Motions in the Sky

How do the motions of

Earth affect our lives?
Earth is always in motion.
The Celestial
Sphere
• The Ancient Greeks
considered Earth to
be enclosed in a
hollow sphere called
the celestial sphere,
where the stars, the
sun, and other
heavenly bodies are
embedded.
 The Celestial
Sphere
• is an imaginary sphere
with the Earth located
at its center. We
imagine that the
‘celestial bodies’ such
as the Sun, Moon, stars
and planets are
placed on the inner
surface of the celestial
sphere just as we would
see them in the sky.
Ecliptic

-The path that the

sun appears to
take around the
celestial sphere .
Ecliptic
-Although it is the
Earth that orbits the
Sun, it appears to us
that the Sun moves
around the celestial
sphere taking one
year to complete a
revolution. This
apparent movement
of the Sun is called
the Ecliptic.
The Celestial Sphere
- Ecliptic Poles.
- If we imagine a line taken
from the center of the
Earth to the Sun, it will be
at right angles to the path
of the ecliptic and where
this line meets the
celestial sphere will mark
the North Celestial Pole
(NCP) and South Celestial
Pole (SCP)
- The points where Earth’s
rotational axis cut this
sphere.
The Celestial
Sphere
- Solstice
- The two points on the
ecliptic with the
greatest distance
from the celestial
equator.
- Summer solstice and
Winter solstice
 The Celestial
Sphere
- Summer solstice
- The point where the sun
is at its northernmost
position above the
celestial equator or at its
highest in the sky is
called the summer
solstice.
- It is sometimes called the
June solstice because it
happens on or near June
21. Day is longest and
night is shortest during
the summer solstice.
 The Celestial
Sphere
- Winter solstice
- The winter or December
solstice occurs when
the sun is at its
southernmost position or
at its lowest in the sky.
This normally happens
on or near December
21. Day is shortest while
night is longest during
the winter solstice.
 The Celestial
Sphere
- Equinoxes
- The two points where the
ecliptic intersects the
celestial equator.
- The Sun crosses the
celestial equator on two
occasions during the
course of a year and these
occasions are known as
the equinoxes. Because
the Sun is on the celestial
equator at the equinoxes,
its0 declination is of course
0 .
 The Celestial
Sphere

- Equinoxes
- The autumnal equinox
happens on or near
September 22.
- The vernal or spring
equinox happens on
or near March 21.
• The ecliptic traces
through a series of star
clusters called
constellation.
• This sequence of
constellation is called
zodiac.
• What do the zodiacs mean?
• In astrology, a sign of the zodiac
refers to one of 12 specific
constellations of the zodiac that
the sun passes through.
 Precession of the equinox

• Precession –
there is a
change in the
orientation of
the rotational
axis of any
rotating body.
 Precession of the equinox

• Earth precession
was historically
called precession
of the equinoxes
because the
position of the
equinoxes was
slowly and
gradually
changing with
respect to some
background stars.
 Precession of the equinox

• Hipparchus of Nicaea
(known today as
Turkey)- was credited
for having discovered
the precession of
equinox.
 Hipparchus of Nicea

• as a Greek astronomer,
geographer, and
mathematician
regarded as the
greatest astronomer of
antiquity and one of the
greatest of all time. He is
best known for his
discovery of the
precession of the
equinoxes and
contributed significantly
to the field of astronomy
on every level.
 Diurnal Motion
• The apparent daily
motion of stars and
other celestial bodies
across the sky caused
by earth’s rotation about
its axis.
• Diurnal motion is
responsible for the daily
raising and setting of the
sun and the stars.
 Annual motion
• Annual motion accounts for
the visibility of a zodiac
constellation at a specific
time of the year. Along with
the tilt of earth’s axis, it is
also responsible for the
seasons.
 Models of the Universe
• The early humans relied on the
skies as their principal means of
telling the time, of navigation,
and of knowing when to start
planting crops.
• Some 3000 years ago, the
Egyptians established a 365- day
calendar based on the track of
the star Sirius. This track also
coincided with the annual
flooding of the Nile River.
Sirius

• Sirius, also called Alpha

Canis Majoris or the
Dog Star, brightest star
in the night sky.
• The expression "dog
days" refers to the
period from July 3
through Aug. 11.
 Models of the Universe
• The Babylonians and the Assyrians
also invented similar calendars to aid
them in determining when to sow
and reap crops.
• Astronomy also influenced
architecture. Around 2560 BCE, the
Pyramids of Giza in Egypt were
constructed in such a way that each
side faced north, south, east, or west
of a compass to within a tenth of a
degree. In addition, the three
pyramids represent the belt stars of
the constellation Orion.
 Orion constellation is
one of the brightest
and best known
constellations in the
night sky. It lies on the
celestial equator.
Orion has been
known since ancient
Orion's Belt or the Belt of Orion, times. The
also known as the Three Kings or constellation is also
Three Sisters, is an asterism in the known as the Hunter,
constellation Orion. It consists of as it is associated with
the three bright stars Alnitak, one in Greek
Alnilam and Mintaka. mythology.
 The Early Universe

• Humans have come up with several models in an effort to

understand the universe. Before the invention of the
telescope, they had to rely on their senses for a picture of
the universe with much philosophical and religious
symbolism.
 The Early Universe

• Around 600 BCE, Thales of Miletus

proposed that Earth is a disk floating on
water.
• Thales of Miletus, (born c. 624–620
bce—died c. 548–545 bce), philosopher
renowned as one of the legendary
Seven Wise Men, or Sophoi, of antiquity.
He is remembered primarily for his
cosmology based on water as the
essence of all matter, with Earth a flat
disk floating on a vast sea.
The Early Universe
 The Early Universe

• In 520 BCE, Anaximander, also from

Miletus, suggested that Earth is a
cylinder and that its surface is curved.
• Anaximander, (born 610 BCE, Miletus
[now in Turkey]—died 546 BCE), Greek
philosopher who was the first to
develop a cosmology, or systematic
philosophical view of the world
Cosmology
studies how the
history of the
universe led to
the stars,
galaxies, and
other features
we can observe
today.
The Early Universe
• Are you interested in the heavenly
bodies? Then maybe you have the
makings of an astronomer or an
astronaut.
• An astronomer is a space scientist
whose main job is to conduct research
about outer space and the universe
through Earth-based observatories.
• A good background in math, physics,
chemistry, computer science, and
astronomy is essential to become an
astronomer. Usually, an astronomer
needs a doctorate degree in astronomy
to be able to work in an observatory.
• An astronaut is a
professional space traveler
trained to do specific
missions in outer space.
Graduates of a bachelor's
degree in engineering
physics, such as
astrophysics, geophysics,
and mathematics, are
qualified applicants for the
job of an astronaut. Can
you be an astronomer and
an astronaut at the same
time? The answer is yes.
 The Early Universe
The Early Universe

• As civilization flourished, several other models were

proposed. These models can be grouped under two
categories: GEOCENTRIC AND HELIOCENTRIC.
• The geocentric model considers Earth as the center of the
universe.
• The heliocentric model assumes the sun to be the center
of the universe
 The Geocentric Models

• The Pythagorean Model • Eudoxu’s Model

• Aristotle’s Model • Ptolemy’s Model

The Pythagorean Model
Pythagoras was acknowledged
to be the first to assert that Earth
is round, and that the heavenly
bodies move in circles.

In his model, Earth is at rest at the

center of the universe, and
everything rotates around it.
 Pythagoras also
considered that the
motions of the planets
were mathematically
related to musical sounds
and numbers. These ideas
are called "The Music of
the Spheres."
 Anaxagoras, a follower of
Pythagoras, was credited with
having determined the relative
positions of the sun, the moon,
and Earth during solar and lunar
eclipses.
 A total solar
eclipse happens
when the Moon
passes between
the Sun and
Earth,
completely
blocking the
face of the Sun.
A lunar eclipse is
caused by Earth
blocking sunlight
from reaching
the moon and
creating a
shadow across
the lunar
surface.
Eudoxus’ Model
• Eudoxus of Cnidus
• Eudoxus was the first to "save the
appearances" that Plato referred to, using
a series of 27 concentric spheres on which
the sun, the moon, and the planets moved
in perfect circular motion.
• The breakdown of the 27 spheres is as
follows:
• one sphere for fixed stars,
• three spheres for the sun,
• three spheres for the moon, and
• four spheres for each of the five known planets
at the time-Mercury, Venus, Mars, Saturn, and
Jupiter.
Eudoxus’ Model
• Note that he did not assign any sphere for
Earth because in his model, Earth is fixed.
Figure 6-8 shows how the four spheres of a
planet around Earth are arranged. The
outermost sphere was aligned with the
celestial poles rotating once a day to give
the effect of rising and setting. The next
sphere was tilted 23.5° and rotating slowly
to simulate the usual west-to-east
movement of the planet about the fixed
stars. The last two spheres produce the
backward motions of the planets. Eudoxus
explained the rotation of the 27 celestial
spheres using the notion of "intelligences."
Aristotle Model
• The Aristotelian model also used the
27 celestial spheres of Eudoxus. In
addition, Aristotle used 27 "buffering"
spheres between the celestial spheres
of Eudoxus and an outermost sphere
that was the domain of what he
called the Prime Mover.
 The Prime Mover rotated this
outermost sphere with constant
angular speed, causing the other
spheres to rotate as well.
According to Aristotle, the order of
heavenly bodies in the universe
was (from Earth out): Earth, moon,
Mercury, Venus, sun, Mars, Jupiter,
Saturn, the fixed stars, and the
firmament of the Prime Mover.

• The Roman Catholic

Church adopted this
idea in Medieval times:
the Prime Mover was
considered as God, and
the sphere of the
firmament as heaven.
• The Roman Catholic
Church adopted this
idea in Medieval times:
the Prime Mover was
considered as God,
and the sphere of the
firmament as heaven.
• How do objects move? How do moving objects stop from
moving? The Greek philosophers sought answers to these
questions about motion. Their ideas on motion were
significantly influenced by Aristotle, a Greek philosopher
and astronomer. His ideas were accepted because they
were based on observations and made logical sense.

• Ancient Greeks believed that the cosmos or the universe

was divided into two realms – celestial and terrestrial. The
motion of the objects in the celestial realm was perfect
and circular. They all revolved around Earth. On the other
hand, the motion in the terrestrial realm was unstable and
imperfect, and there should be a cause for motion.
• Aristotle's model was based on the three
types of terrestrial motion: natural, violent,
and alteration.
• Natural motion is related to the tendency of
an object to seek its natural place in the
universe. Heavy elements move toward
Earth, while lighter ones move up.
• Ancient Greek philosophers believed that all
things in the terrestrial realm were made out
of the four elements – air, earth, fire, and
water. Their natural motion is to go back to
their natural state depending on the element
that composed them. Their motion was either
upward or downward.
• However, terrestrial objects can be compelled to
move in unnatural ways by the application of a
force. This motion is considered violent motion.
Aristotle considered vertical motion as natural, and
horizontal motion as violent. He also considered
motion as a type of change or alteration.
• Alteration is the ability of an object to change. This change
can be generation, corruption, or alteration in quality.
• Generation is "coming to be," while corruption is "passing
away."
• Generation of one object results to corruption of another and
vice versa.
• Aristotle wrote in his book On Corruption and Generation that
"coming to be and passing away take place when a thing
changes, from this to that, as a whole. Passing away takes
place when nothing perceptible persists in the thing, and the
thing changes as a whole.“
• The end of motion is for the object to be at rest. On the other
hand, an object changing its shape from rectangular to
circular is considered alteration in quality
 Ptolemy’s Model
• The Greek mathematician Apollonius,
known in his time as "The Great
Geometer," introduced the idea of an
epicycle to explain planetary motion.
• An epicycle is a circle on which a planet
moves. The center of this small circle in
turn moves around Earth along a bigger
circular path called the deferent.
 Ptolemy’s Model
• To account for the variation in the speed of the
sun during its annual motion, Greek astronomer
Hipparchus refined this model by considering
that Earth was off-center or eccentric in the
deferent where the sun moved. Around 140 AD,
Ptolemy devised a more complex epicyclic
model. He defined a point on the other side of
the deferent's center and called it the equant.
The equant and the center of Earth are
equidistant from the center of the deferent.
When viewed at the equant, the epicycle
orbited Earth at a constant rate. In Ptolemy's
model, each planet has its own epicycle and
deferent. His model of the universe survived for
more than 14 centuries.
The Heliocentric Model
• Before the heliocentric model came about, Greek astronomer
Philolaus initially proposed a pyrocentric model of the universe.
According to him, neither Earth nor the sun was the center of the
universe. Planets and heavenly bodies were supposed to move
around a "fire" located at the center of the universe. In 300 BCE,
another Greek astronomer Aristarchus proposed the first heliocentric
model of the universe by considering Philolaus's "central fire" as the
center of the cosmos. In this model, the sun and the other known
planets revolve around this "central fire." Aristarchus also placed the
other known planets at that time based on their distances from the
sun. However, his theory did not last because of the general
acceptance of the Ptolemaic model.
Copernicus’s Model
• Nicolaus Copernicus asserted that Earth spins on its axis every day
and revolves around the sun just like the other planets; only the
moon orbits Earth. He maintained the concept of uniform circular
motion and Ptolemy's epicycles. He gave reasonable explanations for
the variation of brightness of planets and their retrograde motions.
However, his model had two major scientific flaws: (1) the absence of
stellar parallax and (2) the lack of perceived motion of Earth. Stellar
parallax is the apparent displacement of a star because of a change in
the observer's point of view.
• The Copernican model was not initially accepted because of its
inconsistencies with Aristotelian mechanics and inability to explain
stellar parallax. Copernicus's book Revolutionibus Orbium Coelestion
(On the Revolution of Celestial Orbs) contained his heliocentric theory
and was published in 1543.