Patient astronomers

People are rather ill-designed for astronomy.

It struck our astronomer recently that the normal human perception of time is not very well-matched to his science.  We cannot actually see even such a common thing as the rotation of the Earth, at least without a telescope to magnify it for us.  The best we can do with the naked eye is to mark a direction somehow, and then later notice that a star has moved; or wait until something marks a direction for us, as at sunrise or sunset.  The motions of the planets among the stars are slower still, requiring another exercise in record-keeping (after the much more involved effort of mapping the stars themselves).  It takes more than a working lifetime to follow Saturn around in its orbit twice, or Uranus once; Neptune only recently completed its first orbit since discovery.

The slightly outdated term “fixed stars” tells us that any changes in them are even harder to detect, requiring even more patience.  The fact that their coordinates do change over time was discovered only by comparing one astronomer’s measurements to those from previous centuries.  After the invention of the telescope, many stars were found to be double, orbiting each other (which was a triumph for Newtonian gravity, showing that the laws of physics deduced from Earth apply elsewhere).  But those orbits that can be observed are generally very long, decades at least, and one astronomer’s measurements may only go to help another scientist work out the orbit much later.  In reference to one of these doubles, Sir John Herschel wrote (in 1881), “Thirty of forty years of observation perseveringly directed to the object in view, could not fail to settle the question.”  (Unfortunately, no current source of funding for astronomers would contemplate a project that would only return results after three or four decades.)

Going beyond nineteenth-century astronomy into astrophysics, it gets worse.  The shortest-lived stars hang around for millions of years, longer than our species has existed.  We simply haven’t the time to watch a star form or evolve, or a galaxy respond to a close encounter with another.

We have made some progress since Herschel wrote.  Better telescopes and instruments would reduce the time required for his project considerably, and there is the continual prospect of more improvement.  We can detect the motions of nearby galaxies now, and of stars throughout our own.  And of course there are things in the planets and stars that change on human time-scales, these becoming ever more detailed as our tools improve.

But for the most part we have to use a proxy for time.  We look at similar stars of different ages to work out how they evolve.  And in general we survey the sky to find examples of the same thing at different stages in order to piece together the story.  We assemble the movie from a series of single frames.  Short-lived as we are, astronomers must work out the history of the universe from a snapshot.