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The Last Days of Summer

The equinox ushers in fall

Week’s end finds the waning crescent moon in the company of Jupiter before dawn. Around 6am Friday morning, look for the moon high in the east with Jupiter to its lower left. The same time Saturday the moon shines just six degrees from Old Jove. Then Sunday, the now razor-thin crescent is well below Jupiter, while the first-magnitude star  Regulus, is just six degrees away.
    While you should have no trouble spotting the waning crescent moon and Jupiter in the east before dawn, Venus is a trickier target. This Morning Star rises less than an hour before the sun, and that window of visibility shrinks by about a minute each day.  At best Venus is only 10 degrees above the horizon before sunrise, so you may need to scour the eastern skyline with binoculars to pinpoint Venus’ otherwise dazzling glow.
    This time of year before dawn offers the best chance to spot the eerie zodiacal light also called false dawn. You need dark skies away from any urban glare to see the zodiacal light, which glows like milky pyramid of light rising from the horizon an hour or two before actual dawn.
    Unlike true dawn, the zodiacal light is a pale, ghostly glow devoid of the rosy tint from the coming sun, which is caused by light entering earth’s atmosphere. The zodiacal light is actually sunlight reflecting off countless bits of dust and detritus within our solar system that orbit the sun along the same path as the planets. This time of year the ecliptic — the path of the sun, moon and planets — stands nearly straight up with respect to the eastern horizon before dawn.
    At the other end of darkness, Mars and Saturn shine low in the southwest in the early evening. Of the two, Saturn is slightly brighter and is farther west, setting around 9:30pm. Mars isn’t far behind, setting shortly after 10. But while the ringed planet is weeks away from disappearing amid the glare of the sun, Mars remains a fixture in our early evening skies for weeks to come.
    A clear view to the west-southwest immediately following sunset may reveal Mercury burried in the horizon. Binoculars will help, but don’t confuse it with nearby Spica, which is only a couple degrees away through the weekend. They are so close that both will appear in the same field of view using binoculars or a telescope.
    Monday at 10:29pm EDT, the sun is poised directly above the equator somewhere in the vicinity of New Guinea. On that day or the following, the sun rises due east and sets due west, dividing the day between near-equal amounts of daylight and darkness for everyone around the globe. For the 90 percent of us living in the Northern Hemisphere, this equinox marks the beginning of autumn.
    Because of the earth’s 231⁄2-degree tilted axis, the Northern and Southern Hemispheres each receive more direct sunlight and warmth than the other for half the year. Twice a year, earth’s tilted axis and its orbit around the sun come together just so that the amount of light and dark are equal around the globe. Hereafter our time in the sun will grow shorter each day as the sun creeps ever southward of due east until reaching winter solstice in late December, its farthest point south in our skies.