Gemini displays RA in hours, minutes, and decimal minutes so that the RA value above 

would be represented as 16h 42m9.  Declination is displayed as degrees, minutes, and 

seconds, so the above Dec. value would be represented as +38

 55' 12. 

3.1.2  Precession 

While most stars, galaxies and other deep sky objects do not move relative to each other 

in any kind of time frame we can notice, their RA and Dec. coordinates do change slowly 

over the years.  This is because the coordinate system is based on the poles and equator of 

the Earth.  In addition to rotating on its axis once a day and orbiting the Sun once a year, 

the Earth' s axis is precessing like a top once every 26,000 years taking the RA and Dec 

coordinate system with it. 

This is why RA and Dec. coordinates always have a date associated with them, called the 

Epoch or Equinox of the coordinates.  Most sources now use Epoch 2000 coordinates 

(abbreviated J2000.0), meaning that the RA and Dec. values were correct on Jan 1, 2000.  

In order to accurately point a telescope at an object whose J2000.0 coordinates are given, 

a precession calculation must be applied to determine the RA and Dec. values for the 

current date. 

By default, Gemini assumes that any coordinates you enter are for the epoch of the 

current date.  You can change Gemini' s default epoch to J2000.0 using the 

  Setup

Coordinate Epoch

Epoch J2000.0   menu item, allowing you to enter (or 

transfer from a connected PC) J2000.0 coordinates and letting Gemini perform the 

needed precession calculation. 

3.2  How the Sky Moves 

If you have spent any length of time outdoors looking at the stars, you' ve probably 

noticed that everything in the sky shifts position over time during the night.  Of course 

this is due to the fact that the Earth is rotating on its polar axis.  Recall that the celestial 

poles in the sky lie directly above the Earth' s poles, so just as the Earth rotates around its 

poles, the entire sky appears to rotate around the celestial poles, completing one 

revolution in about 24 hours.  Objects that lie close to the poles make small circles 

around the pole, while objects further from the pole make larger circles, taking them 

below the horizon for periods of time during their revolution.  So, although an object' s 

RA and Dec don' t change significantly during the course of a night, a fixed telescope 

aimed directly at a point on the meridian (the imaginary line passing directly overhead 

from north to south) would see stars drift through it from east to west as increasing RA 

values cross the meridian.  It is important to understand that a fixed telescope points to a 

constant Dec. value, but a changing RA value. 

 There is another coordinate system   one that does not move with the rotation of the 

Earth.  Because it does not move relative to the Earth, it can be used to describe a 

telescope' s position relative to the Earth.  It uses the same Dec. value as does the rotating 

system to measure angles north and south of the poles.  But instead of RA, the so called 

"Hour Angle" is used to measure angles east and west.  It too is measured in hours, 

minutes and seconds (or sometimes just minutes), but uses the meridian as its zero point.  

Gemini Users Manual 

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