The furthest lunar apogee or 2025.

On Thursday 20 November 2025, at 2.49 am GMT, the Moon will be at its furthest point from the Earth in 2025, a distance of 406 692 km. The Moon orbits the Earth every 27.5 days, and like most orbiting bodies, its orbit is not completely circular, but slightly elliptical, so that the distance between the two bodies varies by about 3% over the course of a month. This elliptical orbit is also not completely regular, it periodically elongates then returns to normal, making some perigees closer than others. Because this is an elongating and contracting elliptical orbit, rather than a change in the average distance between the Earth and the Moon, the most extreme Lunar Perigee and Apogee of each year typically happen in the same Lunar Month; this year the closest Lunar Perigee occurred at 10.30 pm GMT on Wednesday 5 November.

Diagram showing the relationship of the Lunar orbit and Lunar month. Southern Astronomical Delights.

Although this is the furthest point from the Earth that the Moon will reach in 2025, it is not exceptional. The Moon reached 406 710 km from the Earth on 29 March 1984, and will reach 406 705 on 1 December 2043. 

The Earth-Moon System to Scale, 650 km/pixel. John Walker/Fourmilab.

However, there is evidence that the Earth and Moon have been moving steadily apart since the formation of the Earth/Moon system, about 4.5 billion years ago. Studies of the shells of Rudists, a sort of Bivalve Mollusc which laid down layers of shell daily, have found that the Cretaceous year was 372 days long. Since the length of a year is unlikely to have changed without the Earth shifting profoundly on its orbit, the most plausible explanation for this is that the days were shorter. Since the length of the day is driven by the closeness of the Moon, that the Moon was significantly closer, with an average distance from the Earth of approximately 383 000 km in the Late Cretaceous (about 80 million years ago) compared to 384 400 km today. 

This fits with measurements that made by the  Lunar Laser Ranging experiments, which work byby bouncing lasers off a mirror left on the Moon. The first of these experiments was set up by NASA's Apollo Program, with additional mirrors being placed by the Soviet Lunokhod remote operated Moon vehicles, and the Indian Chandrayaan-3 mission. All of these experiments have shown that the Moon is moving away from us at an average of 38 mm per year.

Measuring the distance between the Earth and the Moon by bouncing a laser between the Earth and a mirror and the Moon. All distances are to scale, with the light moving in real time for the scale. James O'Donoghue/NASA/Solar System Scope/Wikimedia Commons.

This would seem to imply that the collision which is thought to have formed the Earth/Moon system would have occurred about 1.5 billion years ago, something for which there was no evidence. Studies of Mesoproterozoic Banded Ironstone formations in Australia have shown a 23.3 year variation in tidal cycles, which are also determined by the lunar distance. Today, these cycles follow an 18.6 year cycle, which suggests the average distance between the Earth and the Moon between about 1.5 and 2.0 billion years ago was approximately 332 000 km, suggesting that the rate at which the Moon is retreating from the Earth has increased over time.

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