About eclipses

The night between 27th and 28th of September we are getting the opportunity to experience a total lunar eclipse. The word “we”, doesn’t refer to some common right to see such an event every second year or so. It refers to the fact that a lunar eclipse happens on our moon, sometimes named “Luna”. That means that everyone who happens to be att “the right side” of the Earth will see the lunar eclipse – if the clouds don’t close the curtain. A total solar eclipse though is a dramatic event to be seen from just a tiny spot on the Earth’s surface. It’s time to ask “Why?”

A schematic diagram of a solar eclipse. Image: wikimedia.org/wikipedia/commons

A schematic diagram of a solar eclipse. Image: wikimedia.org/wikipedia/commons

During a solar eclipse, the moon passes in a line between the Earth and the Sun. This is possible only during a “New Moon”. (Why?). The moon’s radius is about 1740km. The radius of the Sun is just under 696000km. In other words, the Sun’s radius is about 400 times the radius of the Moon. While the distance from the Earth to the Sun is about 400 times the distance to the Moon, we find these objects apparently the same size in the sky. In other words, the Moon sometimes fits perfectly in front of the sun to cover it up in a total solar eclipse. If the moon is a bit further from us it appears smaller and – if it then comes in between the Eart and the Sun – we call it an annual solar eclipse.

However, the Earth is much bigger than the Sun, and the Sun’s radius is a hundred times that of the Earth. That’s why the Moon can’t create a shadow covering all the planet Earth. A solar eclipse moves along a narrow strip on the Earth’s surface. In fact, total solar eclipses aren’t that rare. The latest one occurred in March 2015, and next one will be in March 2016. You can see an animation and a map of the coming solar eclipse here. On the other hand, if you want to see a solar eclipse from your hometown you have to be lucky to see one in your lifetime.

A lunar eclipse is not as dramatic as a solar eclipse. The Moon doesn’t vanish for a single moment, it just turns red. The blue color of the sky above us is caused by the scattering of sunlight off the molecules in the Earth’s atmosphere. This scattering is more effective at shorter wavelengths. During sunrise or sunset, though, the sunlight travels a long distance through the atmosphere. Then it’s just the longer red wavelengths that are coming trough, turning morning and evening sky orange or red. To see a “Blood Moon” from a romantic view, it turns reddish because it reflects the light from all the sunrises and sunsets on the Earth – at the same time!

A schematic image of a lunar eclipse. Image: wikimedia.org/wikipedia/commons

A schematic image of a lunar eclipse. Image: wikimedia.org/wikipedia/commons

As I wrote at the beginning, a lunar eclipse happens on the Moon. The Earth’s radius is 3,7 times the radius of the Moon, which means that the Earth’s shadow is big enough to cover the whole Moon for more than an hour. From any spot on the Earth pointed towards the Moon we see the moon get darker at the same time – unlike the situation during a solar eclipse. The night between 17th and 18th September, if you are in a place where you can see the moon, you see the full eclipse begin, when the Moon completely enters the Umbra, at 02:11:12 UTC. In Europe, in Africa, in America.

A lunar eclipse is possible only during a full Moon. (Why?). This September the full Moon is at a point in its orbit when the distance to the Earth is at its shortest. This is called a “supermoon” in the popular press. The radius of the Moon’s orbit varies between 405000km and 360000km. A “supermoon” appears to have a radius about 12% longer, and therefore, to be more than 20% bigger compared to a full moon when it is at its furthest.

Now it is to be decided whether a view of a red full moon, slightly bigger than average, is worth more than a full night’s sleep. There’s always one more question…

9/28 2015 it was a rainy, cloudy morning in western Finland, but 90 minutes of waiting paid off! Photo: Jan Holmgård

9/28 2015 it was a rainy, cloudy morning in western Finland, but 90 minutes of waiting paid off!
Photo: Jan Holmgård

 


JanJan teaches mathematics and interdisciplinary science to pupils 13-16 years of age at Sursik School, Pedersöre, Finland. Space-related science often gives some sort of answer to the question “Why?”, a question quite common in math class. It also triggers curiosity, one key component in progress.

 

 

 

 

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