July 27: The longest total lunar eclipse of the century

Stargazers can look forward to the lunar eclipse of July 27th. It is the only one of the year. And the longest lunar eclipse that can be admired in this century. Even in the last century, there were only two that were longer – in 2000 and in 1982 – but both could not be seen from Europe.

But how does a lunar eclipse occur?

A lot of things happen in space. The Earth revolves around itself, orbiting the Sun as itself orbited by the Moon. This complex game of rotations determines which heavenly body will be lit when and where by sunlight. The moon does not shine; what we perceive as moonlight is reflected solar light.

When the entire lunar disc is bright and round like a full moon in the sky, we look in the same direction on our satellite, from where the sunlight comes. In this constellation, called opposition, the sun, the earth and the moon are aligned; that is, the sun and the moon are on the opposite sides of the earth. In fact, in this case, the earth should hinder the sunlight and darken the moon with its shadow. This happens in fact, but rarely, and there is a lunar eclipse

The cone-shaped Earth shadow, which always points naturally to the Sun, rises to about 1.4 million kilometers from the Earth. The lunar orbit is much closer to the earth, but the earth's ombra is about eight times smaller in the lunar range. Nevertheless, the diameter of the nucleus shadow exceeds that of the moon by almost three times. The dim light, which surrounds the shadow like a ring, is about as wide as a lunar diameter (nearly 3,500 kilometers) at this distance.

The fact that the moon does not normally penetrate the central shadow of the Earth is due to the fact that its orbit is not exactly on the same plane as Earth's orbit (ecliptic) but tilted about 5 °. In extreme cases, the moon is 37,000 kilometers from the ecliptic and passes above or below the earth's shadow. It is only in two points (the so-called node) that its orbit intersects the plane of the ecliptic with each revolution. A lunar eclipse always occurs when a full moon is close enough to one of these two points of intersection. This constellation occurs about twice a year.

Are there different eclipses lunar?

There are different types of lunar eclipses – according to the parts of the lunar eclipse The shadow of the Earth draws the moon:

• Partial Partial Shadow Eclipse : The Moon only crosses the partial shadow of the Earth. In these relatively frequent events, the lunar disc diminishes imperceptibly.
• Total Partial-Shadow Eclipse: In these rare cases, the Moon fully enters the semi-shadow of the Earth, but never touches the Shadow of the Heart. The part of the moon closest to the shadow of the nucleus is much more obscured.
• Partial Nuclear Shadow Eclipse: In more than one-third of the cases, the moon passes with more or less of its diameter the shadow of the Earth, visible on the lunar surface as a circular arc strongly obscured.
• Total Eclipse of Core Shadow: In almost 30 percent of all lunar eclipses, the Trabant enters completely into the shadow of the Earth; The lunar disk becomes cloudy.

The lunar eclipse of July 27 is a total eclipse of the nuclear shadow.

What happens on July 27?

Only about 2.5 hours after the full moon, the moon passes through the ecliptic. At 19:14 it touches the outer edge of the penumbra – but we can not observe this here in Switzerland, since the moon is then still below the horizon. He enters the shadows at 20:24. Also this event is not visible from here.

When the moon rises here at 9 pm – 8 minutes before sunset – it is already deep, but not yet completely in the shadow of the pit. His disc then forms a sickle and it is unlikely that it will be seen at dusk in the summer. The total phase starts at 9:30 pm and lasts 104 minutes. The maximum darkening is reached at 22.22.

Subsequently, the Trabant moves again out of the basic shadow. The total eclipse ends at 23:13 when the moon touches its edge from the inside. At 0.19 clock also ends partial darkness; the moon then completely leaves the shadow of the nucleus. The lunar moon is over at 1:28 when the moon completely leaves the shadow of the Earth.

The total darkness – from the entrance to the exit of the penumbra – lasts 377 minutes, more than six hours. This makes it one of the ten percent the longest. The extraordinary length of this eclipse has to do with the fact that the moon walks in the shadow of the Earth in the middle – if it only drifts through the fringes, the darkness lasts less.

In addition, the distance between the earth and the moon is particularly great this time; The Earthbrabant goes through one of the lowest points on Earth only 15 hours earlier. Although it seems smaller – in fact, it is the smallest full moon of the year – it takes longer to cross the earth 's shadow.

Where do we see better the eclipse?

In Switzerland, not all The darkness is visible, because the moon does not rise here until after the appearance of partial darkness. The total phase in which it is completely in the shadow of the nucleus, but can be observed in good weather conditions. The spectacle of the sky can be seen not only in Europe, but also in South America, Africa, Asia and Australia.

However, the moon is very flat – typical of the summer moon. Therefore, one should visit a slightly elevated place on the outside with the best possible view of the horizon. The best is a hill facing southeast, where the moon rises. In addition, you need to make sure that it also has few light sources nearby.

Of course, if the weather does not play, the best location does not help much either. Clouds could annoy observers. They will then have to wait for the next total lunar eclipse: On January 21, 2019, it is time again. However, a total lunar eclipse of the same length as July 27 no longer occurs until 2123.

And why do we speak of "blood moon"?

Even though the moon is completely in the shadow of the earth, visible, but takes a darker color. This can range from dark yellow to orange to reddish brown. This phenomenon is called the blood moon.

It is caused by the scattered light of the sun, which is broken by the Earth's atmosphere in the cone of the shadow. Molecules in the atmosphere also filter the blue shortwave portions of the light, so that the red light, particularly long, reaches the moon and illuminates.

The icing on the cake:
An opposition Mars

The Bloody Moon is joined, thanks to a nice coincidence, by the Red Planet, Mars. It reaches its opposition – that is, it is in line with the earth and the sun – and is then within 6 ° of the moon. It appears as a bright red dot slightly below the moon. The distance to the Earth is then "only" 57 million kilometers – not much more than the minimum distance of about 54 million kilometers.

Mars will be the brightest star in the night sky and for once Jupiter even points to the field. In the telescope, its disk appears relatively large and it is possible to glimpse the details of the surface of the neighboring planet, for example its polar caps. Unfortunately, the red planet is also in the sky this year.

[Différent avec le SDA]