Tuesday, January 24, 2012

Mars Begins Retrograde Motion: Best Viewing 2012

The red planet, one of our favorites, has begun its literally biyearly occurrence of retrograde motion; the last time it was in such orbit, was in 2009-2010. Apparent retrograde motion is the visible, backward orbit of a planet as viewed from earth, due to the orbit of that planet. Although rather hard to explain without a visual (see the visual below for clarification), retrograde motion is due to the earth's orbit, and thus our perception on things.

On account of the fact that earth orbits quicker than other planets (particularly the outer planets; although the inner planets (Mercury and Venus) exhibit it rather differently), earth, from time to time, will "overtake" a planet of note (such as Mars), causing that planet to look as if it travels in reverse, periodically being motionless for a few days before and after the event. As earth then passes the planet again, we can see that it resumes its old orbit (which is normal: west to east).

As Earth (blue) passes a superior planet, such as Mars (red), the superior planet will temporarily appear to reverse its motion across the sky [Wikipedia]
The retrograde motions of Mars and Uranus in 2003. A composite image created by superimposing images taken on twenty-nine different dates. FROM (Tunc Tezel, apod031216)
"In the illustration above, Mars' images form a large loop, because its orbit is somewhat tilted, and it moves relatively fast. Uranus' images (the fainter line of dots above Mars) form a more compact, straight-line pattern, because it moves far more slowly in its much larger orbit, and because its orbit isn't tilted as much, relative to ours," Courtney Seligman writes in her article of Retrograde motion. The more distant planets retrograde more frequently:
  • Mars retrogrades for 72 days every 25.6 months.
  • Jupiter for 121 days every 13.1 months.
  • Saturn for 138 days every 12.4 months.
  • Uranus for 151 days every 12.15 months and
  • Neptune for 158 days every 12.07 months.
Mars will start its 2012 retrograde motion on January 25, 2012 at 01:00 UT (8:00 pm EST), as it will be closest to the earth during the period. Starting that night and a few following months, Mars' retrograde motion will bring it to its best viewing condition -- perfect for astrophotography and studying it. Rising earlier as time progresses, Mars, in late January, will rise in the evening over the eastern horizon, being up all night in early March 2012. On this January 25 day, Mars will be on the borders of Virgo and Leo, constellations, and will travel across the night sky, retrograde not really being distinguishable just yet. But, it will be noticeable as time wears on. Happy viewing!


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