"Jim Wilkins" wrote in :

"Doug Miller" wrote in message
What part of "one and a half million miles a day" are you having
trouble understanding?

If you are chasing another car at 100 MPH on a straight road across
Texas neither the car nor the horizon appears to move.

True, but completely irrelevant, since neither the Earth nor any object that may be on a
collision course with us move in a straight line.

Likewise an
object approaching along the line of Earth's orbital path may not
shift enough to notice against the starfield.

Nonsense, for two reasons.

First, anything close enough to pose any danger of hitting us certainly will exhibit enough
apparent motion against the star field to be readily obvious.

Second, there is no such thing as "an object approaching along the line of Earth's orbital
path": the orbital path is an ellipse, not a straight line. The only way anything can be
*always* in "the line of Earth's orbital path" is if it is in the same exact orbit -- and if it's in the
same orbit, it can't be approaching us, because two objects in the same orbit are
necessarily moving at the same velocity.

Not to mention the fact that we would have noticed another body in the same orbit a long,
long time ago.

http://en.wikipedia.org/wiki/Blink_comparator

You didn't actually read that article, did you?

"In photographs taken a few days apart, rapidly moving objects such as asteroids and
comets would stand out, because they would appear to be jumping back and forth between
two positions, while all the other fixed stars stood still."

Which is exactly my point: only the stars would appear motionless. The Earth's motion
around the sun changes the direction to any nearby object enough that its motion relative to
the background will be immediately obvious after only a few days. We're not in danger of
being hit by a star. The things that might pose a danger -- asteroids and comets -- move
enough in just a few days to make their position and motion obvious.