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Arc to Arcturus and spike to Spica

Tonight – August 31, 2017 – or any night in the coming weeks, you can use the very bright planet Jupiter to guide your eye to the star Spica in the constellation Virgo. And, in any year, you can rely on the Big Dipper to star-hop to Spica and to the bright star Arcturus. Just follow the arc to Arcturus, and drive a spike to Spica.

Every year, as late summer and autumn comes to the Northern Hemisphere, the Big Dipper is found in the northwest sky at nightfall. From the Northern Hemisphere tropics, it sits quite low in the sky as darkness falls, and then swings beneath the horizon at relatively early evening. But, from middle-to-far northern latitudes, the Big Dipper is easy to see.

Meanwhile, the Big Dipper isn’t visible from far southerly latitudes in the Southern Hemisphere at nightfall, because it’s below the horizon from that part of the world.

By late night, the Big Dipper will reach lower transit – its lowest point for the night – in the northern sky. At lower transit, the Big Dipper plunges below the horizon and out of sight in the southern half of the mainland United States. But north of of 41o north latitude (the latitude of New York City) the Big Dipper is actually circumpolar – out all night long.

Right now, you don’t need the Big Dipper to find Spica at nightfall and early evening. Just look westward as darkness falls and you can’t miss the dazzling planet Jupiter – the brightest starlike object in the evening sky. That nearby star is Spica, the constellation Virgo’s sole 1st-magnitude star. Look for Jupiter and Spica to shine quite close together on the sky’s dome and to follow the sun beneath the horizon fairly early this evening.

Although the Big Dipper may not be visible from the Southern Hemisphere, it’ll actually be easier to view Jupiter and Spica from southerly latitudes. That’s because Jupiter and Spica stay out longer after sunset than they do in the Northern Hemisphere.

Arcturus should be fairly easy to view from the Southern Hemisphere as well. From anywhere worldwide, Arcturus is the brightest star to the north (or right) of Jupiter and Spica at nightfall. At and near the equator (0o latitude), Arcturus sets nearly an hour after Jupiter and Spica do. At the vicinity of the tropic of Capricorn (23.5o south latitude), Arcturus sets at about the same time that Jupiter and Spica set. At temperate latitudes in the Southern Hemisphere, Arcturus sets before Jupiter and Spica.

Click here to find out when the sun, Jupiter, Spica and Arcturus set in your sky.

Bottom line: At this time of year, the Big Dipper is not visible from the Southern Hemisphere at nightfall. But as darkness falls in the Northern Hemisphere, you can use the Big Dipper to arc to Arcturus and to spike Spica.

Bruce McClure