Newbie to AE: How to make objects remain hidden and then appear

[cisco de leon]

Hi, i need help doing something. I'm trying to learn AE on my own and I'm trying to figure out how to do a specific animation in where my graphic will remain hidden on the background but then appear once it goes into a specific object on the screen.

Here is an example:

So i'm trying to do an ET animation for fun, i want the ET graphic to be completely invisible on this back ground but then slowly appear when it moves over the moon. I'm assuming masks will do it but i'm too much of a noob to know how.

Can someone help me? Thanks in advanced.

[Mylenium · Accepted solution]

Apply a duplicate of the "moon" as a track matte to the ET layer. Read the help based on the clues provided or look up a tutorial on the technique.

Mylenium

[cisco de leon]

thank you