher than in the past is not fully understood. As the Moon moves farther away, its apparent size in the sky becomes smaller. According to one explanation, total solar eclipses will begin to decrease in frequency about 620 million years from now [7]. The exact date of the last eclipse is debated, but this figure gives a rough idea of the future.
7. An Experiment to Measure How Slowly the Moon Moves Away
To understand how slow this movement is, you can try a simple experiment at home. Use a ruler to measure 3.8 cm (1.5 inches) and compare it to the tip of your fingernail. Measure your nail length with a ruler and write it down. Then, measure it again one month later. By comparing the Moon’s movement over one year to your nail’s growth over one month, you can see which is faster. It is best to measure before you cut your nails. If you want to explore further, you can choose something else in your house that grows over time and record its change with a ruler over one month. You can compare the two measurements to see how slow a movement of 3.8 cm per year really is.