Then, just for a minute … he turned off the lights…. And then while we all still waited I understood that the terror of my dream was not about losing just vision, but the whole of myself, whatever that was. What you lose in blindness is the space around you, the place where you are, and without that you might not exist. You could be nowhere at all.– Barbara Kingsolver, Animal Dreams, 1990
Where is Mars? The center of our Galaxy? The brightest X-ray source? Where, indeed, are we? Astronomers have always needed to locate objects and events in space. As our science evolves, it demands ever more exact locations. For example, an astronomer discovers, with an X-ray telescope, a source that flashes on and off with a curious rhythm. Is this source a planet, a star, or the core of a galaxy? The exact position for the X-ray source might be the only way to identify its otherwise unremarkable optical or radio counterpart. Astronomers need to know where things are.
Likewise, knowing when something happens is often as important as where it happens. The rhythms of the spinning and orbiting Earth and Moon gave astronomy an early and intimate connection to timekeeping. Because our universe has a history, astronomers need to know what happened when and what time it is now.
The “fixed stars” are an old metaphor for the unchanging and eternal, but positions of real celestial objects do change. Planets, stars, gas clouds, and galaxies all trace paths decreed for them. Astronomers who measure these motions, sometimes only through the accumulated labors of many generations, can sometimes find in their measurements the outlines of nature's decree. In the most satisfying cases, motions reveal fundamental facts, like the age of the universe or the presence of planets orbiting other suns. Astronomers need to know how things move.
Astronomical coordinate systems
Any problem of geometry can easily be reduced to such terms that acknowledge of the lengths of certain straight lines is sufficient for its construction.– René Descartes, La Geometrie, Book I, 1637
Descartes’ brilliant application of coordinate systems to solve geometric problems has direct relevance to astrometry, the business of locating astronomical objects. Astrometry has venerably ancient origins, and it retains a central importance in astronomy.