The Earth turns on its axis, once a day. Objects on the surface of the
Earth, therefore, are moving eastward at a speed which depends on their
latitude. At the north or south poles, an object at rest on the surface
is not moving, while an object on the equator is moving with a speed of
465 meters/second eastward. To go into orbit, spacecraft are nearly
always launched going eastward, to take advantage of the fact that they
are already going in this direction (some spacecraft take a different
path and go over the poles -- it takes more fuel to get into this
orbit, however).
To escape the Earth's gravity entirely, the spacecraft needs to be
going at the escape velocity in a non-rotating frame. Launching the
spacecraft going eastward again increases the speed of the spacecraft
with no additional expense in fuel. Launching westward takes more fuel
to escape.
The speed at which something's going while on the spinning Earth
is that 465 meters per second times the cosine of the latitude
(latitude=0 at the equator, and +- 90 degrees at the poles). That way
you can pick up more speed for free by launching closer to the equator.
But the cosine function doesn't change rapidly near zero latitude, so
you have some freedom to choose a good launch site.
Tom J.
(published on 10/22/2007)
