1. Retrograde Motion of Mars Lab 6

2. Brief Overview of Mars Best views of Mars are at opposition and when it is at perihelion – a favorable opposition – when it is about the size of a head at 1 mile (every 15 years) About 1.5 AU from the Sun Orbital period ~2 years (1.88 yrs) Rotation period ~1 day (24 hrs 37 mins) Obliquity ~ 25.1˚ Diameter ~ ½ earth Pressure ~ 0.0063 bar (same as being 35 km up in the air)

3. Definitions Superior planets – the 7 planets that have have orbits > Earth orbit (Mars, Jupiter, Saturn, Uranus, Neptune, Pluto) Inferior planets – have orbits < Earth orbit (Mercury, Venus)

5. Superior Planet Configurations Opposition – when superior planet is opposite the Earth Conjunction – when superior planet is behind the Sun Eastern quadrature – after opposition, 90˚E of Sun Western quadrature – before opposition, 90˚W of Sun

6. Different Kinds of Movement of Planets All 9 planets orbit Sun in counterclockwise direction (W to E) – orbital revolution All planets also rotate counterclockwise on their axes – axial rotation, except Venus and technically Uranus All the planets exhibit apparent retrograde motion when they are nearest the earth

7. Retrograde Motion of Superior and Inferior Planets The superior planets, whose orbits lie outside that of the earth, appear to move backward at opposition, because the earth is overtaking and passing them Mercury and Venus, the inferior planets, exhibit apparent retrograde motion when they are at inferior conjunction (between Earth and Sun)

8. Retrograde Motion of Mars Every 26 months, Mars appears to reverse its motion – it is an illusion This means it is progressing eastward in sky, or exhibiting direct motion, when all of a sudden, it appears to stop, go backwards for a while, and then resume its normal direction This westward movement is called retrograde motion

9. Explanation of the illusion Since the Earth travels faster in its orbit that do the superior planets, it overtakes and passes them at times during their mutual orbits around the Sun As the Earth begins to overtake Mars, Mars will appear to slow its eastward motion among the stars Then just as the Earth overtakes it, Mars will appear to loop slightly westward for a short time. Once the Earth is well past Mars, that planet will resume its eastward motion among the stars

10. Retrograde motion of Mars animation http://alpha.lasalle.edu/~smithsc/Astronom y/retrograd.htmlhttp://alpha.lasalle.edu/~smithsc/Astronom y/retrograd.html http://www.astro.uiuc.edu/projects/data/Re trograde/http://www.astro.uiuc.edu/projects/data/Re trograde/ http://www.flex.com/~jai/astrology/retrogra de.htmlhttp://www.flex.com/~jai/astrology/retrogra de.html http://mars.jpl.nasa.gov/allabout/nightsky/n ightsky04.htmlhttp://mars.jpl.nasa.gov/allabout/nightsky/n ightsky04.html

12. analogy The retrograde motion effect is similar to passing an automobile on a highway - observers in the faster car see the slower car apparently moving backwards as they overtake it