Let's Talk Physics

A stochasticly updated blog about interesting topics in Physics & Astronomy

A Solar Eclipse, and why it’s so special.

The moon is the most dominant figure in the night sky, whenever it appears. There are a couple of theories about how it came into existence, but what I want to talk about today is what happens when the moon passes between the Sun and the Earth (a solar eclipse) and when the Earth passes between the Sun and the moon (a lunar eclipse).

First things first – a lunar eclipse. The diagram below shows what happens when the moon passes behind the Earth. Below that is a long exposure shot of a lunar eclipse.

A diagram, showing what happens during a lunar eclipse

A diagram, showing what happens during a lunar eclipse

A long exposure shot of a lunar eclipse – the gap in the streak in the middle of the frame is where the moon has been completely blocked, and the gaps in the lower right of the photo are due to passing clouds

Look at the above picture, you’d think that at the middle of the lunar eclipse, the surface of the moon is completely dark. Sadly, you’d be thinking wrong. Below is a multi-exposure shot of the moon during a lunar eclipse. And as you can see, at the moment of total eclipse, the moon is actually tinted red! This is because light from the Sun is being refracted by the Earth’s atmosphere towards the moon. The reason the light that reaches the moon is red is because all of the smaller wavelengths of the visible spectrum (green, blue etc) tend to be scattered by our atmosphere, meaning the only colour that properly reaches the moon is red light.

A multi exposure shot of a lunar eclipse

Now to get onto the really special stuff – a solar eclipse. A schematic for what happens when a solar eclipse occurs is shown below – the moon passes in between the Earth and the Sun, and the moon is EXACTLY the right size and at the EXACT distance that the size of the moon is the sky is the same as the Sun, and it blocks out the sun perfectly! The picture below that shows a total solar eclipse, and shows the corona of the Sun behind it. This is something everyone knows – it’s why a solar eclipse occurs. What we don’t seem to appreciate is how damn unique this is, at least in our own solar system!

Schematic for a solar eclipse

Schematic for a solar eclipse

A total solar eclipse

You see, you could easily imagine that if the Moon were slightly bigger (and at the same distance from the Earth) then it would block out the solar corona. However, if it were slightly smaller, then the light of the sun would still be visible, and would drown out the corona and the detail of the moon. The same applies for changing the distance from the Earth to the moon.And what’s really special, is we haven’t seen this type of eclipse occur anywhere else in our solar system. The solar eclipse that we witness truly is a unique phenomenon.

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About irishphysicist

I'm a PhD student with the Departments of Physics in University College Cork, Ireland and University of Notre Dame, Indiana. I want to try and bring astrophysics to the public, and also would like world domination. But that's a story for another day.

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This entry was posted on March 10, 2013 by in Astrophysics and tagged , , , , , , , , , , , , .
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