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What we know about the universe has taken us
thousand of years
Celestial Bodies
 Celestial Body: a natural object out in space such as a
planet, a moon, an asteroid, a comet, or a star
 Star: like our sun, give off heat and light and are
thousands of years away
 Planets: a celestial body that orbits one or more stars,
is large enough that its own gravity holds it in a
spherical shape, and is the only body occupying the
orbital path.
Constellations
 Constellations: a distinctive pattern in the night sky
formed by a group of stars. It often looks like a
familiar object like an animal.
 Examples: Ursa Major or the great bear, Ursa Minor or
the little bear, Orion the hunter, Cassiopeia the queen
and Leo the lion.
 Asterisms: smaller groups of stars forming patterns
within the constellations. Example: the big dipper
(found in Ursa Major)
Ursa Major
Ursa Minor
ORION THE HUNTER
CASSIOPEIA THE QUEEN
LEO THE LION
Motion of the Sun
 Sun follows the same path through the sky everyday
and the path is called ecliptic.
 Ecliptic: the path followed by the Sun through the sky,
as seen from Earth.
Apparent path of the Sun as seen
from Earth
Motion of planets
 They follow an ecliptic path called orbits around the
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Sun. The orbits are like long, slightly flattened circles.
The planets move at different rates due to their
distance from the Sun.
These orbits result from gravitational forces
As the Earth revolves around the Sun, it appears that
the Sun is moving against the background of stars
As the planets revolve around the Sun, they move at
different rates along an ecliptic path.
Planet motion
Motion of Planets
 Those inside Earth’s orbit (Mercury and Venus) appear
to race through the sky and appear to be close to the
Sun
 Those beyond Earth’s orbit are slower (Mars) and
appear far from the Sun in the sky. Seem to move
against the background of stars in the sky
Motion of Asteroids and Comets
 Both Orbit around the sun
 Asteroids: one of many small rocky bodies in our
solar system, most of which orbit the Sun between
Mars and Jupiter.
Comet orbit
 Comet: Discussed later in Chapter 11, but
basically a “dirty snowball”
 The orbits of comets can be above or below
the orbit of Earth, for this reason we do not
always see the comet on the ecliptic
 They can orbit in different planes (unlike all
other planets)
 Visit to explore the orbits of comets and asteroids
www.spaceplace.nasa.gov/en/educators/comet_or
bits_cnsr.pdf
Comets Orbit the Sun
Motion of the Moon
 The moon orbits around Earth once every 28 days.
Other motion..
 Planets, suns (stars) and moons spin on a central axis.
Earth spins on its axis, which is an imaginary line from
the North Pole to the South Pole
 Example: the Earth spins on its axis towards the east.
Stars, including the Sun, all generally move from east
to west.
Early Models and Evidence
Gathered about the Universe
 Aristotle: saw the universe as geocentric – the Earth
at the center and the sun, moon, planets and stars
revolving around it.
 Patterns of stars does not change so he thought the
Earth was in a fixed place
 During a lunar eclipse, the shadow cast on the moon
had curved edges so Earth must be a sphere.
Aristotle’s Evidence that Earth is
Sphere
Early models...
 Later, Aristarchus proposed that the universe was
heliocentric, revolved around the sun but not widely
accepted at this time
 Erathosthenes of Cyene: first person to accurately
measure the diameter of Earth.
Early models…
Ptolemy: He noticed that the motion of Mars in the
sky over several weeks creates a loop in the sky. This
change in direction is called retrograde motion.
Early models…
 To explain this motion, Ptolemy proposed that each
planet revolved around a point on its orbit, called an
epicycle, to explain its motion through the heavens
Early models…
 Copernicus: he thought the earlier observations
could be explained in a simpler way by a model in
which Earth rotated on its axis once daily and revolved
around the Sun once a year. (Heliocentric model)
 The Earth’s motion around the Sun is relatively recent
(1543)
 Galileo: First person to turn a small telescope towards
the sky and publicly report what he saw.
 Observed craters on the moon, spots on the sun, and
four “stars” orbiting Jupiter.
Galileo...
 Galileo also observed that Venus had phases like our
moon does and this could only happen if Venus
orbited the Sun
 Galileo published his findings and was imprisoned for
his work by the Catholic church because they taught
an Earth centered universe.
 Johannes Kepler: with the help of Tycho Brahe,
made observations of Mars’ orbit and found that it
modeled an ellipse (oval shape)
 Kepler developed three laws of planetary motion
 All planets move in ellipses, with the sun at the center
 The speed of the planet as it revolved around the sun is
not constant. Closer to the sun it speeds up
 The time it takes the planet to revolve around the Sun is
related to how far away it is from the Sun
Sir Isaac Newton
 Sir Isaac Newton: developed three laws to predict
and describe motion and explained how celestial
bodies move through the universe.
 He was the first to show mathematically that the force
of gravity reaches far beyond the surface of Earth and
affects all celestial bodies
 This force causes the bodies to remain in orbit around
larger bodies
 Invented the reflecting telescope which allowed more
precise observations.
Early Technologies for observations
 Stone Circles: possibly used to make simple
astronomical observations. For example, Stonehenge.
On midsummer’s day the Sun rises directly over a
particular stone in Stonehenge. Marking the longest
day of the year.
Sun rising over a stone
Early Technologies…
 Astrolabe: around the time of Ptolemy, they were
used to help locate and predict the positions of the
Sun, Moon and stars
 Using a compass and an astrolabe, they were able to
describe the position of any celestial body in relation
to the direction North as well as in relation to the
horizon.
 ASTROLAB ACTIVITY:
http://cse.ssl.berkeley.edu/AtHomeAstronomy/activit
y_07.html
Technologies…
 Telescope: Refracting telescope (uses lenses to gather
and focus light) was used by Galileo to make his
observations (Sun centered universe)
 Reflecting Telescope: (uses mirrors to gather and
focus light) was developed by Newton to make even
more precise observations and this design is used
today, called the Newtonian design
Core Lab
 Strolling Through the Solar System