Nice online and downloadable tools

[RADAS]

While looking for an animation of an eclipsing binary I stumbled over a US website that has some nice simulation tools that show some of the basic concepts in astronomy.  It's from the University of Nebraska and you can download them as well.

http://astro.unl.edu/ is the homepage, the star simulator [which seems to default to Algol, which is what I was looking for] is here http://astro.unl.edu/naap/ebs/ebs.html or can be found under the NAAP tab.

[impactcrater]

very good...it is often hard to understand all the variables and make sense of what is where and why....