Download Unit 3 - Section 8.9 2011 Celestrial Objects from Earth

Grade 9 Academic Science – Space
Theories of the Solar System
Section 8.9 Pages 338-343
The human understanding of the skies has change throughout history. Stars have been important
culturally, as means to improve and make order in every day life and it has also greatly help to improve
our technology and scientific methods.
Ancients
Constellations are patterns in the sky. Different cultures have seen these as different forms according to
their beliefs or mythologies. Astronomical constellations are regions in the sky used as a map (88
recognized regions)
Throughout the ages the skies have been used to set order. Calendars are based on the star cycles and
monuments have been erected to help determine the growing seasons and such (e.g., Stonehenge,
Machu Pichu)
Seafaring cultures devised methods of using star to navigate the open oceans advancing trade and
leading to globalization. By determining the angles to the North Star (in the north) compared to the
horizon sailors could determine their latitude on the globe.
The use of the stars allowed for greater observations and thus more interest. New models were
developed.
Geocentric Model (geo  earth centered)
Ptolemy (Ancient Greek) discovered that the world was round (sphere – knowledge lost until the
renaissance)
 The sun and all planets orbit the earth in perfect circles
 Retrograde motion was explained by epicycles (apparent backwards movement of the planets
 Accepted as valid until the renaissance
Heliocentric Model (helios  sun centered)
 Copernicus first suggested that the earth and planets orbited the sun
 Galileo used telescopes to show the orbiting moons of Jupiter (Galilean moons)
 Keppler: discover mathematically that planetary orbit were elliptical (predicted planet positions –
of Uranus and Neptune)
 Newton – Work on the law of gravity describes planetary motion
New technologies in astronomy have confirmed and pushed forward our modern galactic theories of the
universe
Questions – Homework
 Pages 339 – 343, Questions 6,7
Grade 9 Academic Science – Space
Planet Retrograde Motion
The word retrograde applies to the apparent backward motion of a planet.
An old encyclopedia of astrology describes this retrograde motion as "…the effect of a slow-moving train
as viewed from another train traveling parallel to it but at a more rapid rate, wherein the slower train
appears to be moving backwards. However, it is not a matter of their actual speed of travel, but the rate
at which they change their angular relationship."
Retrograde motion is the apparent motion backwards in the sky as viewed from the Earth. Normal motion
of planets in the sky is from west to east, but sometimes, outer planets seem to slow to a stop, reverse
direction for a time, and then resume their original direction.
Copernican Revolution
In 1540, Copernicus proposed that the Sun, not the Earth, was the center of the Solar System. Such a
model is called a heliocentric system. In this new ordering, the Earth is just another planet and the Moon
is in orbit around the Earth, not the Sun. The stars are distant objects that do not revolve around the Sun.
Instead, the Earth is assumed to rotate once in 24 hours causing the stars to appear to revolve around
the Earth in the opposite direction
By banishing the idea that the Earth was the centre of our Solar System, Copernicus led to a simple
explanation of both the varying brightness of the planets and retrograde motion.
1. The planets in such a system naturally vary in brightness because they are not always the same
distance from the Earth.
2. The retrograde motion could be explained in terms of geometry and a faster motion for planets
with smaller orbits (i.e., the planets further from the sun are moving more slowly in their orbits
than those closer to the sun). Since the Earth travels faster in its orbit than the superior planets, it
overtakes and passes them at times during their mutual orbits around the Sun. For example 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.
View retrograde motions at
 http://www.go.ednet.ns.ca/~larry/orbits/retrogrd.html
 http://www.lasalle.edu/~smithsc/Astronomy/retrograd.html
 http://www.astro.uiuc.edu/projects/data/Retrograde/
 http://www.flex.com/~jai/astrology/retrograde.html
 http://www.astro.ubc.ca/~scharein/a310/SolSysEx/retro/Retrograde.html
 http://www.scienceu.com/observatory/articles/retro/retro.html
 http://www.mhhe.com/physsci/astronomy/applets/Retro/frame.html
Mercury’s retrograde happens three times a year and it lasts for about three weeks.