Alan Stahler: A sight for solstice eyes
The sun rises and sets because Earth turns – think BBQ rotisserie.
Realizing it’s Earth, not sun, that is moving, we can still pretend that it’s the sun that’s rising and setting.
Sunrise and sunset find the sun low on the horizon. Post-sunrise, the sun climbs upward, away from the horizon; pre-sunset, the sun drops back down toward the horizon.
Midway between sunrise and sunset, the sun reaches its highest point in the sky. That moment of highest sun is noon … high noon.
Earth does not spin straight up-and-down in our orbit around the sun. If we did, one day would be pretty much like another … there would be no seasons. (Though there would be some differences through the year, as Earth draws closer-to, then farther-from, the sun … more on that next month).
Many folks are surprised when first introduced to Polaris, the North Star … the North Star it’s not especially bright. There’s no reason it should be. By luck of the draw, the North Star is the star that happens to lie (almost) directly above the north pole (there’s no South Star over the south pole).
Earth circles the sun at a jaunty angle, mimicked by the globe on the classroom shelf. Earth leans always toward the North Star. Only occasionally does the sun lie in the same direction as the North Star. So in some parts of our orbit, we find ourselves leaning toward the sun; in other parts of our orbit, we lean away.
Once a year, sun and North Star align dead-on, and we lean as far toward the sun as we can. That happens this Friday: the solstice. With the sun as high in the sky as it can get, the solstice is the longest day of the year, and Earth receives the most sunlight (though it’s not the hottest time of year … more on that next month).
The solstice brings with it an unusual twilight — the time before sunrise, or after sunset, when the sun is below the horizon, but still close enough to the horizon that its light spills over … scattered, by air, into the sky above the horizon.
Imagine crossing the street by the shortest route possible – walking straight, from one side other street to the other. Once across, your path will take you quickly away from the street you’ve just crossed.
That’s the sort of path the sun takes, in rising or setting at the time of the equinox, mid-way between solstices, when day and night are of equal length.
Now imagine crossing the street at an angle, and continuing on that angled trajectory after you’re across. Even after some strides, walking at an angle takes longer to get away from the street you’ve just crossed
That’s the path of the sun rising or setting at the time of the solstice. With the sun remaining close to the horizon, our sky remains twilit longer, too.
One caveat: Viewing tomorrow’s true noontime sun — the sun at its highest — requires a correction factor for daylight saving time. When we pushed our clocks ahead an hour last spring, that also pushed the hour of noon ahead. True noon – high noon – arrives at 1 p.m. DST.
Local astronomers will set up scopes for a late-month SkyWatch, Saturday, June 29, at the junction of the old Downieville highway and Highway 49. We’ll gather at 8 p.m. – even though the near-solstice sky will still be twilit, it gives us a chance to chat about the stars before we turn our eyes to the scopes.
It’s free, and children are encouraged. Bring a sweater.
Al Stahler enjoys sharing nature with friends and neighbors. His science stories can be heard on KVMR-FM (89.5 MHz), and he may be reached at email@example.com.
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