Measuring the positions of objects requires the use of a coordinate system of some sort. Here we describe three that you are likely
to encounter in making or reducing astronomical observations.
Equatorial Coordinate System: The Equatorial coordinate system is one of the preferred coordinate systems of
observational astronomers. The Equatorial coordinate system, like latitude and longitude, is based on measures of angular separation
from some arbitrary origin; in the case of latitude and longitude the coordinates are measured from the equator and Greenwich
Observatory in London, respectively. In the Equatorial system, the equator is still used (or, rather, its projection up onto the sky),
but the other reference point is where the Sun crossed the equator on the Vernal Equinox (the point is called the first point of
Aries).
The names of the coordinates in the Equatorial system are Right Ascension (RA  similar to longitude, it measures positions east and
west) and Declination (Dec  similar to latitude, it measures positions north and south). Declination works exactly like latitude,
with the declination of a star being its angular separation from the equator (negative declinations mean the star is south of the
equator). Right Ascension is a bit different from longitude: it is measured in hours, minutes and seconds. There are 24 hours of Right
Ascension around the sky. Each hour corresponds to 15 degrees of arc. Thus there are 360 degrees around the equator. Can you think of a
reason why RA is measured in hours, minutes and seconds rather than degrees, arcminutes and arcseconds? (Hint: How many hours does it
take the Earth to rotate through 360 degrees?) RA is really a measure of time!
NOTE: RA: 1
hour = 15 degrees = 60 minutes and 1 minute = 60 seconds (of time) Dec: 1 degree = 60 arcminutes and 1 arcminute = 60
arcseconds.
Horizon Coordinate System: A coordinate system that is often useful for making astronomical observations is the
Horizon system. In this system the coordinates are related to the individual making the observations. An object is located by its
altitude above the horizon and by its direction from due north. The altitude is given merely by the angular distance measured
perpendicularly from the horizon. The point directly overhead, the zenith has an altitude of 90 degrees, whereas the horizon
itself has an altitude of zero. the direction from north is measured in a clockwise sense, with due east having a azimuth angle
of 90 degrees. South is 180 degrees, west is 270 degrees and north returns us to 0 or 360 degrees. The Horizon system is often also
called the Altitude  Azimuth coordinate system. The control software used to point a telescope must convert RADEC coordinates to
AltAz coordinates so that the telescope will point at the correct position in the sky. The AltAz system is also quite important when
trying to decide if an object is observable from a given location. Clearly if an object has a negative altitude at some given time, then
it is below the horizon and cannot be observed.
Galactic Coordinate System: One final coordinate system you might come across from time to time is the Galactic
Coordinate System. Unlike the Equatorial system, Galactic coordinates are not referenced to the Earth and its spin axis. Instead,
Galactic coordinates are based on the orientation of the Milky Way. The Galactic system uses longitude and latitude, just as we do on
the Earth's surface. In the case of Galactic coordinates, the "equator" is the plane of the Galaxy. The Galactic latitude of objects is
the perpendicular angular distance from the equator, either north or south. The zeropoint for longitude is the Galactic center (in the
constellation of Sagittarius). You won't have to know a lot about the Galactic system to use the telescope, but we have mentioned it for
completeness.
For additional information about astronomical coordinate systems see the Astronomy Notes website. To learn in detail
how to fit an astrometric model, see our astromety page
