Blood Moon - Total Lunar Eclipse
September 2025
Have you ever seen the moon turn red? If you have not, this Sunday you have a good chance to experience it!
On September 7th one of the most beautiful astronomical events will be visible from Austria: a total lunar eclipse. This happens when the Sun, Earth and Moon align in such a way that the Earth is between the other two and casts a shadow over the Moon (see image below right). The Moon is then in the Earth's umbra and no direct sunlight reaches it. If you were standing on the Moon, you would see a total solar eclipse!
Total lunar eclipse (© Flickr - Bernd Thaller)
Geometry of the total lunar eclipse. From top to bottom: Moon without eclipse, Moon in penumbra and Moon in umbra. (© The Urban Astronomer)
The science behind it
But, back to Earth and what we will see from here. One could expect the Moon to become completely obscured, or dark, but that is not what happens. It turns red instead. Therefore the name. This happens due to two different phenomena.
The first has to do with "turning" light red. The sun's rays reach the earth's atmosphere. They contain all the colors of the rainbow, from violet and blue to red. The molecules in our atmosphere, oxygen and nitrogen, scatter or deflect violet and blue light more effectively than red light. That is why we actually have a blue sky on a sunny day, we are seeing blue sun rays scattered by our atmospheric molecules! Red light is not disturbed by these molecules and is able to pass through and leave the Earth's atmosphere, while blue light remains "trapped" in the atmosphere.
The second phenomenon has to do with the bending of this red light that has escaped the Earth's atmosphere so that it reaches the Moon. This is caused by refraction. All light is bent when it enters a different medium. You have probably seen this effect when you put a spoon or pencil into a glass of water, it seems to split at the water's surface. The speed of light changes as it enters the water and we see the bent light rays. Exactly the same thing happens to red light in the Earth's atmosphere, it changes its speed of travel resulting in bending. That is why some of the red light rays are able to reach the moon and color it beautifully.
The refraction of the light causes the light rays to bend and it looks as if the pencil were broken. (© Mohammad Tajer)
How to see it
After this scientific digression, let's get back to practical matters. In Vienna, the moon will rise at 19:20 on this day, with the moon already in the earth's penumbra (the shadow next to the umbra). Only 10 minutes later, at 19:30, the totality of the eclipse will begin, reaching its maximum at 20:11. The total eclipse will end at 20:52, while the penumbral eclipse will last until 22:55. If you are in another part of Austria (or the world), you can check the times of eclipse visibility on the Time and Date website.
You need nothing to see it! Well... just a clear view to the east where the moon will rise. If you have binoculars or a telescope, that is of course a plus. For the more adventurous, you can join the Astropraxis activity organized by the Wiener Arbeitskreis für Astronomie WAA and the Österreichische Astronomische Gesellschaft at the Sophienalpe, where they will be observing and photographing this astronomical event. More info here. Another possibility is to take part in the activities of the Urania Observatory, where there will be an informative lecture and live observations.
I wish us all clear skies on Sunday!