Universal Time

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What is UT?

Universal Time (UT) is the time at the longitude of Greenwich Observatory in England.

Why do I need to know about UT?

By convention, astronomers give the times of planetary events (e.g., eclipses, occultations, etc.) in UT. Each observer at any longitude around the world is expected to know how to convert a date and time from UT into his or her own time. For example, the planet table in every issue of Sky and Telescope lists planetary positions on given dates at 0 hours UT.

How to Convert UT to North American local times

1. Subtract the following number from UT to obtain the indicated North American standard time:
   • East coast: Subtract 5 from the UT to obtain Eastern Standard Time (EST)
   • Midwest: Subtract 6 from the UT to obtain Central Standard Time (CST).
   • Mountains: Subtract 7 from the UT to obtain Mountain Standard Time (MST)
2. From October to April, add 1 to the Standard Time in order to obtain Daylight Savings Time (DST; = Central Daylight Time, CDT).
3. If the result is a negative number, add 24 but use the day before the UT date.

Examples

• 7:00 UT August 1 = 1:00 a.m. CST August 1.
• 5:00 UT August 1 = 23:00 = 11:00 p.m. CST July 31.
• 7:00 UT December 1 = 2:00 a.m. CDT December 1.

How does Universal Time differ from Sidereal Time?

Universal Time has no constant relation with sidereal time (ST). ST is the right ascension (measured in hours and minutes) of any star crossing the meridian at a given moment. A given minute of right ascension will transit the meridian at exactly the same UT only once each sidereal year.

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