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Q & A: Planet with the shortest day

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Most recent answer: 05/14/2008
the planet with the shortest day
- elieace robinson (age 14)
moreno vally CA 92553
Hi Elieace,
The short answer is Jupiter, with a 'Solar Day' of 9 hours and 55 minutes (Earth time).

The long answer is due to the fact that there are two definitions of  'day'. One is called the 'Solar Day', or 'Synodic Period' in astro-speak.   This is the time from noon till noon the next day, averaged over a year.  It is not quite the same from day to day during the year because the orbit of the Earth is not quite a circle but is an ellipse.  The other definition of  'day' is called the 'Siderial Day'.   This is the time difference measured when observing the passage of time when a fixed star passes overhead.   In the case of the Earth the difference is small, only 4 minutes.  In the case of the planet Mercury the difference is HUGE.  
See the web site:
    for a detailed explanation. 
Another web site to visit is
that has a lot of  other interesting information about planets. 


(published on 05/14/2008)

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