Cosmic Distances and Geological Time

What was happening here, when the light left there?

One night not long ago a few of us were looking at Abell 1656, a galaxy cluster 422 million light years away, and trying to find a way to understand such an unthinkable distance.

One way of comprehending these enormous distances is to consider what they mean in terms of time. It's hard to picture the 65 million light years between earth & M104, for example, but we know what a dinosaur was, and we can find the 65 million year old Chicxulub crater on a map. Knowing that the KT extinction was taking place when the light we see today left M104 leads one to understand that object (the Sombrero galaxy) in a different way.

One interesting thing to note in the pages that follow is the lack of any astronomical objects between 179,000 and 2.3 million light years away -- once you exhaust what the Milky Way and the Magellanic Clouds have to offer, you need to go quite a long way before you bump into anything else out there.

So just for reference, here are a dozen popular objects, in order of their distances from us, with a brief account of what was going on here when their light began its journey.

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