3  Important Concepts about Your Mount 

In order to get the most from your mount and the Gemini system, you' ll need to 

understand a few important concepts about both astronomy and telescope mounts.  This 

chapter will introduce you to these concepts and show you how to apply them using some 

of Gemini' s advanced features. 

3.1  Astronomical Coordinates 

3.1.1  Right Ascension and Declination 

Just as any location on Earth can be described by its latitude and longitude, positions in 

the sky can be described by a set of coordinates known as Right Ascension (abbreviated 

RA) and Declination (abbreviated Dec.).  RA and Dec. are the celestial equivalents of 

longitude and latitude, and are actually closely related. 

Imagine a globe of the Earth with lines of latitude and longitude drawn on it.  Now, 

extend those lines out from the globe to the imaginary celestial sphere on which are 

placed all the objects in the sky.  Directly above the poles of the Earth are the celestial 

poles, and directly above the terrestrial equator is the celestial equator.  The star Polaris 

lies very close to the north celestial pole.  Declination is easy since it exactly corresponds 

to latitude, measuring the angle between the celestial equator and the celestial poles.  Just 

as lines of latitude run parallel to the equator and go from 0

 at the equator to 90

 at the 

poles, so do lines of declination.  Declination values are measured in degrees, minutes 

and seconds (or decimal minutes) and are positive when north of the equator and are 

negative when south of the equator.  Thus, a star at 46 degrees 12 minutes 42 seconds 

south of the celestial equator is said to have a Dec. of  46

 12 

 42". 

Right Ascension is analogous to longitude, measuring angles east and west.  And, like 

longitude, RA needs an arbitrary zero point.  While the zero point of longitude is 

Greenwich, England, the zero point of RA is that location in the sky where the Sun 

crosses the celestial equator on its way north in the spring (also called the first point of 

Aries).  However, unlike longitude, RA is measured in hours, minutes and seconds (or 

decimal minutes) from 0h 0m 0s through 23h 59m 59s.  RA values increase as you move 

east of the first point of Aries.  Note that because the Earth is rotating to the east on its 

poles, lines of constant declination remain in constant positions on the sky, but lines of 

constant RA move from east to west in the sky during the night.  We' ll look at this in 

more detail in the next section of this chapter. 

So, the coordinates of an object might look like 16h 42.9m +38

 55 

 12" which means 16 

hours 42.9 minutes east of the first point of Aries and 38 degrees 55 minutes 12 seconds 

north of the celestial equator. You can use the   RA/DEC Coordnt.

Enter RA/DEC   

menu item to input the coordinates into Gemini.  If you then tell Gemini to GoTo those 

coordinates, you' ll find yourself looking at M13 in Hercules. 

Gemini Users Manual 

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