msb@lsuc.UUCP (Mark Brader) (12/29/85)
Rick McGeer (mcgeer@JI) writes: > It will have a very low angular velocity, so much that it will be extremely > difficult to pick out from among the fixed stars. Assume Bode's Law holds > (...it's the only assumption I have...) ... its period [will be about] > 678 years, which works out to an angular velocity of about 32 minutes of arc > (about half a degree) per year. Pluto, by contrast, has about a degree and a > half of arc per year, or about three times as much -- and it took years of > Tombaugh's time on a flicker machine to spot Pluto. Like Don Lynn, whose article did a nice job of answering all the other points that have been raised, I reference the book "Out of the Darkness", written in 1978 by Clyde Tombaugh and (for the historical matter) Patrick Moore. The truth is that it took only a few months for Pluto to be detected on the Blink-Comparator. The computation of 32 minutes of arc per year is irrelevant. The trick is to use the motion of the Earth: instead of detecting the planet's orbital motion, you detect its parallax, which is much larger. For simplicity assume a direct circular orbit coplanar with ours, with 81 AU radius. Then at opposition, Sun, Earth, and planet will be in a straight line, and the Earth-planet distance will be 80 AU. Figure the Earth's orbital angular velocity around the sun as 1 degree per day. Then Earth will be moving with respect to the planet at 1/80 degree per day, or 3/4 minute of arc per day. Note, PER DAY. The planet's net apparent motion will be the difference of this and its much smaller orbital motion. What Tombaugh did, under the direction* of Slipher and Slipher of the Lowell Observatory, was to photograph the sections of the sky that were near to being directly opposite the sun, and to compare (with the Blink- Comparator) pictures taken, usually, 2 days apart. In the case of the actual discovery the pictures were taken 6 days apart because of weather or something, and the two blinking images of Pluto were far enough apart that he had to search (briefly) to find the second one after finding the first. So it should be clear that the slow motion is no obstacle to a Blink- Comparator search. Even if it was, the searcher could simply use plates with a long time interval between them. The dimness, of course, is another matter. Tombaugh said that he could have detected an object 2-3 magnitudes fainter than Pluto, but his eyesight was apparently exceptional. *Tombaugh had no university education at the time -- he was hired as an assistant because he was felt to have potential. Correctly! Mark Brader Usenet readers will see this in net.astro, the proper group by their standards, and net.space. The three earlier articles are in net.space alone, because they originated from the ARPA side, and there's no ARPA gateway to net.astro.