Courtesy of EarthSky
A Clear Voice for Science
Almanacs say the planet Mars is “stationary” today. However, stationary doesn’t mean that Mars stays in the same place in Earth’s sky all night tonight. Mars actually shines in the southern sky at mid-evening, and crosses the sky westward throughout the night. This ruddy world sets beneath the western horizon before dawn tomorrow.
Does stationary mean that Mars is staying still in its orbit around the sun? Hardly! Mars always travels eastward in its orbit. Stationary means that – as viewed from Earth – Mars is staying put relative to the background stars. Tonight, Mars shines about halfway between the Gemini star Pollux and Cancer’s Beehive star cluster. You will probably need binoculars to see this cluster of stars.
As seen from Earth, Mars has been moving in retrograde – or westward – through the stars from December 21, 2009 until today. Mars is said to be stationary as it reverses direction, now about to go prograde – eastward – toward the Beehive star cluster. Look for Mars to meet up with the Beehive cluster in mid-April 2010.
Mars’ temporary retrograde – or westward motion – in front of the stars greatly confused astronomers until the astronomer Copernicus (1473-1543) correctly figured out the riddle of retrograde. Whenever the Earth in its smaller, faster orbit laps Mars (or any planet that orbits the sun outside of Earth’s orbit), Mars gives the illusion of going westward (backwards) for a couple of months as the Earth swings by this more distant planet from the inside track.
Written by Bruce Mcclure
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