Jim Deadman, Age 14, Or Toronto, Ont., Canada, for his question:
Can two full moons occur in the same month?
Our master clock is the sky. We reckon tine from the sun, the moon and the stars as they wheel across the heavens. But these pointers do not keep in step with each other. There are not an even number of months in a year, and there are about 365 days in a year.
Most calendar months have only one full moon. But in 1963, the moon will be full on nov. 1 and again on nov. 30. Every three years or so, we have two full moons in a calendar month, and this has happened only 20 times in the 20th century.
Our calendar months vary in length from 28 to 31 days, and the changing moon does not fit into this system at all. The period of time between one full moon and the next is about 292 days. There are 12 full moons every 354days, and the calendar year is 365 days or 11 days longer than 12 lunar months.
The year is based on the time it takes the earth to travel its 600 million mile orbit around the sun. A day, from midnight to mignight, is the time it takes the earth to rotate once on its axis. But the earth's revolution is not in step with its rotation, and there are not an even number of days in the true year.
We adjust our calendar year to end at midnight, but the earth does not complete its orbit for another six hours. The true year is 365u days long, and every leap year we add an extra day to pick up the lost quarter days. Hut there seems no way in which we can adjust the lunar month to the calendar year. There are always 11, days .lefty over, because there are not an even number of months in the year.
The moon's orbit is almost 1.5 million miles in length. Traveling at 2300 miles an hour, it takes the moon 27 1/3 days to travel this distance, but meantime the earth has moved forward on its orbit. The moon must travel two more days to return to its originalposition in relation to the earth and the sun. This is the lunar month of 292 days, which takes us fran one new moon to the next.
The fast moving sun and moon travel against a background of fixed stars, and what we call sidereal time is measured by the stars. It takes the moon 27 1/3 days to return to its original position in relation to a star, and this is the sidereal month. In a year, there are 13 sidereal months plus 10 days. Even if we used sidereal time, we could not fit the months neatly into the calendar year.