Q: What is the Moon's angular speed through the sky? Express your answer in arcseconds per second. The moon completes one circuit of the sky in about 29 days. Thus, it moves at an angular speed of 360 degrees angular speed = -------------- 29 days (360 degrees) * (3600 arcsec/degree) = ------------------------------------ (29 days) * (86,400 seconds/day) = 0.52 arcsec per second Q: How long would it take for the Moon's limb to cover a star of angular diameter 100 mas? 100 milliarcseconds time to cover star = ----------------------- 0.52 arcsec per second 0.100 arcseconds = ----------------------- 0.52 arcsec per second = 0.19 seconds