NASA/JPL-Caltech

On the evening of July 27, 2018, the Earth will come between the Moon and the Sun. From France, it will be possible to observe this eclipse, which will transform our satellite into a Red Moon for more than an hour.

A few months after the “blood-blue super-moon”, which unfortunately Europe was unable to observe, our natural satellite will once again be involved in a celestial ballet on 27 July 2018. During the second lunar eclipse of the year, sky observers will be able to see a total eclipse – the second of three, spaced 6 months apart.

During this event, the Moon is expected to pass in the centre of the Earth’s shadow: it will be the first central lunar eclipse since the one in June 2011. Even more noteworthy is the fact that on July 27, 2018, we will witness the longest lunar eclipse of the twenty-first century, as it is expected to last nearly 103 minutes.

If all these reasons have made you want to look up to the heavens to admire the passage of the Moon in our shadows, here is some information that can guide you to know where to look at the “Red Moon”, and to better understand the phenomenon that will occur in front of you.

Look up from 9:30 p.m.

From the European continent, we will witness the different phases of this eclipse after sunset, as will northwest Africa and South America. The best view of the eclipse will be reserved for the Indian Ocean, because it will be turned towards the Moon.

In other words, if we imagined that we could go to the moon during this eclipse and land on the moon so that we could look at the earth, we would have a view of that ocean, as well as a large part of Africa and Asia.

Wikimedia/CC/SockPuppetForTomruen

At the maximum of the eclipse, this is what we would see from the Moon. Wikimedia/CC/SockPuppetForTomruen

Above all, France will have time to contemplate the second part of this eclipse. By the time we see the Full Moon appear, it will already be taking on reddish colours.

As for the time at which you should watch the eclipse, it will depend on where you are. In metropolitan France, you will have to start looking up at 9:30 pm (the time the sun begins to set). The eclipse is expected to peak at 22:20, and to end at about 1:30 the next day.

Of course, it is advisable to choose a location where clouds will not obscure the visibility of the eclipse. If you are equipped with a telescope, binoculars or astronomical glasses, you can take them out to enjoy the show. If the sky is clear, it is safe to observe the eclipse with the naked eye.

Why does the Moon turn red?

As with every eclipse, the question is which of all the stars involved passes between the others. On July 27th, we will literally be in the center of attention as the Earth will make its way between the Sun and the Moon. That’s why a shadow will be cast on the moon.

If you are patient enough to wait for the moment when the natural satellite passes through the center of the shadow zone, you will be able to observe the Moon take on a reddish color. This phenomenon is also known as the “Red Moon”, and sometimes incorrectly called the “Blood Moon”.

This peculiarity comes from the fact that we will witness a total lunar eclipse. During a total solar eclipse, the Moon is between the Sun and the Earth, and therefore casts its shadow directly on us. In this situation, the eclipse does not change the colour of the Moon, which has no atmosphere.

Max Pixel/CC0

Max Pixel/CC0

The Earth has a nitrogen-rich atmosphere. As Trust My Science explains, this light – a combination of the different colours of the spectrum – is scattered in blue colours, hence the fact that we see the sky in blue. When the Sun rises and sets, our photonic vision (in bright lighting conditions) captures light waves with greater scattering: we then see them as red, orange and green.

If we return to our total lunar eclipse, we understand that the light from the Sun, which passes through the Earth’s atmosphere, will be projected onto the Moon. By scattering sunlight, the Earth’s atmosphere will cast a colourful shadow on the Moon. The thin layer of dust on the surface of the satellite will then do the rest, since it has the particularity of reflecting this received light.

In other words: the Moon will return to the Earth the light of the Sun, which has previously passed through the Earth’s atmosphere. And if this colour tends towards red, it is directly due to our human activities (including pollution) and other natural activities (such as particles sent back up to the top during volcanic activity).

On the same day, Mars will be “in opposition”

On July 27, the Moon will not be the only star in the sky to attract attention: you will also be able to observe Mars. Indeed, the Red Planet should appear much brighter than usual from this date until July 30. As NASA explains, Mars will be in opposition on the same day that the Moon takes on its red colour.

NASA/JPL-Caltech

No, Mars won’t look as big as the Moon on July 27th. // Source: NASA/JPL-Caltech

This phenomenon, which occurs every 780 days, refers to the moment when the Sun, Earth and Mars are aligned. On July 27, the planet that is (perhaps) home to the Opportunity robot will also be closer to us than it has been since 2003: 57.6 million kilometres from Earth. The next time this will happen, it will be 2020.

Although Mars will be much brighter, it would be wrong to think that it will appear bigger to us. If you read somewhere that the red planet will look as imposing as the Moon in the sky that night, NASA invites you not to believe it: it’s an urban legend. “If this were true, we would have a big problem, given the gravitational forces of the Earth, Mars and our Moon!,” the space agency warns.

Now you know why the Moon will be red on the evening of July 27th, and why Mars will also appear brighter (but not as big as our natural satellite). All that remains is to wish you a good observation of the sky – if possible clear of clouds from 9 p.m. onwards.