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 Ancient Greeks noticed that certain celestial objects changed their locations from time to time They called them “wanderers”; ‘planet’ comes from that Greek word 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 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) 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) 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) For superior planets (further from the Sun than Earth) Earth orbit is faster than these planets Earth orbit is faster than these planets Earth catches up to a planet in its orbit Earth catches up to a planet in its orbit Planet appears to stop and move backwards (West to East) Planet appears to stop and move backwards (West to East)

YouTube “Retrograde Motion and the Opposition of Mars” YouTube “Retrograde Motion and the Opposition of Mars”

The Ecliptic Line

The Ecliptic Apparent path of the Sun and planets through the night sky 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? 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 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) 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 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 Broken into 12 constellations (groups of stars) or signs each covering 30 degrees of sky What’s your ‘sign’? What’s your ‘sign’?

SignSymbolDates Aries RamMarch 21 - April 19 Taurus BullApril 20 - May 20 Gemini TwinsMay 21 - June 20 Cancer CrabJune 21 - July 22 Leo LionJuly 23 - August 22 Virgo VirginAugust 23 - September 22 Libra BalanceSeptember 23 - October 22 Scorpio ScorpionOctober 23 - November 21 Sagittarius ArcherNovember 22 - December 21 Capricorn GoatDecember 22 - January 19 Aquarius Water BearerJanuary 20 - February 18 Pisces FishFebruary 19 - March 20 What’s your sign?

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