Porcupyn's Blog

October 9, 2015

How many times does the Earth rotate about its axis in a year?

Filed under: Uncategorized — Porcupyn @ 7:41 am

To be more precise, how many times does the Earth rotate about its axis in 365.25 days? Guess what! Well, if you say “The answer is 365.25, of course” then you would be wrong. The earth actually rotates about its axis once more than the number of days it takes to complete one complete revolution around the sun. To generalize this further, each planet has one less “day” than the answer you would get when you divide its revolution time by its rotation time.

It is ironic that I did not independently understand this fact. I had to read this question and (incorrect) answer combination, and then do some analysis. As an aside, I hope that “nik” has gone on to do higher things in life and hopefully Nicolle understands that she was wrong.

Here is my simple experiment:

Assume that a planet rotates about its axis every 24 hours and takes 48 hours to revolve around its star. Draw the star in the centre and draw four positions of the planet at 90-degree intervals around the star. Pick a spot on the planet and draw the positions of the spot in those four locations of the planet. You will see that though the spot will see only one each of the following,

– star-rise
– noon
– star-set
– midnight

Thus the planet revolves around the star in one day instead of the two days that you would intuitively assume. In fact, here is a table:

Rotation Revolution

Does not rotate Any One
X hours X hours None (zero)*
X hours 2X hours One
X hours 3X hours Two
X hours 4X hours Three
and so on… and so on… and so on…

When I google the question, most of the answers that come up are INCORRECT!! I did find one (and I am sure there are others, but they are all much deeper down in the search reults) that is correct.

Wait a minute though! That is not the final answer (and depending on how you did your modelling of your star-planet combo, you might have realized it as well). The answer actually depends on the direction of the revolution and the direction of the rotation. If the planet rotates clockwise and revolves around the star clockwise as well, the above scenario holds true, as it does for the Earth and most of the planets; however, some planets rotate in the opposite direction of their revolution, i.e., the rotate counter-clockwise and revolve clockwise, or vice versa. For such planets, whose rotation is termed retrograde, the Days/Year in the above table would be One, Two, Three, Four, Five and so on.

PS: I wonder … which is the closest planet to the Sun where the stars can be seen during the daytime?


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