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near Virgo's brightest star, Spica. As evening deepens, watch for Mars, Saturn and Spica (and the constellation Virgo) to descend westward, to follow the sun beneath the horizon by mid to late evening. This apparent westward motion of the planets and stars is caused by the earth rotating from west to east on its axis.
• To get a better idea of the planets' true motions, we need to watch their change of positions relative to the background stars. Both Mars and Saturn are traveling eastward through Virgo right now, heading for the star Zubenelgenubi in the constellation Libra the Scales. Mars will reach this stellar lamppost in mid-September 2012, but Saturn won't hit this milestone until mid-November 2013.
• Libra? Here's your constellation
• For now, enjoy the rather fleeting alignment, as Mars passes in between Saturn and the Virgo star Spica during these next few nights.
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