Chapter 22
Origin of Modern Astronomy
Sec. 1 Early Astronomy
• Astronomy—the science that studies the universe
• Properties of objects in space and the laws of the universe
• Greeks were the first to study the sun, moon, and stars
• Used geometry and trigonometry to measure
sizes and distances
• Aristotle was the first to believe the Earth was
round based on the curved shadow on the moon.
• His beliefs were abandoned in the Middle Ages.
The Geocentric Universe
• By the 2nd Century B.C., the Greeks believed the Earth was a
motionless sphere in the center of the universe.
• Geocentric Model—(geo-Earth, centric-centered) the moon,
sun, and known planets (Mercury, Venus, Mars, & Jupiter)
orbit the Earth.
• Orbit—the path of an object as it goes around
another object in space
• Beyond the planets was a hollow sphere where
the stars travelled daily around the Earth.
Early Astronomers
• Eratosthenes was the first to attempt to
establish the size of Earth.
• By comparing the noontime sun and shadows in 2
Egyptian cities, and measuring the difference in angles
between them, he was able to estimate the
circumference of the Earth.
• Hipparchus was the first to create a star catalog.
•
Divided almost 850 stars into 6 groups according to their
brightness.
Ptolemaic System
• Claudius Ptolemy’s model of the universe (141 A.D)
predicted the motion of the planets around the Earth.
• Each planet moves slightly eastward among the stars, but
periodically each planet appears to stop, change direction
for a while, then continue eastward again.
• Retrograde Motion—the apparent westward drift of the
planets.
• Ptolemy believed it resulted from planets moving along smaller
circles, which then moved around the Earth.
• His theory predicted planetary motion well, and went unchallenged
for 13 centuries.
Heliocentric Universe
• Ptolemy was WRONG! The planets do not orbit the Earth.
•Aristarchus was the first to propose a
heliocentric universe.
• Helio-Sun Centric-Centered
• He used geometry to calculate distances
and sizes of the sun and moon (although
his calculations were too small)
• Heliocentric Model—Earth and the other planets orbit
the sun.
Modern Astronomy
• We now know that the universe is not heliocentric,
but our solar system is.
• Nicolaus Copernicus (2000 yrs after Aristarchus) proposed a
model of the solar system with the sun at the center.
• It made more sense that Earth, rather than the sky, rotated once a day.
• He used perfect circles to represent the orbits of the planets, even
though they strayed from his predicted positions.
• This model also explained the retrograde motion of the planets
• The combination of the motion of Earth and the planet’s own motion
around the sun.
• Different sized orbits make the planets appear to change speed and
direction.
https://www.youtube.com/watch?v=zHUWP9zu4W8
Tycho Brahe
• Danish, born 3 years after Copernicus died.
• Built instruments & angle-measuring devices
which he used to measure the locations of the
heavenly bodies (Before the telescope was
invented)
• His observations, especially of Mars, were far
more precise than earlier measurements.
https://www.
youtube.com
/watch?v=x3
ALuycrCwI
Johannes Kepler
• Originally Brahe’s rival, became his assistant.
• Discovered 3 laws of planetary motion:
1. The path of each planet around the sun is an
ellipse, not a circle.
• The sun is 1 focal point, the other focal point is on the
opposite side.
• The further the focal points are apart, the more
stretched out the ellipse.
2. Planets revolve so that a line connecting to the
sun sweeps over equal areas in equal time.
• It must move faster closer to the sun and
slower farther from the sun.
Kepler’s Laws Continued
3. The square of the time it takes a planet to orbit the sun is
proportional to the cube of its mean distance to the sun.
• The larger the orbit of a planet,
the slower its speed and therefore
the longer the orbital period.
• T2 = d3 (T is period in earth years
d is solar distance in AU)
• Astronomical Unit (AU)—the average distance between
Earth and the sun (About 150 million km)
• Used to measure the distance from the sun to any planet/object
in our solar system.
Galileo Galilei
• Italian scientist, contributed descriptions
of the behavior of moving objects.
• Constructed a telescope to view the
universe and discover the following:
• Discovered 4 satellites (moons) orbiting
Jupiter
• Planets are spheres, not just points of
light (as was previously thought)
• Venus has phases, just like the moon
• Our moon’s surface was not smooth
• The sun has sunspots, or dark regions.
Isaac Newton
• Born Dec 25 1642 (the year Galileo died)
• First to formulate and test the law of universal
gravitation.
• the force that holds the moon in orbit around the Earth.
• Universal Gravitation—every body in the universe
attracts every other body
• 𝐹𝑔 =
𝑚1 𝑚2
G 2
𝑑
𝐹𝑔 : Force of Gravity (in N)
2
−11
𝑚
G: Universal Gravitational Constant = 6.67 × 10
𝑁∙
𝑚1 𝑎𝑛𝑑 𝑚2 : Masses of the 2 objects (in kg)
𝑑: Distance between the 2 objects (in m)
𝑘𝑔2
Gravitational force
• Gravitational force decreases with
distance
• Gravitational force increases with
mass
• Weight and mass are not the same.
• Mass—the amount of matter of an object (units: lbs, kg)
• Weight—the force of gravity acting on an object (unit:
Newtons, N)
• W = m x g m: mass, g: acceleration due to gravity
• Mass is constant, while weight can vary if gravitational
force changes.
Orbits
• A planet (or moon, or other object) has
inertia which gives it a tendency to move in
straight line motion.
• The force of gravity pulls an object towards
the larger object.
• Newton concluded that the combination of
forward motion and “falling” motion defines
the elliptical orbits that Kepler discovered.