1. Part 1: Historical Models
Astronomy!
Part 1: Historical Models

2. What is Astronomy?
Astronomy is the study of the universe!
(everything we see in the night sky)

3. Astronomy is very different
from astrology:
Astronomers are scientists who use
telescopes to see the night sky.
Astrologers are people who believe
the planets and stars can
tell your horoscope.
(a prediction of your future)

4. Long ago, people who looked at the
night sky realized Earth was not alone
in the universe...
Earth is part of a greater system
including the sun, moon, and stars.

5. They saw that the sun and moon
seemed to move against the background
of stars...

6. and some of the stars also seemed
to move...
They called these planets!
(Which means ‘wandering star’)

7. In 140 AD, Greek astronomer Claudius
Ptolemy created a model of this system.
His model only had 5 planets, and it was
accepted as true for over 1400 years!

8. He put Earth at the center since we were
thought to be the most important.
This is called geocentric (Earth centered).

9. The planets went in circles around
Earth, doing loopty-loops to explain
their retrograde motion in the sky!

10. In the 1500’s, Polish astronomer Nicolai
Copernicus proposed a heliocentric
(sun centered) system of 6 planets.

11. His system was rejected, especially by
the churches! Earth not the center?!

12. In the late 1500’s Danish astronomer,
Tycho Brahe made detailed observations
of the planetary motions.
He had a silver nose,
because it got cut in a
duel over mathematics!

13. He came up with a slightly different
model, a mix of Ptolemy and Copernicus.
Earth was still the center of the universe,
but other planets went around the sun.

14. Brahe had to really work to get this
model to match what he observed!

15. In the 1600’s, Italian astronomer Galileo
Galilei was the first astronomer to use
a telescope to support heliocentricity.

16. He saw moons orbiting Jupiter, which
showed not everything revolves around
the Earth.

17. He also saw that Venus had phases like
our moon, which would only happen
with the sun at the center, not Earth!

18. Then the Copernican model was finally
accepted as true, and made more sense!
Let’s
compare...

19. A man named William of Ockham also
helped support the Copernican model.
Ockham’s Razor is the theory that the
simplest explanation is usually correct.

20. In the 1600’s, German mathematician
Johannes Kepler analyzed Brahe’s data.

21. He discovered 3 laws of planetary
motion that, with Newton’s laws, made
the model that we still use today!
The planets orbit the sun in ellipses
not circles, with the sun off-center
making them go faster when closer!

22. Our Solar System (the system revolving
around our sun named Sol!)