Moon going far from Earth: Scientists explain how it could bring longer days and end total solar eclipses

Earth's only natural satellite is gradually moving farther away at a measured rate of 3.83 centimetres per year, a phenomenon confirmed through decades of high-precision observations. While the change is negligible over a human lifetime, scientists say the long-term recession could lead to longer days, weaker tides and eventually the end of total solar eclipses over geological timescales.

The figure, with a formal uncertainty of about 0.09 millimetres, is based on the best interpretation of five decades of lunar laser ranging data collected from observatories across three continents.

Lunar laser ranging confirms Moon's gradual recession

Scientists have measured the increasing Earth-Moon distance by firing laser pulses at retroreflector arrays placed on the Moon by Apollo astronauts between 1969 and 1971. The reflected signals allow researchers to calculate the distance with remarkable precision.


Apollo 11 astronauts Neil Armstrong and Buzz Aldrin installed the first reflector array at the Sea of Tranquillity in July 1969. Additional arrays were deployed during the Apollo 14 and Apollo 15 missions in 1971. The Apollo 15 reflector, containing 300 corner-cube prisms, remains the primary instrument for modern lunar ranging. Soviet Lunokhod 1 and Lunokhod 2 rovers also carried French-built retroreflector panels.

According to NASA's Jet Propulsion Laboratory, four observatories currently conduct routine lunar laser ranging from New Mexico, France, Italy and Germany. The Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) in New Mexico provides the highest precision, measuring the Earth-Moon distance to about one millimetre by timing laser pulses with picosecond accuracy.

According to Science Blog, the measurements have consistently shown a slight increase in the Earth-Moon distance each year, with sub-millimetre precision maintained for nearly three decades.

Although the Moon moves away at a rate comparable to human fingernail growth, the change amounts to only about three metres over an average human lifetime and roughly 190 metres over 5,000 years. On geological timescales, however, the cumulative effect becomes significant.

Moon's distance from Earth

The Moon is, on average, 238,855 miles (384,400 kilometres) away from Earth. Scientists describe this as being about 30 Earths away, using Earth's diameter as a unit of measurement. Since the Moon follows an elliptical orbit rather than a perfect circle, its distance from Earth changes throughout the month. At its farthest point, known as apogee, it is about 252,088 miles (405,696 kilometres) away, or nearly 32 Earth diameters. At its closest point, called perigee, it is about 225,623 miles (363,104 kilometres) away, which is roughly 28 to 29 Earth diameters from our planet.

Tidal friction is driving the Moon away

Scientists attribute the Moon's gradual recession to tidal friction, a process involving Earth's rotation, the Moon's gravitational pull and the interaction of Earth's oceans and crust.

The Moon's gravity creates tidal bulges on both the near and far sides of Earth. Because Earth completes one rotation every 24 hours while the Moon takes about 27.3 days to orbit the planet, Earth's rotation carries these tidal bulges slightly ahead of the Moon's position.

This offset produces a forward gravitational pull on the Moon, transferring energy into its orbit and causing it to move farther away each year.

Under the conservation of angular momentum, the Moon's gain in orbital energy comes from Earth's rotational energy. As a result, Earth's rotation gradually slows, the length of a day increases and the Moon continues to recede.

Long-term effects of the Moon moving farther away

Scientists say the Moon's gradual retreat could have several long-term consequences for Earth over hundreds of millions of years.

Longer days

Earth's rotational day is increasing by approximately one to two milliseconds per century. Evidence indicates that around one billion years ago, a day on Earth lasted about 19 hours when the Moon orbited closer to the planet. As the Moon continues to recede, Earth's rotation is expected to slow further, leading to progressively longer days.

End of total solar eclipses

Total solar eclipses are possible because the Moon and the Sun appear nearly the same size in Earth's sky. Although the Sun is about 400 times larger than the Moon, it is also roughly 400 times farther away, creating a near-perfect alignment.

As the Moon moves farther from Earth, its apparent size will decrease. Current calculations indicate that the last total solar eclipse visible from Earth's surface could occur about 600 million years from now. Beyond that point, the Moon will no longer completely cover the Sun's disc.

Weaker ocean tides

The Moon's gravitational influence on Earth's oceans will gradually weaken as the distance between the two bodies increases. Spring and neap tides will continue to occur, but the difference between them is expected to diminish over time, reducing the tidal patterns that influence coastal ecosystems, marine reproduction and fisheries.

Scientists note that the Moon has been moving away from Earth for approximately 4.5 billion years and is expected to continue doing so as long as Earth keeps rotating.

(With inputs from TOI)