At 10.30 pm GMT on Wednesday 5 November the Moon will be at its closest point to the Earth in 2025, at a distance of 356 832 km. This will fall nine hours and eleven minutes after the Full Moon, at 1.19 pm on the same day, making it particularly large in the sky. The Moon completes one orbit about 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. These cycles mean that the Moon often reaches its furthest point from the Earth (apogee) of the year in the same lunar cycle, with the furthest Lunar Apogee of 2025 falling on 20 November.
Although this is the closest point to the Earth that the Moon has reached this year, it is not exceptional. The Moon reached 356 589 km from the Earth on 21 January 2023, and will reach 356 649 km from the Earth on 24 December 2026. The closest the Moon can come to the Earth is currently about 356 375 km, while its maximum distance is about 406 720 km.
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 day 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, and 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 bouncing lasers off a mirror left on the Moon by the Apollo Program astronauts, which have shown that the Moon is moving away from us at an average of 38 mm per year.
However, this would 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.
See also...
.png)
.png)
