Mars Retrograde

All of the planets, except for the Sun and Moon, which are not actual planets, have a period of retrogradation when they “appear” to move backward in the sky. Mercury turns retrograde when it is about 14 degrees ahead of the Sun and it is retrograde for 3 weeks 3 times each year. The other planets turn retrograde when they are on the opposite side of the zodiac from the Sun. Often, a planet forming a trine to the Sun will also be retrograde. Mars is retrograde for slightly approximately every 2 years. When a planet is retrograde, it does not work in its normal manner.

If you were to look up in the eastern sky at the same time each night and note where Mars appears to be compared to the constellations of stars, you would find the planet a little farther east with each viewing. That is, Mars appears to move from west to east from one night to the next. Every two years or so, there are a couple of months when Mars' position from night to night seems to change direction and move east to west. This strange behavior was very puzzling to early skywatchers. Did the planet really stop, back up, change its mind, and then continue to move forward? Did it have some weird, mystical meaning?

Today we know what's going on. It's an illusion, caused by the ways that Earth and Mars orbit the sun.

Mars Retrograde Happens Every Two Years

The two planets are like race cars on an oval track. Earth has the inside lane and moves faster than Mars -- so much faster, in fact, that it makes two laps around the course in about as much time as it takes Mars to go around once.

About every 26 months, Earth comes up from behind and overtakes Mars. While we're passing by the red planet this year, it will look to us as though Mars is moving up and down. Then, as we move farther along our curved orbit and see the planet from a different angle, the illusion will disappear and we will once again see Mars move in a straight line.

This apparent erratic movement is called "retrograde motion." The illusion also happens with Jupiter and the other planets that orbit farther from the sun.

Just to make things a little more odd, the orbits that Earth and Mars follow don't quite lie in the same plane. It's as if the two planets were on separate tracks that are a little tilted with respect to each other. This causes another strange illusion.


References:

http://mars.nasa.gov/allaboutmars/nightsky/retrograde/

http://www.lynnkoiner.com/



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Final Exam for Electromagnetism Lab: 23/04/2018

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