So at lunch we were idly wondering whether the Hubble Telescope could resolve the Mars lander Phoenix.

The resolution of the HST is approximately 0.1 arcseconds.

The time delay to the Mars lander is 15 minutes.

So Google: “0.1 arcseconds * 15 light minutes” and the answer is 130.8 km…hence the lander is not resolvable at that distance (coulda guessed that).

Another one of my favourites is: “1 sidereal day in seconds”.