12/01/1998 08:00:00

Caltech Question of the Month: When a plane flies from New York to San Francisco, why can't it just idle in midair and wait for the earth to spin San Francisco around underneath it?

Submitted by Norman Arce, San Marino.

Answered by Dr. Andrew Ingersoll, Professor of Planetary Science, Caltech.

We don't feel it, but the planet is rotating eastward at a rate of about 1,000 miles per hour. Thus, it might make sense that you could rise in the air, stay in one spot, and wait for the West Coast to rotate underneath you in about three hours.

But practically speaking, this is what supersonic jet planes already do. If you were in a plane, you'd still have to contend with the winds that would be hitting you in the face at 1,000 miles per hour. This is true because friction causes the atmosphere to be dragged around by the solid surface of the planet. Bucking that sort of headwind is difficult, which is why you need the Concorde to do it.

But even if you got above the atmosphere, you'd still be carried eastward at 1,000 miles per hour by your own inertia. In effect, this would be just like jumping straight up while inside a moving train. If you've ever tried this, you know that you land in the same spot rather than several feet rearward. Thus, to go westward, you'd still have to fire your rockets to undo the eastward motion.

So there's no easy way to do it. Either way you have to burn a lot of jet fuel or rocket fuel to get there.