Equinox offers skywatchers a lunar show
Earth’s northern hemisphere tilts toward the sun in summer. High in the sky, the sun rises early, sets late, and gives more hours of daylight than night.
Streaming down from the sun high above, summer sunlight hits the ground almost straight on, rather than at a slant, as it does in the winter. Long days of direct sunlight warm the ground, and the ground warms the air.
The sun was highest, its rays most direct, and the days the longest in the middle of June. Since then, the sun’s been getting lower, its rays hitting us at more of a slant. The days have been growing shorter.
The sky is a calendar. We can measure the time of year by noting where and when the constellations appear.
The constellation Pisces now rises in the middle of the night. At 9:55 p.m. next Monday night, a line from the center of the sun through the center of the Earth, extended out into space, will run through Pisces. On that day, the hours of darkness will equal the hours of daylight. Earth will have arrived at the autumnal equinox (from Latin for “equal night”).
At sunset this Saturday, skywatchers will see the full moon rise in the east. The rising moon will likely be yellow, perhaps even orange, and will loom very large.
As white light passes through a diamond, the stone bends different colors by different amounts, causing the diamond to sparkle with “fire.”
Air does the same thing. When bright stars lie close to the horizon, they twinkle with every color of the rainbow.
Sunlight is similarly dispersed by air. Blue light is especially bent, scattered in every direction, giving color to the whole sky. The effect is exaggerated when the sun is close to the horizon; with most of the blue (and green and yellow) scattered away, the setting sun looks red.
The rising moon also looks red, especially at this dry time of year, with no rain to wash the light-scattering dust and dirt and smoke out of the air. But the moon can look red any time of the year.
The rising moon looks large; try this experiment: Photograph the full moon as it appears, large and low, over the horizon; make another photograph of the moon, now higher and smaller, a few hours later. When the photographs are returned, hold one over the other.
The moon is exactly the same size in both shots. Its large size near the horizon is an optical illusion.
And the illusion can happen any time of the year.
For millennia before the advent of electric light, the full moon, rising at sunset, has illuminated the fields of farmers, working against time to bring in the harvest before the rains. And for several nights after the moon was full, the nearly full moon would again shine a significant amount of light to Earth.
The moon rises later from one night to the next, however, forcing field workers to suspend their work until they could see again. Depending on the moon’s phase and the time of year, the delay in moonrise could be as little as 20 minutes, or it could be well over an hour.
Thanks to a fortunate geometry among Earth, sun and the full moon, the delay of the almost-full moonrise from one night to the next is at a minimum at the time of the autumn equinox. By rising early, night after night, the succession of full and almost-full moons allowed our ancestors to harvest the food that would sustain them through the winter, and fully deserves its appellation: the “Harvest Moon.”
There will be a benefit for the Nevada County Imaginarium and the Sierra Friends Center, including a talk and space images, Friday, Sept. 27, at 7:30 p.m.
Alan Stahler is a trained biologist and amateur astronomer.
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