Using shapes as a mask

[Indie767]

I have a NASA video of Earth against a black background. I want to replace the background with a star field. I thought the eclipse I drew around Earth would act as a mask but I cannot get it to work. Is there a tutorial someone can direct me to?  

[Mylenium]

Well, start by actually reading the online help. the rest is not clear. Why shouldn't a mask not work beyond the fact that your Earth potentially isn't perfectly round? A screenshot may help to explain things.

Mylenium

[Indie767]

The Earth is perfectly round. I finally pulled off the shot after I got the hang of elipse tool and how to re-size it.