00:15the early models of the universe
00:18geocentric and heliocentric models
00:33during ancient times the earth was
00:35perceived to be flat
00:38early babylonian chinese and egyptian
00:41civilizations believed that earth had
00:45it was then believed that if you set to
00:47sail straight in the ocean
00:49you would get to the edge of earth and
00:53the greek philosophers also investigated
00:56on the shape of earth
00:58all of them described earth and the
01:13here are the ideas of some greek
01:15philosophers about the sphericity of the
01:20pythagoras started the idea of the
01:33eutyxis of nidis another greek
01:37constructed a planetary model based on
01:39the thought that the earth is spherical
01:54also a greek philosopher educated his
01:57students on the sphericity of the earth
01:59but made no justifications
02:13also aristotle noted that there were
02:16egypt that could not be seen on the
02:18other parts of the earth
02:21this phenomenon was only possible if the
02:24earth had a curved surface
02:33eratosthenes a greek philosopher and
02:38was able to estimate the circumference
02:54before the telescope was invented
02:56ancient astronomers only used their
02:58unaided eyes to observe the sky and the
03:02eventually they created models of the
03:07geocentric meaning earth-centered
03:11and heliocentric which means
03:16here are the philosophers who propose
03:18the geocentric model of the universe
03:22eudexus aristotle claudius ptolemy
03:36while those who believed in the
03:37heliocentric model are
03:40aristarchus nicolaus copernicus
03:43galileo galilei and johannes kepler
03:56for eutexus the universe was composed of
03:59the earth and five other planets that
04:01are visible with the unaided eye
04:04namely mercury venus mars
04:16he was considered as the first
04:18astronomer to explain the retrograde
04:20motion of the planets in the sky
04:23also he found out the differences in the
04:26motion of each planet
04:27that should be considered to give an
04:29accurate description of the universe
04:36retrograde motion is an apparent change
04:39in the movement of a planet through the
04:43from night to night they gradually
04:44drifted in the sky relative to the stars
04:48generally toward the east but
04:50occasionally they stop their eastward
04:52and reverse direction moving toward the
04:57the planets are apparently changing
04:59direction during the year
05:02this phenomenon can be observed by the
05:04greeks but they still do not know how to
05:17aristotle patterned his model for
05:21it showed that the universe was
05:23spherical and finite
05:26he also perceived that earth was at the
05:28center of the universe
05:37he believed that the earth is too big to
05:41it could not rotate other celestial
05:44bodies were built up symmetrically
05:46in concentric spherical rings moving
05:57aristotle believed that the earth was
05:59composed of four elements
06:02earth water air and fire
06:06beyond earth are the planets and the
06:09which revolved in spherical rings
06:17he further described that each ring was
06:20in physical contact with one another
06:22which means the motion of a heavenly
06:26will affect the motion of a nearby
06:37it was aristarchus of samos
06:40a greek astronomer who made the first
06:43to create a heliocentric model of
06:57based on aristarchus model
07:00the sun and the fixed stars were at rest
07:03while earth and other planets revolved
07:06around the sun in a circular path
07:27aristotle and many philosophers do not
07:32since stars did not show parallaxes
07:35which they must if the earth goes around
07:39stellar parallax is the annual shift in
07:42the apparent positions of the fixed
07:47a moving earth should produce stellar
07:51this means the star's position in
07:53january is different from its apparent
07:56position in july of the same year
07:59also distant stars will have smaller
08:03while closer stars have larger parallax
08:07so he argued that the stars were very
08:10thus no parallaxes could be observed
08:14and since stellar parallax is only
08:17detectable with telescopes
08:19his accurate speculation was unprovable
08:27claudius ptolemy in his model
08:31he assumed that earth was at the center
08:33of the universe while the other
08:35celestial bodies revolved around the
08:38in perfect circles with constant
08:47ptolemy's model was considered more
08:50than previous geocentric models
08:54his model could explain the motion of
08:56the celestial bodies accurately
08:59like the retrograde motion of the
09:07in his model ptolemy assumed that
09:10planets revolved on epicycles
09:12the small spheres which in turn moved
09:27in this illustration the red planet
09:30revolves on epicycles
09:31the small green circle which in turn
09:34revolve around the different
09:36the large blue circle
09:39the center which is earth of the
09:42is called the eccentric
09:47as seen on earth the movement of planet
09:51is direct motion while the movement of
09:55is retrograde motion
10:08notice here the epicycle moves slowly
10:11and the planet moves rapidly eastward
10:14then later as it moves along the
10:17it moves rapidly westward
10:28during that time ptolemy's epicycles has
10:31explanation to the retrograde motion of
10:48the planets revolve in ptolemy's
10:49epicycles only during the months when
10:52the retrograde motion can be observed
10:56in this case it's during august first
11:08tycho brahe a danish astronomer who also
11:12made planetary observations
11:15brahe believed in a geocentric universe
11:18but his idea of the geocentric universe
11:21is slightly different from ptolemy's
11:28in the ticonic system earth was at the
11:31the sun and the moon revolved around it
11:34and all the other planets orbited the
11:38such a model was a type of
11:40geoheliocentric system
11:48here is an illustration of the
11:49geoheliocentric model of the universe
12:08nicolaus copernicus during the 16th
12:12copernicus a polish astronomer revived
12:16the heliocentric model of aristarchus
12:28he was actually hesitant to publish his
12:31because he was afraid of condemnation by
12:36but it was still published a year before
12:48copernicus strongly believed in the
12:52because there were loopholes in the
12:53ptolemac model in terms of predicting
12:55the positions of the planets
13:08in copernicus model heavenly bodies
13:10exhibited constant circular
13:12and perpetual motion along their
13:17this is a large epicycle wherein the sun
13:42order of planets from the sun is mercury
13:45venus earth mars jupiter
13:49saturn and the fixed stars
13:52the earth is constantly rotating on its
13:54axis while it is continuously revolving
14:10also now the heliocentric model can
14:13clearly explain the retrograde motion of
14:18since the earth revolves faster than
14:20mars there are times that mars seems
14:31however the copernican model was still
14:34because the movement of the earth cannot
14:38the model is also still unable to
14:40explain stellar parallax
14:51galileo galilei was a pivotal figure in
14:54the development of modern astronomy
14:57he invented the telescope and proved the
14:59copernican hypothesis
15:11galileo believes that the earth rotates
15:14and revolves around the sun
15:18he also stated that the very distant
15:20stars do not exhibit parallax
15:31in galileo's model the sun is the center
15:36our solar system is a part of a larger
15:40using a new invention the telescope
15:43galileo was able to view parts of our
15:45solar system in motion
15:51here are galileo's observations new
15:54the milky way is made up of stars
15:56mountains and valleys on the moon
15:59four moons orbiting jupiter now called
16:11sunspots which are rotating around the
16:13sun about once a month
16:16the rings of saturn and that planets are
16:20not pinpoints of light like the stars
16:31johannes kepler a german astronomer who
16:34lived at about the same time as galileo
16:38he also believe in the heliocentric
16:40model of the universe
16:51in kepler's model he showed
16:53mathematically that copernicus idea of a
16:57worked well if the uniform circular
16:59motion was replaced with uneven
17:02but predictable motion along off-center
17:23here are the three laws of planetary
17:26first is the law of ellipses the orbit
17:30of the planets is an ellipse
17:32with the sun at one of the foci of the
17:34ellipse the closest point to the sun in
17:37is the perihelion the farthest point is
17:51second is the law of equal areas
17:54the line joining the sun and the planet
17:57sweeps over equal areas in equal times
17:59as the planet travels around the orbit
18:03this means the planet moves fastest at
18:07and slowest at the aphelion
18:19third is the law of period or harmonic
18:23the square of the period of revolution
18:25of a planet around the sun
18:27is proportional to the cube of the
18:29average distance of the planet from the
