At one time, astronomers thought that Mercury might rotate with the same period as it revolves around ___________________. During the 1960s,
however, astronomers used __________________ to determine that Mercury takes __________________ to rotate. The period of rotation is _______________________ of the period of revolution, which turns out to be a stable ratio. Even more strange, the time it takes the Sun to return to the same position in Mercury's sky, turns out to be 176 days, which is equal to two Mercury _______________________
Draggable Items
1/2
2/3
59 days
365 days
laser reflectors
radar measurements
the Sun
Venus
weeks
years