QUESTION:
By knowing how long it takes for signals to go to different locations 
in space, how can this help with the study of the Solar System?

ANSWER from Steve Wall on January 31, 1997:
That's a really good question.  When you're curious about the way something
works, the first thing to think about is what ways you have of finding out 
about it - what you can see, or smell, or measure that has to do with it.

We use the time it takes for signals to travel in several different ways. 
Most directly, the time tells us how far away objects are.  If the signals 
come from an object (like the light or radio signals from stars, or the 
radio signals from our spacecraft), we also can use the way that signal 
changes to tell us what direction it comes from. We can tell how far away 
something is because we know how fast signals travel and we can use the 
time it takes to tell us the distance.  We can tell the direction by 
listening to the  pitch, or frequency, of the signal to tell if the object 
is coming closer or going farther away.

Once we know how far away things are and where they are, we can start to 
think about how the positions of things could tell us what they are made 
of and where they might have come from.  We have been watching the way 
objects move in the sky for a long time now, and we can even predict 
where they will move in the future most of the time. That's how we know 
when eclipses will happen, when we will see a comet, and even when tides 
will come.  The more of how things move we understand, the more we can 
start to guess what forces make them move, and what has made them become 
the way they are.

Steve Wall