Contribution: Thomas Baer, ORION editorial team

The last magnificent total lunar eclipse observable from Switzerland was quite some time ago. It occurred on January 21, 2019, but due to cloudy weather, it could only be seen south of the Alps. Before that, on the evening of July 27, 2018, the totally eclipsed Moon rose into the evening sky, as it will again next September 7.

Lunar eclipses, like solar eclipses, occur in pairs about every six months. However, they are not always total and are often not visible from Europe. When the sun, Earth and Moon align, as will be the case on September 7th, the Earth's shadow crosses the full moon and darkens it. Normally, however, the full moon moves above or below the Earth's shadow because the Moon's orbit is tilted by a good 5° to the plane of Earth's orbit. Then we experience a completely "normal" full moon without an eclipse. This time, however, the full moon crosses the Earth's orbital plane just a few hours after the full moon, meaning the Earth is in front of the Sun. For a few hours, it passes through the southern part of the Earth's shadow.

When dusk begins to fall on the first Sunday in September, shortly before 8 p.m. in the east-south-east, a pale, coppery-brown orb shines in the sky instead of a bright full moon disc. Of course it is the full moon, but this time it is completely immersed in the Earth's shadow. Only a small amount of residual sunlight is able to make the satellite appear a faint reddish-brown, less strongly scattered light that is deflected by the deep layers of the Earth's atmosphere into the dark core shadow and makes the lunar surface at 368,744 km shimmer in the colours of earthly dawn and dusk; around 10,000 times weaker than in full moonlight. The darker the sky becomes, the more visible the characteristic colouring becomes. As the moon travels deep through the southern umbra, it should be visible at the centre of the eclipse at 8:13 p.m. CEST.

The total phase ends at 8:53 p.m. CEST. Now the south-eastern lunar edge shines for the first time this evening in direct sunlight, which, seen from the moon, slowly emerges from behind the earth and gradually returns the night star to its usual appearance. The umbra will retreat to the upper right in just over an hour. It will also be nice to observe how the stars fade with the increasing moonlight and how the shadows in the neighbourhood become more and more clearly visible.

How dark or bright a total lunar eclipse is depends largely on the transparency of the Earth's atmosphere. Extremely dark and colorless eclipses have occurred after major volcanic eruptions, such as the one in December 1982 following the eruption of El Chichón, or in 1992 due to the massive ash cloud from Pinatubo that spread around the globe.
These suspended particles and aerosols further attenuate the long-wave residual light that passes through the lower layers of the atmosphere. But gigantic clouds of smoke from forest and bush fires, such as those we experienced last June from Canada to Europe, also make a lunar eclipse appear darker than usual. Geometric factors also play a decisive role in the brightness distribution. The more centrally the full moon passes through the Earth's shadow, the less residual light it receives. If the moon is close to Earth, the shadow cone is less evenly illuminated due to the angle of incidence of the residual radiation. The upcoming lunar eclipse is likely to be classified as a darker eclipse. According to the five-point scale of the French astronomer André Danjon, which ranges from 0 (very dark) to 4 (very bright), we can expect a similarly dark lunar eclipse of magnitude 2 (deep red) as in September 2015. With a magnitude of 1.367 (136.7%), the northern limb of the moon will only just miss the center of the shadow. Furthermore, the moon will pass through the point in its orbit closest to Earth just three days after the eclipse.

Those who want to experience the total lunar eclipse on Sunday evening, ideally from moonrise onwards, should seek out a hill with a clear view to the east-southeast. If a high hill or mountain in this direction blocks the view, they may not experience the total darkening until much later, or perhaps only the final phase of the eclipse. It should be mentioned at this point that, weather permitting, numerous observatories throughout Switzerland will be open to the public.

The third lunar occultation of the Pleiades visible from Europe will occur shortly after moonrise in the late evening of September 12th. By 11:00 PM CEST, the waning gibbous moon will have settled into the "Mini Dipper." During this event, the moon's orbit will pass centrally in front of the cluster of stars. Unfortunately, the moon will completely outshine the Pleiades this time. Therefore, binoculars or a telescope are necessary for observation; the "Seven Sisters" will be almost impossible to see with the naked eye.

Shortly after 10:00 p.m. CEST, a veritable stellar occultation parade will occur. While the stars on the eastern edge disappear behind the bright limb of the Moon, they reappear on the opposite dark limb after a maximum of one hour. The table above lists the exact occultation times.
You may also have wondered why, in autumn and winter, a Pleiades occultation always occurs through a "brightly illuminated" Moon and never through a crescent Moon. This depends on the Sun's position in the ecliptic. The Pleiades are at 83° ecliptic longitude. In mid-December, the Sun reaches 263° (63° + 180°) ecliptic longitude and is exactly opposite the Pleiades in Ophiuchus. This explains why the fourth and final Pleiades occultation that can be observed here will occur on the morning of December 4, shortly before the full moon. Like the Pleiades, the Moon is opposite the Sun. 

Occultations of planets by the Moon only occur every few years, and even more rarely at a time of day that is convenient for us. If the events occur in the daytime sky, they can only be observed with a telescope. This will be no different on September 19th.
However, since it is the bright Venus, we can observe the occultation quite well despite the daytime brightness, provided we have a perfectly clear sky. The crescent Moon, illuminated by 5.5%, will be located just over 27° northwest of the Sun at 2:00 PM CEST.
Since it is often difficult to see objects in the daytime sky, it is worth covering the Sun with your hand. This should make it easier to spot the Moon at roughly the same altitude to the right of the daytime star.

In Zurich the bright edge of the moon will capture the 11.48" large and three-quarters illuminated planetary disk at 2:11:27 p.m. CEST. The entire occultation will last exactly 30 seconds. At 3:27:32 p.m. CEST Venus will begin to emerge again - now at the dark edge of the moon. This time it will take 27 seconds until we can see Venus in full again. The circular section below shows the situation shortly before and after the occultation. The crescent moon is drawn in the correct position in relation to the horizon.
Anyone who misses this year's event will be able to enjoy another occultation of Venus next year on September 14th shortly after moonrise, this time due to the waxing crescent moon at a slightly greater distance from the sun.