Astronomy Calculations Tool I took many images like the following:



As many of you may recognise, this is a picture of Jupiter and its four Galilean moons. Using these images, I am aiming to calculate the mass of Jupiter by finding the orbital distance and orbital period of each of these moons, and then plugging those values into Kepler's Third Law. 

However, this requires a seemingly tedious process of the following sort:

1) Locate the centre of mass of Jupiter

2) Measure the distance to each of the planets

I have a lot of data (many, many pictures). I am hoping that there is a relatively easier (or even more accurate) method of doing all of the above than using software like Gimp to do everything for each image manually. Can anyone recommend anything?
Try WinJupos. It can be used to measure Jupiter (as well as other planets)