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Backyard Astronomy

The night sky in autumn

Quamrul Haider, pii.d
TO astronomy buffs, nothing could be more exhilarating than gazing at the night sky in the autumn months. So let us immerse ourselves in the wonder of the Universe by taking a fantasy trip to a vast open space in a mid-northern latitude with an unobstructed view of the sky and no light pollution.
Assume that it is early evening (8:00 p.m.) in late September or early October and the sky is cloudless with the moon in the waning crescent phase. As we wait for the sky to darken, we look in the twilight for three planets - Mercury, Venus and Jupiter, low in the western sky. Mercury is always hard to see. Venus and Jupiter can be seen as bright lights in the southwestern sky. Venus is so bright in the evening that sometimes we mistakenly think of it as an UFO.
As time elapses, the sky gets darker. Moon is not yet up; it will adorn the eastern sky at early dawn. At first only brightest stars are visible, but soon we can see hundreds of stars. Straight overhead is one of the brightest stars in the sky, Vega in the constellation Lyra.
As the sky becomes darker, we will see a pale band of light which rises from the horizon in the southwest and pass almost overhead before it sinks to the eastern horizon. This is our galaxy, the Milky Way. Saturn can now be seen with binoculars very low in the west, far to the right ofVenus.
Now turn toward the north. We will see the seven brightest stars of the constellation Ursa Major or Great Bear. It is the Big Dipper. The handle of the Dipper is the Great Bear's tail and the Dipper's bowl is the Bear's flank. Now use the Big Dipper to find the North Star. To do this, first find the two stars - Merak and Dubhe which are on the end of the bowl farthest from the handle. These are often called "pointer stars.” Start from the bottom of the bowl and extend an imaginary line upward through the top star and continue 5 times the spacing between the pointer stars. You will arrive at Polaris, the North Star.
Look at Venus and Jupiter again. They are now closer to the horizon and Jupiter is sinking toward the western horizon. Now fast forward to night sky at 2:00 a.m. Jupiter and Venus have set
hours earlier. Vega, which was overhead, is now about two-third of the way to the western horizon. Orion and its brilliant retinue climb over the horizon directly in the east. Right above Orion are the Pleiades, a small cluster of stars known as seven-sisters.
Turn north again and you will find Polaris 40 degrees above the horizon and directly north. Big Dipper is directly below Polaris and is lying along the northern horizon. The Big Dipper isn't going to set at all. It is as low in the sky as it gets and is beginning to climb again. Its motion is counterclockwise on a circle that is centered near Polaris. Any star or constellation closer to Polaris than the Big Dipper also moves in circular path around Polaris. These stars are called circumpolar stars. They never dive below the horizon but trace complete circles above it. The center of these circles marks the celestial pole.
The altitude of the north celestial pole is the same angle as the observer's latitude. The more northerly the observer is, the
higher is the north celestial pole. At the North Pole, it is at the zenith. Polaris lies close to the north celestial pole. That is why it is called the North Pole star.
In addition to planets, countless stars and constellations, the night skies of September are decorated enticingly with many more celestial entities. Very soon the Sun will rise, the sky will light up and the stars will disappear from the heavens.
These are just a few things a person with no previous knowledge of astronomy should look out for in the autumn sky. Astronomy is much more fun when we are amateur astronomers. So go outdoors, explore the wonders of the autumn night sky, and gear up for a good winter ahead. As Plato said, "As tronomy compels the soul to look upwards and leads us from this world to another.”
The writer is Professor, Department of Physics & Engineering Physics, Fordham University, New York.