How to Calculate Sidereal

Sidereal time is an astronomical concept used to track and observe stars. It is based on the sidereal day, which is about four minutes shorter than a regular, solar day. The reason for this is that the earth, as it rotates the sun, needs to rotate a bit farther each day to reach the same point in its revolution relative to the sun, but since the stars we observe are so far in the distance, this does not apply to the sidereal day. The best way to calculate the sidereal time is to use the standard calculator provided by the U.S. Naval Observatory.

Instructions

1. • 1

     Go to the U.S. Naval Observatory site, listed under Resources.

   • 2

     Enter your city or town in the space indicated; you may also enter your zip code. You do not need to enter your longitude if you do not know it.

   • 3

     Enter your two-letter state abbreviation in the space indicated.

   • 4

     Click on "Compute" to calculate the sidereal time.