3.3.1.2  Computed Tracking Rates 
In addition to the Earth' s rotation, the computed tracking rates take many additional 
factors into account such as the Earth' s orbit around the Sun, the Moon' s orbit around the 
Earth, and an object' s position relative to the horizon.  
Lunar and Solar 
Neither the lunar nor solar tracking rates are constant.  Since the orbits of Earth and 
Moon are slightly elliptical, the apparent speed of Moon and Sun will change.  When you 
select the lunar or solar tracking rates, the tracking speed will be calculated based on the 
actual position and the position one hour later. 
Because the Moon orbits the Earth, its RA and Dec change during the course of the night.  
The lunar tracking rate compensates for the RA component of that change; there is no 
compensation for the changes in Dec.  While sidereal rate can certainly be used for visual 
lunar observing, the lunar rate will do a better job keeping the Moon centered in the 
telescope' s field of view. Since the Moon is relatively near the Earth, even the location 
on Earth where you are observing must be taken into account.  The lunar tracking rate 
therefore takes the parallax into account by calculating topocentric instead of geocentric 
positions.  
Gemini computes the tracking rates by calculating the current position of the Sun or 
Moon and the position one hour later.  While the tracking rate for the (distant) Sun will 
not change much during an observing session, the tracking rate for the Moon might vary 
significantly as its parallax changes moving from the horizon to higher elevations and 
vice versa.  You can reselect the lunar tracking rate periodically throughout the observing 
session to allow Gemini to recalculate the correct rate as needed. 
Because the Earth is orbiting the Sun, the Sun' s RA and Dec. do change during the 
course of the day, but much more slowly than the Moon.  The solar tracking rate again 
compensates for the RA component of that change, but not for the Dec.  Since the 
difference between solar and sidereal rate is only about 4 minutes a day, you will see 
little difference between the two over a short period of time. 
Adaptive King 
In the absence of the Earth' s atmosphere, sidereal rate tracking would be all that is 
needed to accurately track objects with fixed RA and Dec. coordinates (any object 
outside the solar system).  However, because the Earth' s atmosphere refracts (bends) 
light, it complicates the situation.  The light from an object close to the horizon must pass 
through more atmosphere than an object higher in the sky.  It is therefore subject to more 
refraction, causing the object to appear to be slightly higher in the sky than it actually is.  
This is called refraction.   As the object climbs higher in the sky, it is subject to less and 
less refraction, and its image catches up to where it actually should be.  This means that 
objects appear to move at a rate slightly slower than sidereal rate when they are close to 
Gemini Users Manual 
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