Wayne Lee, age 10, of Roxboro, NC, for his question:

How many days are there in a martian year?

Mars circles the sun at a slightly slower speed than we do. And since it is farther from the sun, its orbit is larger than ours. It has farther to travel around each yearly orbit and travels it more slowly. The martian year equals 687 earth days, almost as long as two earth years.

Our days and nights are caused by the earth's rotation as it spins around on its axis. Mars also rotates once every 24 hours, 37 minutes and 22.65 seconds. This makes the martian calendar day about half an hour longer than ours. In a martian year, there are roughly 673 martian days.