COLOUR OF MOON DURING LUNAR ECLIPSE

Lunar eclipse is one of the most common celestial phenomenon.Yet,not many know some of the simple scientific principles behind its occurance.

      

What is a lunar eclipse?    
Lunar eclipse occurs when the shadow cast by earth falls on the moon. The moon revolves around our earth once in a month. Well, if that is the case, then lunar eclipse should be a monthly phenomenon right? But, a lunar eclipse occurs like twice a year max. Why?  Reason is, the moon has an orbit which is inclined to the earth’s rotation axis by about 5 degrees. Thus, the sun, earth and moon do not come in a straight line often. But when it does, earth does cast its shadow on the moon and we have an eclipse.

If the shadow of earth falls on the moon, it should not be visible right?

It would not have been, but for the earth’s atmosphere. What does the Earth’s atmosphere do? To know why, let us go back to the 17^th century when Newton performed one of his famous optical experiments.

This is the prism experiment conducted by Issac Newton. Basically, Newton forced sunlight through a prism and observed the light coming out of the prism in a dark room Let us take a closer look into the experiment.

Two import observations from this experiment are:

1. The direction of sunlight has changed after passing through the prism.
2. Sunlight is broken up into 7 colours after passing through the prism

Now let us apply  result  (1) on the lunar eclipse model.

2        

           Observe the ray of sunlight AB.

If the earth’s atmosphere had not been present, then the ray of light would take the path ABE and never hit the lunar surface. But, the earth’s atmosphere acts as a prism and bends the light towards the moon and eventually the light hits the moon. This can be seen if the ray of light ABCD is observed.Thus we successfully applied result (1) to the lunar eclipse model. 

Now let us apply result (2).If we apply the second result, it would imply that we would observe a rainbow like pattern on the moon. That is hilarious, isn’t it? Such a phenomenon has never occurred.

Why is the moon red during lunar eclipse?

This can be explained by a phenomenon named scattering of light. Sunlight is a mixture  of 7 colours. Colour is defined by wavelength.(blue has shortest wavelength, red has longest wavelength).The moment sunlight enters the earth’s atmosphere, it encounters air molecules and it starts getting scattered. Turns out, scattering is much more for blue light than red light. This is the reason why we see the sky as blue most of the time. But the longer sunlight travels through the atmosphere, the more blue light is scattered and only red light remains. This is the reason why only red light hits the moon.

Thus, the moon appears red during lunar eclipse.

 On similar lines, you can get an answer to why the sky appears red during sunset.We looked at a relatively simple phenomenon today.After all, Science has always been simple.