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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