First a quick geometry lesson:
this is the circumference of a circle
this is the radius
the diameter is two times the radius
you can calculate the circumference from the diameter by multiplying it with pi
lets first create a new comp
then we’ll add a 400 by 400 solid
double click on the circular mask tool to create a circle mask inside the solid
we’ll precompose this just to be able to add some detail inside the circle
this will make it easier for us to see how it is rotating
next we’ll need the diameter of our circle in order to calculate the amount of pixels it should move during a 360 degree rotation
we already know it is 400 but you can also measure it by dragging out two guides and subtracting the smaller value from the bigger one
then we multiply our diameter with pi
finally we can animate our circle one full rotation and for the position we’ll add the calculated pixel amount
let’s also adjust our motion path so that the animation begins off screen