Review – Frames of Reference A frame of reference is a set of axes of any kind that is used to describe the positions or motions of things What does this mean again? Can someone give me a REAL life example?

Stars as a Frame of Reference It is VERY hard to see how planets move if you use Earth as your frame of reference Example To track the motion of Mars, you would find a few bright stars right near Mars and then mark down the location of Mars The next night you would find the same stars and mark down Mars’ location Continue this and you will be able to see the direction Mars moves.

What our Ancestors Saw Ancient people (Greeks, Babylonians, Hindus and Egyptians) watched the motions of celestial bodies move around our sky. Sun Moon Stars Planets

What did they learn? 1. The stars make unchanging patterns in the sky. These patterns looked like objects so people grouped them and named them The groupings of celestial objects are called constellations

Constellation Examples Orion – The Great Hunter Ursa Major – The big Dipper

2. On each successive day, a star rises and sets FOUR minutes earlier than the day before This means that over months, there are different stars in the night sky So we can only see certain constellations at certain times of the year This helped ancient people develop calanders

3. The sun rises and sets at a rate different from stars

4. The moon also rises and sets at a rate different from stars The moon also shows phases

5. Five other bodies (Mercury, Venus, Mars, Jupiter and Saturn) rise and set at different rates from stars. The ancients called these planets Planets is the GREEK word for “wanderer”

FYI Ancient people developed stone circle technology to observe the rising and setting of the Sun and Moon. This helped the people mark important dates like summer solstice (the longest day of the year!)

Ancient Legends There were many ancient legends about the stars Ancient Romans – Jupiter was the king of the Gods Aboriginal people – Ursa Major was a running bear from hunters and since it was so low in the sky in fall, the blood from the bear painted the leaves red.

Models of the Universe Earth Centered Model (Geocentric) Sun Centered Model (heliocentric)

Geocentric Aristotle made the geocentric model and it was believed to be true for almost 2000 years! He knew the stars were “fixed” so he placed them outside of the planet spheres He arranged the planets (they only knew about five of them) and the sun according to how they were observed to set and rise

Heliocentric Model Copernicus proposed a much different model. He said the sun was “fixed” and in the center and a rotating earth revolved around it. People disagreed with this model for many years because: they thought if the Earth was actually moving people would be thrown off. They didn’t want to believe that they weren’t the center of the universe.