Retrograde Motion
You’re going the wrong way!
History of Retrograde Motion



Ancient Greeks noticed that certain
celestial objects changed their
locations from time to time
They called them “wanderers”;
‘planet’ comes from that Greek word
Greeks charted stars and planets,
noticed that planets seemed to stop
and change direction sometimes
Retrograde Motion


On Earth, planets appear to move from
east to west due to our rotation (prograde
motion)
Once in a while, planets appear to stop and
move west to east (retrograde motion)
Time lapse of Mars
Retrograde Motion




For superior planets (further from
the Sun than Earth)
Earth orbit is faster than these
planets
Earth catches up to a planet in its
orbit
Planet appears to stop and move
backwards (West to East)

YouTube “Retrograde Motion and the
Opposition of Mars”
http://www.youtube.com/watch?v=72FrZz_zJFU
The Ecliptic Line
The Ecliptic

Apparent path of the Sun and planets
through the night sky
The Ecliptic



Q: How far (in degrees) would the
Sun move along the Ecliptic each day
over the course of 1 year?
Hint: There are 360º in a circle and
365 days in a year
A: about 1 degree (width of your
baby finger at arm’s length)
The Zodiac



16 degree wide band centered on the
Ecliptic is called the zodiac
Broken into 12 constellations (groups
of stars) or signs each covering 30
degrees of sky
What’s your ‘sign’?
What’s your sign?
Sign
Symbol
Dates
Aries
Ram
March 21 - April 19
Taurus
Bull
April 20 - May 20
Gemini
Twins
May 21 - June 20
Cancer
Crab
June 21 - July 22
Leo
Lion
July 23 - August 22
Virgo
Virgin
August 23 - September 22
Libra
Balance
September 23 - October 22
Scorpio
Scorpion
October 23 - November 21
Sagittarius
Archer
November 22 - December 21
Capricorn
Goat
December 22 - January 19
Aquarius
Water Bearer
January 20 - February 18
Pisces
Fish
February 19 - March 20
Star Charts

Let’s check out the star charts!