Science

Supermoon: no more, no very red – Science et Avenir

This article is taken from Les Indispensables de Sciences et Avenir #210 July/September 2022.

On Friday, July 13, 2022, a full moon rose, which some call a supermoon. Kesako? Does he seem bigger than usual? On that day, our satellite was at a distance of 363,666 kilometers from the Earth … that is, very close to its perigee, the closest point of its orbit to our planet (357,263 kilometers), which it reached at 9:30 in the morning earlier.

Compared to the full moon in January, which was 400.779 km away, this gives a difference of about 10% in distance and therefore in apparent size, or about 3 minutes of arc (1/20 of a degree). Therefore, it is impossible to see with the naked eye without landmarks. So a supermoon is just a big moon to… astronomers who measure its size with precision instruments.

Our brain plays tricks on us

But is the Moon more impressive when it is low on the horizon than it is when it is high in the sky? Nope! Try it yourself, with your hand outstretched: it’s always about half a finger. So why this optical illusion? Just because our brain plays tricks on us. When two objects are at different distances but in very close lines of sight, our vision tends to expand the one in the background. Thus, the Moon behind hills, trees, or buildings appears more massive to us than when it is high in the sky and cannot be compared with the foreground.

Only yellow and red rays reach our satellite.

The question of color is a completely different matter! When the moon rises or sets, the light rays of the sun that illuminate it pass through the denser layers of the earth’s atmosphere. The most energetic ones (blue, green) are then absorbed and scattered in the sky. Only yellow and red rays reach our satellite. Hence its visible orange color.

This is also the case during lunar eclipses, when this hue is all the more pronounced because dust is present in the Earth’s atmosphere. On the other hand, the Red Moon of April has nothing to do with this phenomenon. It is called so because the leaves and buds are then often burned – scorched – by late frost … Especially when the sky is clear and the Moon appears in all its splendor.

Back to top button

Adblock Detected

Please consider supporting us by disabling your ad blocker.