Measuring distances in ds9
When clicking around on ds9 images, you may occasionally leave behind a green circle; this is a "region", and they are ultimately very helpful, but when learning things, they can be very annoying. To make accidental regions go away, pick the region, and hit backspace or delete on your keyboard or from the top regions menu.
Very commonly, you need to be able to measure distances. Measuring distances in ds9 is basically creating a special 'region' that is a ruler, so you may find it clunky. From the menus on the top, select Region/Shape/Ruler. Click on one end of what you want to measure, then move to the other end and click again (or click-and-drag; you may need to experiment to see what your system wants). A line with arrows will be drawn connecting the two, along with the distance in text and dotted lines completing the triangle. By default, the distance will be in physical units (pixels of the image you are viewing), but by accessing the region's Get Information panel (top menu: Region/Get information; buttons in the middle of the ds9 screen: Region/Information), you can change both the endpoints and (more usefully) distance units to WCS so that the units will be in degrees, or minutes, or seconds.