Hanging Free

Forty years ago men and women first saw a picture of the Earth, suspended like a blue marble, hanging freely in space. That picture was taken by an Apollo spacecraft and has since become a symbol of the fragility of the world we hold in trust for future generations.

But men and women knew for thousands of years that they would see the Earth this way. The astronomer Anximander speculated five hundred years before Christ that the Earth hung unsupported in space, and this was his reasoning:

There are stars that are always in the sky. The stars of the Big and Little Dipper and of Cassiopeia are like that. These, and a few near them are called the circumpolar stars. They circle the North Star endlessly, and never rise or set.

Well, reasoned Anaximander, all stars must be like the circumpolar stars. They, too, must circle the North Star. When they set, they are still continuing their circular path, but are now below our horizon. And that could only be true if the Earth were hanging free in space.

That leads us to the equinox just past and the coming short days of fall and winter. The stars close to the North Star are always in the sky. Other stars in the north, but not as close, are in the sky most of the time. Vega, for example, is in the northern sky. It rises at 9 A.M. and sets at 4 A.M. It is above the horizon for 19 hours.

On the other hand, Rigel, in the southern sky, rises about midnight and sets at 11 A.M. It is above the horizon for only 11 hours. (Of course, for both of these stars, much of the time they're above the horizon is during daylight. But they're there, all the same.)

Where is the Sun in all of this? Well, during the year the Sun moves along a path in the sky called the ecliptic. Sometimes it's in the northern sky, like Vega, and the days are long, up to fifteen hours long here in Vermont. But sometimes it's in the southern sky, like Rigel. Then it can be above the horizon for as little as nine hours.

On the day of the equinox, the Sun entered the southern sky. Of course, this apparent motion of the Sun is only a reflection of the Earth's own orbit around it. But it has consequences that are very real. The days get shorter and shorter. Winter is on the way!

(09/29/04)