Science

Don’t miss: total lunar eclipse on the night of May 15-16!

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To observe this event, you need to be quite early: on Monday, May 16, at 5:29, the lunar eclipse will very accurately reach maximum darkness. This is the first total lunar eclipse of the year and will be partially visible from most of the globe, including France. The second total eclipse will occur on November 8, but, unfortunately, it will not be visible from Europe. So this event is not to be missed!

We remind you that lunar eclipses occur when the Earth is between the Sun and the Moon; it then casts a shadow on the lunar surface. These eclipses can last for several hours. When the three celestial bodies are perfectly aligned, the Earth’s shadow completely covers the Moon: a total eclipse. Some of the sunlight is then refracted to the Moon through the Earth’s atmosphere, giving the Moon a reddish color.

This Sunday, May 15, moonrise is scheduled for 20:49 (Paris time); she goes to bed at 5:46 the next morning. The Moon will enter Earth’s penumbra at 3:32 AM, then into shadow at 4:28 AM. Maximum darkness will be reached at 5:29 am – so you only have about 15 minutes to capture the moment. Note, however, that the observation may be compromised by the first rays of the Sun, which will rise on this day at 6:08 am. The weather should still be good…

The moon that turns red

There are three types of lunar eclipses, depending on the position of the Sun, Earth, and Moon at the time of the event: an eclipse can be total, partial, or penumbral, depending on how the Earth blocks the sun’s rays reaching its satellite. When the three bodies are perfectly aligned, the Moon goes into shadow and a total eclipse occurs; otherwise, the Earth blocks only part of the light rays, and the Moon is in partial shade, as shown in the following diagram:

A lunar eclipse is considered total or penumbral, depending on the position of the Sun, Earth, and Moon at the time of the event. ©NASA

During a total eclipse, the Moon goes through different stages: first, it enters the penumbra of the Earth, then gradually goes into the shadow (the eclipse is then partial), until it completely covers its surface (the eclipse is then total); then the shadow of the Earth gradually moves away from the Moon, the eclipse again becomes partial, then penumbral.

Note that during a total eclipse, the Moon turns red, which is why it is sometimes referred to as the “Blood Moon” in this case. This is due to the interaction of solar radiation with our atmosphere. Indeed, even if the Earth’s shadow completely covers the lunar surface, an indirect part of the sunlight always reaches the Moon, because it is bent by our atmosphere – and this is also why there are penumbra zones.

The Earth’s atmosphere absorbs and then scatters most of the visible light it receives from the Sun; violet-blue light with a short wavelength is most affected by this phenomenon. In contrast, the longer orange/red wavelengths pass through our atmosphere without being absorbed or scattered; these rays are refracted outwards, casting an indirect reddish light onto the Moon. This indirect light, which is therefore only a fraction of what constitutes white light, is obviously much weaker than the ordinary luminosity of the Moon.

Note that the exact color of the Moon during a total eclipse depends on the amount of dust, water, and other particles present in the Earth’s atmosphere at that time, as well as other factors such as temperature and humidity.

No other total eclipses will be visible until 2025

Lunar eclipses occur much more frequently than solar eclipses: on average, there are about three lunar eclipses per year, 29% of them are total. They are also much easier to observe, as they can be seen with the naked eye as soon as the Moon is above the horizon (and without the risk of eye damage, unlike solar eclipses!).

Keep in mind that lunar eclipses visible from Europe are relatively rare: the last total eclipse seen from France was in January 2019, and the next one will occur on March 14, 2025! In the meantime, we can enjoy the partial lunar eclipse on October 28, 2023.

To better observe the eclipse, plan an observation site with a clear horizon. Binoculars or a telescope will obviously allow you to admire the phenomenon in more detail. The eclipse will be total throughout South America, Central America and the eastern half of the United States. This way the French West Indies as well as Guyana will be able to take full advantage of the show.

It should be noted that these eclipses are also an opportunity for scientists to learn more about our satellite: for example, data collected by the NASA Lunar Reconnaissance Orbiter (LRO) probe in 2011 made it possible to calculate the cooling rate on the eclipsing side of the satellite. Moon; these data then allowed a better characterization of the Moon’s topography (a flat surface cools faster than a rough surface).

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