Almost a year ago, Eris – the, uh, most massive known dwarf planet -- passed directly in front of an otherwise anonymous star, momentarily causing the star to disappear, as seen from the earth. By carefully measuring the length of time that the star disappeared, astronomers made a very precise measurement of the size of Eris. I care about the size of Eris for many different reasons, but the most trivial yet emotional for me is the fact that, 5 years ago, I measured the size of Eris myself. We used a much more difficult and less accurate technique than watching a star disappear and timing it. We looked at Eris with the Hubble Space Telescope and carefully compared the tiny disk that we saw with a picture of a star (which should show no disk at all) and we claimed that we could tell that Eris had a diameter of about 1.3 pixels on the HST camera. Only 1.3 pixels! It’s hard to imagine that you could tell the difference between something 1.3 pixels across and 1.2 pixels. In fact, it had never been done before. Even we were not convinced at first that our technique was as accurate as it appeared to be. So we spent months on a careful analysis to make sure we had done nothing wrong. In the end our measurement technique passed every test we could dream up for it, and we became convinced that it was correct. We wrote the paper to announce it to the world. The diameter of Eris, we claimed, was 2400 km with an uncertainty of 100 km in either direction (I’ll be writing this as 2400±100 km).
