But how do we, sitting on earth know how rapidly a planet like Mercury which is around 48 million miles away is rotating ?
Doppler effect
This is a very interesting example of Doppler effect.
Radio waves are shot from the earth towards the surface of mercury, one side of the planet will be red shifted (since it is moving away from you) and the other will be blue shifted (since it is moving towards you).
By measuring this apparent change in frequency, we can find out how rapidly mercury is rotating.
Using this method we have found out that the rotation period of mercury is approximately 58.6 days.
Piotr Olejnik, from a
Zdzisław Beksiński’s illustration. I love his art so much, but It’s probably the first time I see one of his works reproduced as a tattoo…
