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
- 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
- Mountains: Subtract 7 from the UT to obtain Mountain
Standard Time (MST)
- From October to April, add 1 to the Standard Time in order to
obtain Daylight Savings Time (DST; = Central Daylight Time,
- If the result is a negative number, add 24 but use the day
before the UT date.
- 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.