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Name: Tom
Status: educator
Grade: other
Location: CA
Country: USA
Date: N/A 


Question:
I'm compiling a list of NEOs for my kids and I to track. I have all their orbital parameters calculated from JPL

(http://neo.jpl.nasa.gov/ca/).

Most of their orbits I have been able to model if they are within a year away. But for example, Apophis which is scheduled to swing by in 2029 and 2036, I can only get it to be as close as .3AU, and not within our Moon's orbit. I have just had some trouble in determining what the precise variable is for Earth's year, (period-yr). I have 365.256363004 days as the most precise sidereal year that I can find. (http://hpiers.obspm.fr/eop-pc/models/constants.html) Is there a more precise number for the number of days that make up Earth's Sidereal Year?



Replies:
Dear Tom,

Our sidereal year, according to Wikipedia, is 365.256363004 days.

It is the amount of time it takes Earth to revolve about the Sun once with respect to "fixed" stars.

Sincerely,

David H. Levy


The precision with which the Sidereal Year is known is greater than the accuracy, due to small but measurable variations. These variations result from the location of other planets, the moon and changes in the Earth's gravitation. You will also notice in the website you site http://hpiers.obspm.fr/eop-pc/models/constants.html that some "constants" are "exact" that is they are taken as "fixed" and other related "constants" are adjusted to keep those parameters "exact".

Vince Calder



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