Assign a Number to it, said Lord Kelvin

“When you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and  unsatisfactory kind:  it may be the beginning of knowledge, but you have scarcely in your thoughts, advanced to the stage of science, whatever the matter may be.” – Sir William Thomson, FRS, FRAS, etc., “Electrical Units of Measurement,” May 3, 1883 in Popular Lectures and Addresses, MacMillan, London, 1893.

Oddly enough, for all of the numbers in the hobby of astronomy, we observers report very few of our own. We have the Bortle scale of relative darkness. And of course, stellar magnitudes are commonly referred to, mostly in the context of what is the dimmest you can see tonight? We know that for eta Cassiopeiae, the main star is +3.44 (apparent) and +3.45 absolute and that the companion is +7.51 because that is what is in the references, the same as their classes: G0 for eta Cass A and K7 for eta Cass B. But we do not measure them and report those facts ourselves. Someone might take a picture of them. No one subjects them to a spectroscopic investigation. I, too, am remiss: I only have noted visual impressions and subjective evaluations.

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Measuring Your Universe: Alan Hirshfeld's Astronomy Activity Manual 

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