Hilchos Kiddush HaChodesh 11:12
Therefore, if you need to to subtract 200 degrees, 50 minutes and 40 seconds – abbreviated 200, 50, 40 – from 100 degrees, 20 minutes and 30 seconds – abbreviated 100, 20, 30 – you start by adding 360 to 100, which yields a result of 460. You then subtract the seconds. Since you can’t subtract 40 from 30, you must first convert one of the 20 minutes into 60 seconds. Adding these 60 seconds to our initial 30 gives us 90 seconds. Subtracting 40 from this leaves us with 50 seconds. You must next subtract 50 minutes from the 19, since one of the original 20 minutes was converted into seconds. Since 50 can’t be subtracted from 19, you must convert one degree into 60 minutes. When you add the 60 minutes to the 19, you get 79 minutes. Subtracting 50 from this leaves you with 29 minutes. Finally, subtract the 200 degrees from the 459, since one degree was converted into minutes. This leaves 259 degrees, so the result is 259, 29, 50. All such subtractions are conducted in this manner.
Hilchos Kiddush HaChodesh 11:13
There were seven celestial bodies traditionally used in such calculations; each of these travels in its orbit at a set rate of speed, neither slowing down nor speeding up. The speeds at which they travel today are the same speeds at which they traveled yesterday and will travel tomorrow. This will always be the case. While all these things (appear to) travel around the Earth, Earth is not the center of their orbits.