This is something I have not talked about much. There is a reason.
I needed to derive some geometrical and statistical results to get us this far. Our hardest work is now over. The thrust of this chapter is actually very simple. Since I know how big the planets look in my photos, and I now know how far away they are, I can calculate their sizes.
I also need the stuff in Chap. 4 about converting lengths on photos to angles in arcminutes, which I showed you how to do in (4.1)–(4.7). The only slight difference from this treatment is that when taking planet photos, I used a Barlow lens. You have to multiply the image size you calculate in arcminutes by the magnification of the Barlow. Do remember to calibrate your Barlow: the quoted magnifications are not exact.