You can also measure the size of certain astronomical objects with nothing more than a clock. Imagine a variable star with a circular nebula around it. At some point the star hiccups and emits more light than usual. However far away the star is from us (and it doesn't matter for this calculation, nor do we have to know how far it is), the increase in light will arrive at Earth to be observed at some point in time, call it T. Meanwhile, some of the light from the same flash will have left the star traveling perpendicular to our line of sight, eventually reach the ring nebula after an elapsed time of X (i.e., that's the amount of time it took the flash to travel from the star to the nebula), and then the light of the nebula suddenly being illuminated will travel to Earth and become visible to us at time T+X. Voila, we now know that the nebula is X multiplied by the speed of light in radius. QED.
Note that in order to get the correct answer we needed to know *nothing* but the elapsed time between our seeing the star brighten, and the time we saw the nebula brighten. Well, that and the speed of light. We didn't have to know how far they were from us, or how big we thought they were, or anything else.
Also note that now that we know for sure how big across the nebula is, we can directly calculate its distance from us by simple algebra given how big the nebula looks in a given telescope of a known magnification.