Retrograde Motion of Mars Lab 6 Brief Overview

Retrograde Motion of Mars Lab 6

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)

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

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

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

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)

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

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

Retrograde motion of Mars animation • http: //alpha. lasalle. edu/~smithsc/Astronom y/retrograd. html • http: //www. astro. uiuc. edu/projects/data/Re trograde/ • http: //www. flex. com/~jai/astrology/retrogra de. html • http: //mars. jpl. nasa. gov/allabout/nightsky/n ightsky 04. html

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