Description:
This calculator has been developed primarily to extract the Greenwich Hour Angle, GHA, of a celestial body from MICA (Multiyear Interactive Computer Almanac) in a convenient way, but it can be used in combination with other almanac software or printed almanacs as well.

Unlike the Nautical Almanac and ICE (Interactive Computer Ephemeris), MICA only provides the Right Ascension, RA, of a body which is the equivalent to the Siderial Hour Angle, SHA, with the exception that RA is measured in hours (0 - 24) eastward from the Vernal Equinox (first point of Aries) whereas SHA is measured in degrees (0 - 360) westward from the vernal equinox.

SHA[°] = 360 – 15 · RA[h]

Converting RA to GHA requires Greenwich Siderial Time, GST, (also provided by MICA), the Greenwich hour angle of the vernal equinox measured in hours (0 - 24) westward from the Greenwich meridian (1h is equivalent to 15°).

GHA_Aries[°] = 15 · GST[h]

GHA = SHA + GHA_Aries

When using MICA, RA can be accessed through the following menus and submenus:
Calculate
Position
(select object)
Apparent
Geocentric
Equator of date

Greenwich apparent siderial time is available through:
Calculate
Time and orientation
Siderial time (App.)

This program is freeware and may be copied and distributed in unmodified form and on a non-profit basis. For your private use, feel free to modifiy the source code according to your requirements. Any commercial distribution requires the author's permission.

Henning Umland

N53-20.1 E9-53.6

e-mail: mail@celnav.de

Check this web site for updated versions: http://www.celnav.de