Al Stahler: Sky calendar
It’s been getting dark earlier these days.
It’s also been getting dark faster.
The world globe on teacher’s desk never sits upright. The globe is always catawampus, twenty-three-and-a-half degrees off vertical. Were it not for that jaunty tilt, Earth would be a much less interesting planet.
Earth leans always toward Polaris – the north star – 24/7, 365.
In some parts of our orbit around the sun, the tilt toward Polaris also tilts us toward the sun; we lean most toward the sun at the summer solstice, in June.
On the opposite side of our orbit – while still leaning toward Polaris – we lean away from the sun; we lean away from the sun most at the winter solstice, in December.
Were we not to lean toward or away from the sun – if we had no tilt – Earth would have no seasons. Sure, we’d feel slightly warmer when our orbit brought us a bit closer to the sun, every January … and we’d feel slightly cooler, when our orbit took us a bit farther from the sun, in July. But a slightly warmer, closer-to-the-sun January, slightly cooler, farther-from-the-sun July would be nothing like summer and winter. Without Earth’s tilt, we’d spend our lives forever in spring.
Eternal springtime sounds rather pleasant … but would we really appreciate spring, if it did not follow winter?
Our planet does tilt, and we tilt most toward the sun at the June solstice, bathing us in sunlight, and making days long, nights short.
This past Wednesday, daytime was neither shorter, nor longer, than night. Wednesday marked the autumn equinox (“equal night”), with night as long as day, day as long as night … twelve hours each.
The equinox has an effect, too, on twilight – the time between day and night.
Earth circles the sun in rotisserie mode: We spin, making a complete rotation in twenty-four hours. Sometimes turning us toward the sun, sometimes turning us away, this spin gives us day and night.
Between day and night is twilight, when the sky is neither bright blue nor dark black. At the end of day, twilight provides just enough light to pick up our tools, pick up our toys, and get inside.
Twilight happens after the sun has sunk below the horizon … but not very far below. The sun is still close enough to the horizon for some sunlight to leak over the horizon and light up the sky (if not the ground).
The sun goes down because Earth spins. If we did not spin at a slant – if Earth did not tilt – the sun would head nearly straight toward the horizon, and keep going, below the horizon, every time it set. But because of our tilt, for every “step“ the sun downward, it takes a step sideways. At the solstice, summer and winter, that sideways step is a large one.
Stepping sideways so much, at solstice, the sun, after it’s set, moves only slowly, down away from the horizon, giving sunlight plenty of time to leak up over the horizon, giving us a long twilight … plenty of time to pick up our toys.
But at the time of the equinox, spring and fall, Earth’s tilt is mostly cancelled, and the sun moves through the sky with the smallest of side-steps – nearly straight down.
Once below the horizon, the sun at equinox, continues moving quickly downward, down from the horizon, cutting twilight short.
Next Saturday night, Sept. 25, at 7:30 p.m., local astronomers will gather where the old Downieville Highway meets State Route 49, just outside Nevada City (well before Newton Road), to set up scopes and share the sky with our neighbors. It’s free – bring the kids, and bring a sweater.
Maintaining a space station – crew and equipment – requires a lot of electricity, and thus, a large area of solar panels. Covered with glass, the panels reflect a lot of sunlight. Brighter than any other satellite, a station flyover is unmistakable.
Next Tuesday morning, Sept. 28, at 5:47a.m., the foothills will enjoy such a station flyover, almost directly overhead (get out a couple minutes early – moving 17,000+ miles an hour, the station doesn’t remain long in the sky).
This flyover will not be by the International Space Station, crewed by astronauts from the U.S. and other countries (and for which I’ve many times put out a heads-up). This Tuesday morning, the space station flying over the foothills will be Tiangong (“Heavenly Palace”), now under construction by the People’s Republic of China.
Al Stahler enjoys sharing science and nature with friends and neighbors in The Union and on KVMR-FM. He teaches classes for both kids and grown-ups, and can be reached at firstname.lastname@example.org
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