Sky Power Blog Post #7 — sun / earth / TIME rarity!

Photo by Geoff CADDICK / AFP

Here’s a climate-related item for you — but not with the usual associated political / economic issues. The connection to climate is perhaps rather whimsical, as a tangent to consideration that the nature of seasons
will depend on geography and climate — and today is a seasonal milestone (if you’re reading this 6/20/21).

Here in the Pacific time zone — and on the central coast of California in particular — something’s happening today that (although I’d have to go back and check, to be sure) is likely unprecedented in our lives:  the time of sunset hereabouts, today, is almost exactly (within 1 minute!) the same as the actual moment of solstice!

(To refresh your memory, solstice is when the sun is directly overhead at some location along the Tropic of Cancer — in this year’s case south of Japan in the western Pacific Ocean somewhere.)

So if you manage to find a clear NW horizon view at sunset time (ideally over the ocean — and IF the marine-layer “high fog” ever clears here today!), you get to see the sun setting at a position along the horizon that’s not only its northernmost (farthest to your right, looking seaward) for the whole year, but even the very northernmost in our lifetimes! 

The difference in that position angle between this year and any other year at summer solstice is actually so slight as to be undetectable by our eye-brain system, but still kind-of fun to at least think about!

HOWEVER, … IF we had much longer lives — with time-scales more like the ~ 41.000-year cycle of the changing tilt of the earth’s axis — then we would be able to easily notice substantial shifts in the horizon positions of solstice sunsets!

The tilt = “obliquity” of the earth’s axis, with respect to the plane of our
orbit around the sun, changes at a “glacial” pace — very appropriate adjective there:  it’s related to the ice ages! — and as it does so there’s this gradual, periodic widening and then narrowing of the range or “fan” of sunset / sunrise positions along their respective horizons. 

By the way, the same-but-symmetric reflected cycle of shifts would also be observed for winter solstices, and for sunrises as well as sunsets at both of these “extreme points” of the year.

Finally (and very briefly), you may wonder why solstice this year falls (for us out here in California, anyway) on the 20th instead of 21st as usual.  The periodic leap years in our calendar are the primary factor playing havoc with such timing “issues” (although 2021 is not a leap year).  More can be said about that, some other time — but you can now at least see a hint of the kinds of “complications” involved.

 … Happy (northern-hemisphere) summer !  ***

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