December 19, 2007 – The Earth passes Mars every 26 months, overtaking it in an "orbital race" as both bodies go around the Sun and tonight it will make its closest approach to Earth until 2016. 

Earth comes close to Mars because our planet is moving faster in its orbit, catching up to and passing Mars," said Jaymie Mark Matthews, an astronomer at Canada's University of British Columbia.

Tonight's passage happens while Mars is in retrograde motion, or appearing to move westward across the night sky.

"Thus, for the three months around closest approach, the yellowish-orange planet will appear to move slowly backward from the constellation Gemini into Taurus," said Edward Murphy, an astronomer at the University of Virginia.

The exact distance between the two worlds varies during a close encounter, because the planets' orbits are elliptical.

Murphy calculates that today Earth is roughly 55 million miles (88 million kilometers) apart from Mars, a figure backed by Matthews.
But experts say tonight's glimpse of the red planet will be nothing compared to the show-stopping passage of August 27, 2003, when a mere 35 million miles (56 million kilometers) stood between the two bodies.

"That was when the red planet came closer than it had ever been since the time Neanderthals walked the Earth," Matthews said.
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