This sky map shows how Saturn, Mars and the bright star Spica will form a sky triangle on Monday night (Aug. 20, 2012). This map shows their location with the moon at 8:30 p.m. local time from mid-northern latitudes.CREDIT: Starry Night Software
As soon as it gets dark on Monday night (Aug. 20) — say about an hour after sunset — look low toward the southwest horizon and you will see something rather striking: three "stars" forming an equilateral triangle, tilted a bit to the left. What will make this a rather eye-catching sight is that the three objects forming the triangle are of similar brightness, but of contrasting colors.
In fact, at first glance the night sky triangle may look a patriotic red, white and blue. Each side of the triangle will measure about 4 degrees in the night sky. The width of your clenched fist at arm’s length measures roughly 10 degrees, so the triangle will appear roughly "half a fist" in apparent size.
This triangle will remain in view until the trio sets less than an hour later. So if you wish to see it for yourself, you need to make sure you have a clear and unobstructed view (no tall buildings or trees) toward the southwestern part of the sky.
At the onset I dubbed these three objects "stars" because only one of the three is indeed a true star, the other two are planets.
The brightest of the three is the ringed wonder of the solar system, the planet Saturn; the object occupying the upper right corner of the triangle. Shining with a yellowish-white glow, Saturn has been enjoying a nice run as a conspicuous evening object, though in the coming weeks as it dips lower into the sunset glow and sets progressively earlier it will be more difficult to observe.