Imagine you wake up in the middle of a flat desert and you see that your pet turtle is gone, but its tracks lead directly away from you and, with binoculars, you see it 1 mile away from you. You happen to know your turtle moves at a constant speed of 1/2 miph. Thus, you now kinow that it left your camp 2 hours earlier.
Once astronomers were able to produce reasonably accurate distances to other galaxes, then their other discovery of how to determine how fast they are traveling away from us allowed them to calculate when we were, essentially, all together at one tiny spot.
It's not quite that simple, but that is the basics to it.
It was the discovery that certain variable stars (Cepheids) produce known values of brightness based on on fast they go from bright to dim. This allowed them to determine the distance to the nearer galaxies. Refinements came when they discovered that a certain type of supernova (Type Ia) was another "standard candle", which extended their distance measurements greatly -- billions of light years.
That, alone, only provides the distance. The rate of travel away from us for these galaxies became known by determining the redshift of light. It is similar to Doppler used on police radar. Wavelengths change when the source of light, or sound, comes from an object moving toward or away from the observer.
Many other techniques combine to confirm the accuracy of their other measurements. The result provides extremely strong arguments for an expanding universe. I think it was around 1995 when they, surprisingly, discovered that the expansion rate is now getting faster and faster, slightly.
Since this is your first post, it is a little hard to tell how fast to throw the balls to you. I hope this helps.
Last edited by George; 2011-May-14 at 04:13 PM.
Reason: 2 gramms
We know time flies, we just can't see its wings.