Specifications

A brief explanation of time
To simplify matters, nations have generally agreed to employ, on an earth wide basis, only 24
varieties of time, differing by whole hours from one another. We subdivide the 360 degrees of
longitude around the equator into 24 intervals of 15 degrees, one for each hour. Thus the ZERO
MERIDIAN, the meridian of Greenwich, is centered on one of these 15-degree strips. Within
each segment of the earth, time remains constant; across the boundary of the neighboring 15
degree segment, time shifts by one hour, and so on, in orderly progression. A glance at a map of
standard time zones, however, shows that the boundaries actually adopted are often quite
irregular, to suit the convenience of individual communities. Eastern Standard Time is 5 hours
earlier than Greenwich Standard Time, Central Standard Time, 6 hours earlier, Mountain
Standard 7, Pacific Standard 8, and so on. These figures, which represent the difference
between Greenwich Standard Time and your own standard time, we shall refer to as Standard
Longitude Difference, abbreviated as SLD.
Greenwich Mean Time (GMT)
Which is also called Greenwich Standard Time, Greenwich Civil Time and Universal Time (UT), is
the basic reference time used for most astronomical work. To avoid the cumbersome A.M. and
P.M., astronomer's number the hours from 0 to 24, starting with midnight.
To derive GMT, add to your own Standard Mean Time (SMT) the number of whole hours
corresponding to the Standard Longitude Difference (SLD): GMT = SMT + SLD. For example if
you live in the zone of Central Standard Time. SLD = 6hours. If your SMT is 21h 32m, GMT =
21h 32m + 6h OOm = 27h 32m. Subtract 24 hours if necessary, in order to give a result in the
normal range from 0 to 24: 27h 32m = 3h 32m. If you live east instead of west of Greenwich, the
SLD is negative.
To calculate your Local Mean (solar) Time (LMT) the following formula is often useful:
LMT = SMT + SLD - LLD. Suppose you live in Cleveland Ohio, longitude 81 degrees 45min W.
Divide by 15 to obtain the Local Longitude Difference of 5h 27m. Since Cleveland operates in the
Eastern Standard Time, SLD = 5h. So, if the standard clock (Greenwich time) reads 3:49 PM
(15h 49m), the Local Mean Time is:
LMT = (15h 49m) + (5h) - (5h 27m) = 15h 22m. If necessary, add or subtract 24 hours, as
explained above.
Sidereal Time
The earth's revolution in its orbit causes the sun to drift eastward approximately 1 degree per day
with respect to the stars. Thus, at the end of a year, when the earth has completed 365.2422
rotations with reference to the sun, it has made exactly 1 additional rotation, with reference to the
stars; and so our year contains 366.2422 sidereal, or star, days. A Sidereal Day, in
consequence, is about 4 minutes shorter than a solar day (24 hours divided by 366.2422).
On or about March 21, when the sidereal and solar years start, Sidereal "noon" (or zero hours
sidereal time) occurs as solar noon (or about 12 hours on the solar clock). Hence the sidereal
and solar clocks, whose time scales run consecutively from 0 hours to 23 hours 59 minutes, do
not really coincide until 6 months later, at the autumnal equinox, on or about September 21. By
this time the sidereal clock has gained 12 hours on the solar clock and both read midnight, or Oh
OOm. The sidereal clock continues to gain on a mean time solar clock by 3m 56.555s of sidereal
time for each solar day.
27