Kathy Stewart, age 12, of Winston Salem, N.C., for her question:
HOW FAR DOES THE EARTH TRAVEL FROM JULY TO DECEMBER?
In six months, the earth has time to travel half its yearly orbit around the sun. Its average orbital speed is estimated to be 18 1/2 miles per second and naturally we go along on the trip. But this is not the whole story. Far from it. Meantime our whirling world spins on its axis and also travels with the sun on a star trek through the galaxy.
The 12 months fit into the calendar year, and the year is based on the time it takes the earth to complete one orbit around the sun. For those of us who yearn to travel, it's nice to know that the orbiting earth whizzes us around the sun at an average speed of 66,600 m.p.h.
This means that each 24 hour day we travel an orbital distance of about 1,598,400 miles around the sun. The total distance around the earth's orbit is about 595 million miles. This is the distance we travel in one year, so naturally we would expect to travel half this distance between July 1 and Dec. 31. However, things do not work out quite this way.
As we know, the months are uneven. Some have 30 days, some have 31 and one has 28, plus an extra on leap years. When we count them up, we find that the second half of the calendar year has three more days than the first half, and two more on leap years. Since we travel more than one and a half million miles every 24 hour day, we have time to go several million miles farther during the second half of the year.
Meantime our dizzy old planet also spins on its axis, making about one complete turn every 24 hours. During the yearly orbit, we make 365 and one quarter rotation turns. Meantime our solar system is embarked on a most fantastic spin through the Milky Way, on this part of the heavenly hoedown, the sun, the planets and their moons travel together in neat formation. The average speed is estimated to be 43,000 m.p.h. or more than a million miles during a 24 hour day.
It takes us about 200 million years to complete one of these circuits through the starry galaxy. So during an ordinary stay at home day we spin around with the earth, travel more than one and a half million miles around its orbit, plus more than a million miles through the galaxy. From July 1 to Dec. 31, we repeat this fantastic mileage 184 times.