Why write another astronomy program? Here’s Dan’s answer.
To be useful to most of my students, a simulation program has to be (a) free; (b) delivered through a web browser, with nothing to download or install; (c) easy for beginners to understand; and (d) convenient for showing the motions of the stars and other objects with respect to earth’s horizon.
It’s a lot of fun to play with, and I like how Dan notes that his UI was partially inspired by H.A. Rey’s stargazing books, of which I too was, and remain, a loyal fan.