The planet Mars, which has been growing bigger and brighter in the northeastern sky each evening for months, tonight makes its closest approach to Earth since 2003, and the nearest until 2016.

A forecast for "mostly cloudy" skies threatens to obscure the view tonight. But Mars will continue to dominate the evening sky for several more months, even as Earth's faster orbital track begins to speed us farther and farther away.

When the clouds do part, anyone can spot Mars, gleaming high above the northeastern horizon after sunset. Its reddish hue sets it apart from the bright stars of the winter constellations in that part of the sky.

Because of peculiarities of Mars' orbit, tonight's close approach comes six days before Mars reaches "opposition,"  that is, opposite the sun as seen from Earth's perspective. That means Earth's orbit around the sun has brought it around to the same side of the solar system as Mars, which is farther from the sun, and therefore moving more slowly along its orbital track. Earth is, in effect, "lapping" Mars on its race around the sun.

So, at sunset tonight, we can look inward toward the sun at the center of the solar system as the sun sets in the west, and then turn east and look outward and watch as Mars rises.

Mars oppositions occur once every 26 months. They are prime time for planetary scientists to launch probes toward Mars, because the time and distance the craft need to traverse are shortest. NASA launched the Mars Phoenix mission toward Mars in August, and they hope to land it in the Martian polar north in May.

This 2007 opposition marks Earth's closest approach to Mars since the historic opposition in August 2003, when the planets came within just 35 million miles of each other - the nearest in all of recorded human history. Tonight's approach will bring them within 55 million miles. The next one closer than that - 47 million miles - will be in 2016.

Astronomers using the Hubble Space Telescope have been taking advantage of the shrinking distance to Mars, snapping a series of photographs of the Red Planet, and stitching them together into a video animation of a rotating Mars. The third video ("Close Encounters...") offers a nice explanation of Mars oppositions, and features Baltimore's own Streetcorner Astronomer Herman Heyn.