Poetic and precise

How do you describe a color?

Our astronomer continues to read the books of nineteenth-century astronomers, those who flourished back in the days when all observations were made with the human eye and recorded by pen or pencil.  They are interesting in part because they describe the sky very much as it appears today when looking through an amateur-sized telescope.  What attracts our astronomer is that the observers had no astrophysics to tell them what was improbable, impossible or to be expected, so they recorded just what they saw.  Sometimes the description is inaccurate or the observer was in some way misled.  But he thinks they are well worth study.

Especially, he says, their records of the colors of stars.  Of course the stars have colors.  If you take a little time to look at them carefully from a reasonably dark place, you’ll notice that some of the bright ones are orange or reddish, some white, and some definitely have a bluish tinge.  (The fainter ones all look white, because our color vision needs a certain level of brightness before it can function.)  If you ask an astronomer, “What color is that star?” she is likely to answer with a number: “That one’s pretty red, with a B-V of almost 2.”  What she means is that the star’s brightness was measured with two filters, one passing blue light and one green, and the result turned into a number on a specific scale going from blue to red.

But suppose you had neither the measuring equipment nor the calibrated filters to produce such a number.  As important, suppose you also did not know that a single number along the blue-to-red line would be useful.  After all, any painter could tell you that it takes at least three primary colors to have a hope of replicating the colors you see in nature, so you would expect to need at least two or three numbers.  So if you describe the components of a triple star as “pale white, violet and dusky” and distinguish two others because one is “golden yellow” and the other “bright yellow” you are not trying to be so much poetic as precise.  And very possibly the difference in shade of yellow could be important.

If we could trust it.  A certain double star was described by one observer as “yellowish and bluish,” while another said the second star was “certainly not bluish,” and a third said it was “sometimes yellow, sometimes ashy;” finally the first observer concluded the second star was “sometimes ruddy, sometimes bluish.”  The discordance among experienced observers (together with what we know about the variable response of the eye) makes modern astronomers discount any but the most obvious visual color observations as unreliable.  There seems hardly any point in trying to distinguish between smalt blue, cerulean blue and sky blue.

Our astronomer, however, is not yet willing to consign all these descriptions to the poets.  He thinks that maybe, with enough of the old descriptions and modern data, he can disentangle the effects of telescope, observer, atmosphere and all, and pull out useful information from these poetic terms.  He won’t know for sure until after he’s tried it, but that’s what science is all about.