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Venus dominates sky in September

By Mike Shanahan

Bishop Museum

LAST UPDATED: 9:08 p.m. HST, Aug 5, 2011

I'm happy to be back writing the monthly "Skywatch" column, which Bishop Museum started producing back in the 1980s. We'll run this article in the Star-Advertiser on the last Sunday of each month, and will provide night sky information and a star map for the upcoming month. The focus of these articles will be on things that you can see in the skies above the Hawaiian Islands without a telescope or binoculars. Our star maps are good for anywhere in the main Hawaiian Islands.


Venus has been blazing away in the west all summer and will continue to dominate the western sky at dusk in September. You can find Venus on the western edge of the September star map. Venus looks like an airplane coming in for a landing (except that it doesn't blink or land at the airport). Venus is often the nearest planet to earth, as in this September; is a decent-sized planet, roughly the same size as Earth; and is covered in shiny, reflective clouds. These factors combine to make Venus the brightest dot of light in the sky.

To see Venus, just look west as it gets dark. At dusk in early September (about 7:20 p.m.), Venus is about a quarter of the way up the sky and sets about 9 p.m. Each night, Venus will be a little lower at dusk. By the end of September, Venus is only about 7 degrees above the horizon at dusk (7 p.m.), which is about the width of three fingers held at arm's length. By that time, you'll have only about 45 minutes to see Venus before it sets at 7:45 p.m. Shining at around minus 4 magnitude, Venus is by far the brightest dot of light in the sky.

The brightness of objects in the sky is measured in magnitude. A medium-bright star, like any of the seven stars of the Big Dipper, is second magnitude. A really bright star such as Vega, in the constellation of Lyra the Harp, is first magnitude. A first-magnitude star is 2.5 times as bright as a second-magnitude star.

A few objects in the sky are so bright that they are given negative magnitude numbers. Venus is the leader of the pack at minus 4 magnitude. In fact, Venus will reach its maximum brightness toward the end of September, when it will blaze at minus 4.5 magnitude.

Mars will keep company with Venus all September long, much as the gods Mars and Venus kept each other company in Roman mythology. You can find Mars next to Venus on the western edge of the September sky map. Shining at a mere 1.5 magnitude, Mars is 300 times dimmer than Venus next month. Look for Mars as a little orange dot to the right of Venus. Mars starts out the month almost exactly side by side with Venus; toward the end of September, Mars will be a little higher in the sky than Venus.

From today through Friday, the bright star Spica will be right in between Venus and Mars. The crescent moon will appear close to Venus, Mars and Spica on Sept. 10 and 11.

Saturn hugs the west horizon at dusk at the start of September but is gone by the middle of the month.

As Mars and Venus set in the west, Jupiter rises in the east. Look for Jupiter on the eastern edge of the September sky map. Jupiter shines at minus 2.9 magnitude, brighter than any other dot of light in the sky except for Venus.

Mercury pops into the morning sky by mid-September. Look for it in the east at about 5:30 a.m. from Sept. 13 to the end of the month. Its brightness will increase as the month goes on; from 1 magnitude in the middle of September to minus 1 magnitude by Sept. 30.


The Bishop Museum star map for September is good for 9 p.m. at the start of September and 7 p.m. at the end of September.

On this map, the constellation of Scorpius the Scorpion is visible in the southwestern sky. To find Scorpius, look for Antares, the star that marks the heart of the Scorpion. It is a first-magnitude star and has a reddish-orange hue.

Scorpius has a distinctive shape, made up of about a dozen stars in the second-magnitude range. These bright stars of Scorpius make a pattern that really does look like a scorpion.

Overhead is the "Summer Triangle." This triangle is made up of three bright stars from three different constellations: Vega, the brightest star in the triangle, in Lyra the Harp; Altair, in Aquila the Eagle; and Deneb, the dimmest of the three bright stars, in Cygnus the Swan. We've shown both the individual constellations and the triangle on our map.

Turning to the north, the North Star is visible 21 degrees above the horizon. Whatever your latitude in the Northern Hemisphere, the North Star is that far above the horizon. Honolulu is at 21 degrees north latitude.

The North Star is in the Little Dipper. The official name of the Little Dipper is Ursa Minor, the Little Bear. Despite the myth that it's the brightest star in the sky, the North Star is actually only about the 50th brightest, and is a rather average second-magnitude star.

Mike Shanahan is director of Education, Exhibits and Planetarium. For more information, go to


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