Some Terms You Should Know: the Sun
These are some terms you will come across in your lab class dealing with
the sun and timekeeping.
Apparent Time (AT) or Solar Time
The time calculated using the Sun's true motion. A sundial reads the Apparent
Equation of Time (E/T)
The Equation of Time gives the difference in Right Ascension between the
Sun (used to reckon AT) and the ficticious Mean Sun (used to reckon MT).
That means it also gives the correction between the AT from a sundial and
the local MT.
Mean Time (MT)
Sometimes called Civil Time, it's the time calculated using the Mean Solar
day (precisely 24 hours) and assuming that the Earth's orbital speed is
constant throughout the year. This is the time you normally use.
Sidereal Time (ST)
The time claculated with respect to the stars rather than the sun. The
Siderial Day is about 23 hours 56 minutes.
Universal Time (UT) or Greenwich Mean Time (GMT)
The Mean Time at 0 degrees longitude. (It is sometimes called GMT because
this is the longitude of the Royal observatory in Greenwich, England.)
Back to the definitions page
This page was last updated on 18 January 1998
and is © 1998 by Jim McDonald.