Retrograde MotionRetrograde Motion

You’re going the wrong way!You’re going the wrong way!

History of Retrograde MotionHistory of Retrograde Motion

Ancient Greeks noticed that certain Ancient Greeks noticed that certain celestial objects changed their celestial objects changed their locations from time to timelocations from time to time

They called them “wanderers”; They called them “wanderers”; ‘planet’ comes from that Greek word ‘planet’ comes from that Greek word

Greeks charted stars and planets, Greeks charted stars and planets, noticed that planets seemed to stop noticed that planets seemed to stop and change direction sometimesand change direction sometimes

Retrograde MotionRetrograde Motion

On Earth, planets appear to move from On Earth, planets appear to move from east to west due to our rotation (prograde east to west due to our rotation (prograde motion)motion)

Once in a while, planets appear to stop and Once in a while, planets appear to stop and move west to east (retrograde motion)move west to east (retrograde motion)

Time lapse of MarsTime lapse of Mars

Retrograde MotionRetrograde Motion

For superior planets (further from the For superior planets (further from the Sun than Earth)Sun than Earth)

Earth orbit is faster than these Earth orbit is faster than these planetsplanets

Earth catches up to a planet in its Earth catches up to a planet in its orbitorbit

Planet appears to stop and move Planet appears to stop and move backwards (West to East)backwards (West to East)

YouTube “Retrograde Motion and the YouTube “Retrograde Motion and the Opposition of Mars”Opposition of Mars”http://http://www.youtube.com/watch?vwww.youtube.com/watch?v=72FrZz_zJFU=72FrZz_zJFU

The Ecliptic LineThe Ecliptic Line

The EclipticThe Ecliptic

Apparent path of the Sun and planets Apparent path of the Sun and planets through the night skythrough the night sky

The EclipticThe Ecliptic

Q: How far (in degrees) would the Q: How far (in degrees) would the Sun move along the Ecliptic each day Sun move along the Ecliptic each day over the course of 1 year?over the course of 1 year?

Hint: There are 360Hint: There are 360º in a circle and º in a circle and 365 days in a year365 days in a year

A: about 1 degree (width of your A: about 1 degree (width of your baby finger at arm’s length)baby finger at arm’s length)

The ZodiacThe Zodiac

16 degree wide band centered on the 16 degree wide band centered on the Ecliptic is called the Ecliptic is called the zodiaczodiac

Broken into 12 constellations (groups Broken into 12 constellations (groups of stars) or of stars) or signssigns each covering 30 each covering 30 degrees of skydegrees 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

What’s your sign?What’s your sign?

Star ChartsStar Charts

Let’s check out the star charts!Let’s check out the star charts!