An object in the east has a negative hour angle.  This value is interpreted as the number 

of hours it will take for the object to arrive at the meridian.  If it is due east, its Hour 

Angle (abbreviated HA) will be  6h, meaning it will need 6 hours to arrive at the 

meridian.  Objects in the west have a positive hour angle, indicating the number of hours 

the object is past the meridian.  Gemini displays the hour angle of an object (in minutes, 

together with the current elevation) at the time you select it from one of the databases.  

3.3  Tracking 

One of the jobs of a telescope mount and control system is to move the telescope to 

compensate for this constant change in RA of the sky.  In order to stay pointing at a 

constant RA value, the mount must move at the same rate that objects in the sky appear to 

move.  This is called tracking.  If your mount is polar aligned, then its polar (or RA) axis 

is parallel with the Earth' s axis, and tracking should involve simply driving the mount in 

RA at the same rate the Earth is turning.  While this is a very close approximation for 

stars, there are a few complications that need to be addressed. 

3.3.1  Rates 

Gemini can drive the telescope at several different tracking rates depending on the object 

you are observing and how close your mount is to being polar aligned.  The currently 

active tracking mode is displayed briefly whenever you press the MENU button to enter 

Telescope Control Mode. 

3.3.1.1  Predefined Rates 

The two predefined tracking rates drive the RA axis at a constant speed to compensate for 

the Earth' s rotation.   The sidereal rate assumes that your mount is accurately polar 

aligned. 

Sidereal 

Sidereal rate is simply the rate that compensates for the Earth' s motion relative to the 

celestial sphere.  It keeps the mount pointed at a constant RA and Dec. coordinate in the 

sky and will probably be the rate that you use the vast majority of the time.  With sidereal 

tracking selected, the telescope makes one revolution per sidereal day (86164.0905 

seconds). 

None/Terrestrial 

This rate turns tracking off completely and allows you to observe stationary objects such 

as points on land, or geosynchronous satellites.  It is also useful for keeping the telescope 

from exceeding its safety limits when the Gemini must be left on for extended periods of 

time between observations. 

Gemini Users Manual 

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