Yesterday was our autumnal equinox (no, I will not surrender my boreal chauvinism to call it “September equinox”). I’ve seen this day from Berkeley’s latitude for decades now, and it never really feels like fall. While the days are getting shorter and the light is slanting in more acutely day by day, it’s the warmest month of the year there, non-fall. The real herald of the seasons in the northern half of California is the arrival of the first substantial rain, and that can happen any time from now through the end of October in what we like to call a “normal” year.
Still, on every equinox, I go through the same exercise in my head of trying to imagine our planet in space, its axis tilted roughly 23 and a half degrees to the plane of our orbit around the sun (I think I have that right). And while I can recite what’s supposed to be happening out there from equinox to solstice to equinox to solstice, I honestly have a hard time wrapping my brain around it (believe me, I have done the kitchen table demonstrations of the axial tilt and how first one pole and hemisphere, then the other are inclined toward the sun (and how the inclination accounts for our terrestrial seasons). And I’ve played those demonstrations out mentally hundreds of times; maybe I have trouble imagining all this happening in 3D or something.
Anyway, it struck me yesterday that it might be amusing to compare how the equinox was defined by, say, Samuel Johnson when he compiled his dictionary in the second half of the 18th century and maybe compare that to some contemporary definition. I looked–by way of Google Books–and I didn’t think it was that amusing. But I found something better: an article on astronomy from the 1771 edition of the Encyclopaedia Britannica. And in the course of the long, long recounting of late 18th century astronomical knowledge–hey, they knew a lot back then–I came upon this: “Chapter VIII. The different Lengths of Days and Nights, and Vicissitudes of Seasons, explained.”
The chapter starts:
“The following experiment will give a plain idea of the diurnal and annual motions of the earth together with the different lengths of days and nights and all the beautiful variety of seasons depending on those motions. Take about seven feet of strong wire and bend it into a circular form, as abcd, which being viewed obliquely appears elliptical, Plate XLI fig. 3. Place a lighted candle on a table and having fixed one end of a silk thread K, to the north pole of a small terrestrial globe H, about three inches diameter, cause another person to hold the wire circle so that it may be parallel to the table and as high as the flame of the candle which hould be in or near the centre. …”
Read the whole thing for yourself, or just as much as you can handle, and let me know how you make out causing another person to hold your stiff wire circle. Just for fun, up above is the plate referred to in the suggested experiment, the description of which goes on and on.