# Re: CONCULTURE: dual planets

From: | Roger Mills <rfmilly@...> |

Date: | Thursday, November 18, 2004, 7:15 |

Michael Poxon wrote:
> You can easily work out the apparent size any object would have as seen
> from
> another. The formula is
> S = 2 * arctan (R/d)

(snip explanation)
There's another version of this formula (using kms. not miles), plus
examples and a table of the necessary tangents, at the website I've been
touting:
http://curriculum.calstatela.edu/courses/builders/lessons/les1/moons/see_moon.html
(should all be on one line of course)-- perhaps a little simpler for the
math-challenged and fancy-calculator deprived. It reaches the same answer:
> about 0.5 degrees, which *is* the
> angular diameter of the Moon as seen from Earth.

Isn't there some rule of thumb, whereby you can determine such angles
roughly by holding up fingers at arm's length? Next time there's a full
moon, I'll have to try it. By the way, is that 0.5 degree when the moon is
high up? I assume so, because the just-risen (full) moon near the horizon
seems substantially bigger (atmospheric effect IIRC).
I'm futzing around with Cindu's moons; I want the larger one to have a
viewing angle of around 2 degrees-- it's much bigger than our Moon.