IDA Membership - Astronomy Club

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Time and Astronomy

LOCAL TIME is what the general public accepts as a 24 hour day.  We are in the Pacific Time Zone and by definition it is the Local Mean Time at 120 degrees longitude. 

UNIVERSAL TIME (UT) is the time zone at zero degrees longitude.  This longitude is, by definition, that of a line engraved in a brass plate in the floor of the Old Royal Observatory at Greenwich, England.  We are in the Pacific Time Zone and we subtract 8 hours from Universal Time to convert to Pacific Time.  During Daylight Saving Time, we subtract seven hours from Universal Time.  If you get a negative number, add 24 hours and you will be on the date before the Universal Time date given.

1/2/04 at 06:00 UT is 1/1/04 at 22:00 PT (6 minus 8 is minus 2, add 24 hours is 22 hours on the previous date)

SIDEREAL TIME is the right ascension of stars on our local meridian at any moment.  It is based on a sidereal day or the time required for a complete rotation of the earth, measured as the interval between two successive meridian transits of the vernal equinox.  A sidereal day is about 23 hours, 56 minutes.  An example is if the sidereal time is 5:35 a.m., then right ascension 5h 35m is on the local meridian and that is where the Orion Nebula (Messier 42) would be found.

To better understand sidereal time, come up to the Observatory and see how the time changes on the computer program, The Sky.  Also, learn how Polaris and the two pointer stars in the Big Dipper show the approximate sidereal time.

For a very technical explanation, do a search on 'sidereal time' in your web browser and scan for the sites that have the spreadsheets with all the calculations to convert to sidereal time.