Paramount GEM User Guide
16 | P a g e
Azimuth
The azimuth (az for short) of an object is generally reckoned from north, increasing in the clockwise
direction, and ranges from 0 to 359 degrees. North is 0 degrees, east is 90 degrees, south is 180 degrees
and west is 270 degrees.
When the Paramount is controlled by TheSky Professional, attempting to slew the mount below the local
horizon is not permitted and results in an error message.
The Equatorial Coordinate System
The horizon coordinate system is not convenient for directing a telescope to a celestial object because,
due to the rotation of the Earth, the coordinates continually change with time.
The exception occurs with objects near the north and south celestial poles. These positions are unique
since they are near to the axis of rotation of the Earth and therefore move only in small circular paths.
The altitude and azimuth of Polaris, for example, do not vary significantly. In the equatorial coordinate
system, the coordinates of celestial objects remain fixed over time
2
.
An object's equatorial coordinates remain the same regardless of the position on Earth the object is
viewed. This allows astronomers to create star maps that apply to any place on Earth or publish the
anticipated position of an upcoming comet so that astronomers everywhere know where it is located
among the stars.
The equatorial coordinate system used to specify the positions of celestial objects is directly analogous to
the latitude-longitude coordinate system used on Earth. In fact, if you were to expand the latitude and
longitude grid of the Earth so that it is out beyond all stars, you would have a sphere with identical
geometry to the celestial sphere.
We suppose that all stars and deep-sky objects are located on a very large sphere (out beyond all stars).
We call this the celestial sphere. For purposes of describing the positions of celestial objects, we consider
all stars and deep sky objects to be on the celestial sphere, when they are all positioned at varying
distances from the Earth.
2
Equatorial coordinates change over extended periods due to precession (wobbling of the Earth). TheSky
computes this change in stars’ position for the current date. Precession, however, does not change the
relative positions of objects with respect to one another.
Right Ascension
The geometry of the right ascension (RA) lines on the celestial sphere is the same as the longitude lines
on Earth. Longitude lines divide the Earth into 360 degrees. Right ascension lines divide the celestial
sphere into 24 hours, based on one revolution of Earth. Therefore, one hour of right ascension equals 15
degrees (360 divided by 24). See the definition of Local Sidereal Time for additional information on why
24 hours are used for right ascension instead of 360 degrees.
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