Alan Stahler: The sky is changing!
September 12, 2018
The days are getting shorter, the sun is rising later, it's getting a lot easier to get up before dawn.
Looking up at the stars after sunset, we're looking back — back in the direction from which we came, as our planet circles the sun.
Observing the stars before dawn, we're looking ahead, in the direction Earth is moving.
After sunset, we see the constellations of the season past — tonight, we'll see the constellations of early summer. Before dawn, we see the stars of the season ahead. Looking south before dawn, tomorrow, we'll see Orion, not just above the horizon, but above the trees.
Orion, with his belt of three stars, is the signature constellation of winter.
Not only are the days growing shorter, but so is twilight, the time between sunset and the dark of night. Days will continue growing shorter, up to the winter solstice, but twilight is shortest at the equinox (Sept. 22 this year).
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Venus and Mars, Jupiter and Saturn could not care less about seasons on Earth. It's only by luck of the draw that these four planets shine brightly in our sky tonight.
Saturn, whose rings are visible in our SkyWatch scopes, will rest to the right of the moon on the night of Monday, Sept. 17.
Mars, one or both of whose polar caps may be visible in our scopes, will lie beneath the moon on the night of Wednesday, Sept. 19.
Jupiter has dozens of moons, four of them bright enough to see in our scopes. Bright enough, too, to be seen by Galileo's primitive telescope in 1610. They were obviously orbiting Jupiter, which earned Galileo a summons to appear before the Inquisition, for the sin of announcing that Earth was not the center of the universe.
And then there's Venus, intensely, unmistakably bright in the western sky, just after sunset. Venus mimics the moon (as Galileo discovered): Venus goes through phases, fully-lit to crescent to invisible, then back to crescent and fully-lit.
Crescent Venus will be visible in our scopes.
As the Venusian crescent narrows to invisibility, Venus reflects less light towards us, and should, by rights, grow dimmer. But even as it grows more narrow, Venus draws closer to us, allowing us to capture more of the light it reflects our way
As its crescent grows thinner, but Venus draws closer, there comes a cross-over point, when thinness (and dimming) overwhelm closeness (and brightening). That point comes on the night of Sept. 24, when Venus will shine at its brightest.
Local astronomers will set up scopes for a SkyWatch, at 7 p.m. Saturday. It's free, and kids are welcome.
Note that the constellations of summer are setting, and the constellations of autumn and winter are rising — dress warmly.
Alan Stahler enjoys sharing his love of nature with students of all ages. His science stories can be heard on KVMR-FM (89.5 MHz), and he may be reached at firstname.lastname@example.org.
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