Transcript Retrograde Motion of Mars

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
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
ightsky04.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