I am at the moment in the middle of the classic "determining the mass of jupiter by the motion of the moons" experiment. I have, however ground to a halt at a certain point, and am unsure as how to proceed. I have a series of photographs of the moons. We have a beautiful 14" in our University obs, but this gives too small a view, so we use a 500mm camera lens on the mount. Seems like a waste to me.. So as you can imagine, Jupiter appears pretty small on these images. The next difficulty comes with measuring the distances moved by the moons. Some pictures appear to have different scales. They were not all taken by myself, they were taken by lots of other people over a period of about a month. This means I have no way of relating the distance travelled in one picture to that of a different scale in another. The only almost suitable method is to compare the distance of the moon to the diameter of Jupiter. But the small size of the pictures means that if I use a picture viewer, I can never measure to the middle of the moon, only to the edge of a pixel either side. We've been given a spreadsheet with a SOLVER routine to output more accurate measures of a and P which would be needed for the graph which gives the mass, but until all the images are at the same scale, this is pretty useless. Also, the routine calculates assuming you used millimetres in the measurement, which is pretty silly! I have about five or six data points for each moon. If anyone could impart any words of wisdom to help me with this, it would calm me down an awful lot, It's due in in three days!