Aristotle believed motion was either natural or violent. Natural motion involved straight upward or downward movement, while violent motion resulted from a push or pull. Ptolemy altered Aristotle's model, placing Earth stationary at the center of the universe with planets orbiting in loops. Copernicus formulated a model with Earth and planets revolving around the Sun in circular orbits, contradicting the accepted model. Kepler discovered through Brahe's observations and his own that planets move in ellipses with the Sun at one focus.

2. Motion according to Aristotle was either NATURAL
or VIOLENT;
1. Natural Motion on Earth was thought to be either
straight up or straight down, like the rising of
smoke, and the falling of stone to the ground. Fire
rose naturally to the Sun. Earthly objects like
wood and pieces of metal all fall toward the
Earth.
2. Violent Motion was conceived to be the result of
a push or pull. Motion was imparted to the
objects.

3. Aristotle’s model of the universe was
conducted in layers with Earth
located at the center. Earth it self
was layered with earth, water, air
and fire. The Planets were attached
to transparent spheres whose
centers were located at the center of
Earth.

4.  A Greek astronomer Claudius Ptolemy, altered
Aristotle’s model. He advocate the Ptolemic
System where Earth lies stationary at the
center of the celestial sphere. In his system,
each planet is supposed to travel around the
Earth in loops while the orbits of the Sun and
Moon are circular. In his model, the Sun was
though to move around the Earth once a day
while the Moon was much slower than the Sun.

5.  Aristarchus, a Greek mathematician presented the
first known model of the Universe placing the Sun
at its Center. His model had Earth revolving
around the Sun. The ancient thinkers then thought
that if this were true and Earth were moving, then
the distant stars ought to change positions with
respect to one another. However, the apparent
relative motion among the stars was too small that
no available instruments during those time could
detect them.

6.  In 1543 an astronomer Nicolaus Copernicus
formulated his theory of the moving Earth.
He explain that Earth rotates on its axis. It
also revolves with the other planets around
the Sun in circular orbits. His theory was so
controversial during his time that he had to
work secretly to avoid persecution. He
received the first copy of his work, De
Revolutionibus, on the day he died.

7.  Tycho Brahe, observed 777 stars and several
planets using only a large sextant and a compass.
He was able to find the accuracy in defining the
position of the planets Jupiter and Saturn. He also
found 1577 comets moving in space among the
planets which meant that these heavenly bodies
were not supported on fixed spheres. Through
Brahe’s observation of Mars, Kepler discovered
that planets do not move in circles but in ellipses
wherein one focus is the Sun.

8.  In 1609, Galileo Galilei built his first telescope and
began making observations of the heavenly
bodies.
- He discovered the lunar craters of the moon.
- He observed the 4 largest moons of Jupiter.
- He discovered the sunspots on the Sun which
confirms that the planet orbits around the sun.
- He also confirmed that the orbits of the planets
were circular in shape which was later proven to
be wrong by Kepler.

9. - He gave public lectures on the supernova
when he was able to observe one in 1604.
- He explain the nature of the New Star.
- Using Parallax, he was able to demonstrate the
new star was well beyond the moon.
- He also explained why the size of stars were
identical as seen by the naked eye.
- His observations showed that Venus was
observed to exhibit a complete range of
phases from crescent to full, which would not
be observed if the distance between the Earth
and Venus was constant.

10.  Even before the telescope was invented, accurate
measurements and observations of the positions
of stars, sun, moon and planets were already
made by Tycho Brahe, a Danish astronomer.
When he died in 1601, his assistant Johannes
Kepler obtained all his recorded data and analyze
them. He formulated the three laws of planetary
motion based on the vast amount of data gathered
by Tycho Brahe.

11. 1. The planets move in elliptical orbits.
2. An imaginary line from the planets to
the Sun sweeps equal time intervals.
3. The ratio of the squares of the periods
(t) of the planets is proportional to the
cubes of their mean distances (R) from
the Sun.