Mary Ann Recktenwald, age 14, of Louisville, Kentucky, for her question:

How fast does the moon orbit the earth?

The moon's average orbital speed is slightly more than half a mile a second. This is faster than 2,000 miles per hour, or more than 50,000 miles while the earth rotates through a day and night period. The moon's orbital speed varies for several reasons. Its path around the earth is not a perfect circle. Its orbital speed is slightly faster when it comes closer to the earth and slightly slower when we are farther apart.

The moon completes its orbital distance around the earth in about 27 1/3 days. But during this time, the earth has moved along almost 1/12 of its orbit around the sun. So the moon has farther to go to complete a circle around the earth. This is why it lags behind and rises 50 minutes later each night. The time lag adds up to the lunar month of about 29 1/2 earth days.