Yes; the moon is drifting away, but don’t worry about it

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

20 April 2025

Amina Saidi says that she read somewhere that the moon is drifting away from the earth and wonders if one day, it will escape from the us completely. The answer to the first part of her question is yes. Our moon is slowly drifting from us at the rate of about 3.8cm ever year.

Now, given that the moon is hundreds of thousands of kilometres away, one wonders how it is possible to establish that every year it drifts by just a few centimetres. To put this into perspective, one kilometre has 1,000m and one metre has 100cm; so, one kilometre has 100,000cm and 100,000km have 100,000 x 100,000 = 10,000,000,000,000cm; that is, 10 billion centimetres. How can astronomers possibly measure the distance to the moon to such a high level of accuracy?

Well, it is all thanks to the men who landed on the moon some 50-odd years ago. They mounted a shiny reflective metal plate on its surface. By shining a powerful laser from earth and timing how long it takes the light to bounce back, the distance to the moon can be determined with accuracy of a few millimetres.

Regular measurement of the distance have revealed that the moon is slowly drifting away at the rate of 3.8cm per year. Should we worry about this? Well, this works to about 3.8m per century and 38m in a millennium. From the time Jesus was born, 2,025 years ago, to date, the moon has drifted away by just 77m from earth.

Now, the moon is about 384,400km away, so 77m is approximately 0.00002 percent increment of this distance. We may want to know how long it will take for this distance to increase by 1km.

Every year it increases by 3.8cm; 1km has 100,000cm; so, it will take 100,000 divided by 3.8 = 26,316 years! That’s a very long time. And one kilometre is still a small fraction of the distance: 1 divided by 384,400 = 0.00026 per cent.

Assuming that the drifting rate has remained stable for millions of years, it turns out that when dinosaurs roamed the earth 100 million years ago, the moon was 100,000,000 x 3.8 = 380,000,000cm closer than it is today. This is equal to 3,800km nearer – approximately one moon diameter nearer. That is, it was 380,600km away instead of the current 384,400.

Is that a significant change? It is just one percent difference. In short, we have other things to worry about instead of the moon drift away completely.