Watch: ancient distance and apparent size of the Moon

Today, on average, the Moon is 384,400 km (238,000 miles) away from the Earth. But that was not always the case. Our satellite was much closer in the past.

Now, Dr. James O’Donoghue (@physicsJ on Twitter) created the animation below showing how close was the Moon to the Earth and how was its apparent size in the sky in ancient times (and then now).

The Moon currently moves away from us at a rate of 3.8cm/year (1.5″/yr). BUT this ‘lunar retreat’ rate has varied over the last 4.5 billion years. This is a rough idea of what it may have looked like. Causes of variation: (1) meteor impacts on Earth or Moon. (2) reconfiguration of landmasses with earthquakes that generate changes in the rotational axis of the Earth.

The Moon is drifting away from the Earth

According to the giant impact hypothesis, a widely accepted theory, the Moon was created as a result of a catastrophic impact between Earth and a Mars-sized planet (called Theia) about 4.5 billion years ago.

The Moon had orbited much more closely in the past and it is drifting away from the Earth since its formation. This drifting was also confirmed by American and Soviet experiments, using laser ranging targets placed on the Moon.

From year to year, the moon never seems to change. Craters and other formations appear to be permanent now, but the moon didn’t always look like this. Thanks to NASA’s Lunar Reconnaissance Orbiter, we now have a better look at some of the moon’s history. Learn more in this video!

It is predicted that the lunar distance will continue to increase until (in theory) the Earth and Moon become tidally locked to each other, just like Pluto and Charon (today, only the Moon is tidally locked, that’s why we see only one side of the Moon).

This would occur when the duration of the lunar orbital period equals the rotational period of Earth. The two bodies would then be at equilibrium, and no further rotational energy would be exchanged. When that happened, you’d see Earth in roughly the same spot from the Moon forever. In other words, you’d see the same face of Earth from the Moon.

However, models predict that 50 billion years would be required to achieve this configuration, which is significantly longer than the expected lifetime of the solar system.

The Moon is tidally locked to the Earth so it always shows us the same face. But the Earth is slowing down so that it’ll eventually lock to the Moon as well. What are the forces involved and when will this happen?
New Horizons poster
Pluto and Charon are tidally locked to each other. Image: Artist’s conception depicting New Horizons spacecraft near Pluto. Pluto’s moon Charon, also known as (134340) Pluto I is in the background. New Horizons is an interplanetary space probe that was launched as a part of NASA’s New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research Institute (SwRI), with a team led by S. Alan Stern, the spacecraft was launched on January 19, 2006 from Cape Canaveral Air Force Station by an Atlas V rocket directly into an Earth-and-solar escape trajectory with a speed of about 16.26 kilometers per second (58,536 km/h; 36,373 mph). Its primary mission was to perform a flyby study of the Pluto system in 2015. It has a secondary mission to fly by and study one or more other Kuiper belt objects (KBOs) in the decade to follow. It is the fifth of five artificial objects to achieve the escape velocity that will allow them to leave the Solar System (others being Pioneer 10 – launched in 1972, Pioneer 11 – launched in 1973, Voyager 2 – launched in August 1977, and Voyager 1 – Launched in September 1977). Image: NASA Goddard Media Studios

Sources

How far is the Moon? on Space.com
How far away is the Moon? on Our Planet
Lunar Distance on Wikipedia
When will Earth Lock to the Moon? on Universe Today

M. Özgür Nevres

I am a software developer, an ex-road racing cyclist, and a science enthusiast. Also an animal lover! I write about the planet Earth and science on this website, ourplnt.com. You can check out my social media profiles by clicking on their icons.

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