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Chapter 18 Notes
I. Astronomy: The Original Science
a. People in ancient cultures used the seasonal cycles of the stars,
planets and the moon to mark the passage of time.
b. Astronomy is the study of the universe
c. Years, months and days are based on the observation of bodies in
our solar system.
i. Year= The time required for Earth to orbit once around the
ii. Month= roughly the amount of time required for the moon to
orbit once around the Earth.
iii. Day= the time required for the Earth to rotate once on its axis.
II. Astronomy’s Founding Fathers
a. Aristotle
i. Greek philosopher who lived from 384 BC – 322 BC and
believed that the universe was geocentric.
ii. Geocentric Model= the Earth is the center of the solar system
and all other bodies in space, including stars, planets and the
sun revolved around the Earth.
b. Ptolemy
i. Greek astronomer who published a book in 140 AD that
combined all of the ancient knowledge of astronomy.
ii. Ptolemy agreed with Aristotle that the Earth was the center of
the universe.
c. Copernicus
i. Polish Astronomer who was trained as a medical doctor
ii. In 1543, he published a theory that the sun was the center of
the solar system.
iii. Heliocentric Model= the sun is the center of the solar system
and the planets, including Earth, revolve around it.
d. Tycho Brahe
i. Danish Astronomer
ii. Recorded very precise observations of the planets and stars.
iii. In a duel over who was the best mathematician, Brahe had a
part of his nose cut off and wore an artificial nose made of
e. Johannes Kepler
i. Discovered that all the planets revolved around the sun in
elliptical shaped orbits
ii. He also discovered that the sun is not the exact center of the
f. Galileo Galilei
i. Italian Astronomer
ii. Became one of the first people to use a telescope to observe
objects in space in 1609.
1. He discovered craters and mountains on the moon, 4 of
Jupiter’s moons, and sunspots on the sun.
iii. Galileo was tried for heresy in 1633. His heliocentric view of
the solar system went against the geocentric view of the
Roman Catholic Church.
1. He was found guilty of heresy and sentenced to a number of
punishments. First, he was required to denounce the theory that the
Earth moved and the Sun was at the center of the solar system.
Second, he was imprisoned on house arrest in his villa near Florence
and was allowed no visitors for the 7 years before he died.
3. Lastly, his work was forbidden to be published.
g. Isaac Newton
i. Showed that all the objects in the universe attract each other
through gravitational forces.
ii. The force of gravity depends on the mass of the objects and
the distance between them.
iii. His law of gravity explained why all of the planets orbit the
sun- the most massive object in the solar system.
h. Edwin Hubble
i. Proved that other galaxies existed beyond the Milky Way in
III. Telescopes
a. An instrument that gathers electromagnetic radiation from objects in
space and concentrates it for better observation.
b. Optical Telescopes
i. Most common type of telescope, includes reflecting and
refracting telescopes.
ii. Used to study visible light from objects in the universe.
c. Refracting Telescopes
i. Uses lenses to gather and focus light.
ii. Have 2 disadvantages, which is why most astronomers use
reflecting telescopes.
1. Images cannot be perfectly focused
2. Their size is limited, if they get too large, the image will
be distorted.
d. Reflecting Telescopes
i. Uses a curved mirror to gather and focus light from distant
ii. Have several advantages
1. Mirrors can be very large
2. Flaws in the glass do not affect usage
3. Images can be perfectly focused
e. Light gathered by telescopes on Earth is affected by the atmosphere.
i. Causes starlight to shimmer or blur
ii. Light pollution from large cities can limit the ability to view far
away objects.
iii. Mountain tops are good locations to view the stars, because
the air is thinner and there is less air and light pollution.
f. Scientists have put telescopes in space, such as the Hubble Space
telescope to avoid interference by the atmosphere.
g. Electromagnetic spectrum
i. made up of all the wavelengths of electromagnetic radiation.
ii. Visible light is a small part of it, most is invisible.
iii. Earth’s atmosphere blocks most of the invisible radiation from
objects in space.
iv. Non optical telescopes can be used to study the invisible
1. Radio telescopes- larger than optical telescopes because
radio wavelengths are longer than visible light
2. X-ray telescopes in space are much more sensitive than
optical telescopes.
IV. Mapping the Stars
a. Constellations
i. Sections of the sky that contain recognizable star patterns.
ii. Different civilizations had different names for the same
iii. Today, astronomers have agreed on names and locations.
iv. Every star or galaxy is located in one of 88 constellations.
v. The apparent locations of the constellations change as Earth
revolves around the sun.
vi. Constellations are different in the southern hemisphere.
b. An instrument known as an astrolabe is used to describe the location
of a star or planet.
c. Celestial Sphere- an imaginary sphere that surrounds the Earth.
i. Astronomers use the terms right ascension and declination to
plot positions in the sky much like we use latitude and
longitude to plot positions on Earth.
d. As the Earth spins on its axis, stars and planets appear to move.
i. Circumpolar Stars- can be seen at all times of the year and all
times of the night.
e. Light Year
i. Unit of length equal to the distance that light travels in one
ii. Equivalent to 9.46 trillion kilometers
f. Doppler Effect
i. Explains how we know the universe is expanding.
ii. If a light source is moving quickly away from an observer, the
light looks redder than normal. This is called redshift.
iii. If a light source is moving quickly towards an observer, its light
looks bluer than normal. This is called blueshift.
iv. Edwin Hubble analyzed light from distant galaxies.
1. He discovered that all the galaxies, except our closest
neighbors, were affected by redshift, indicating that they
are moving away from us and that the universe is