Chapter 22Origin 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=x3ALuycrCwI

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 Continued3. 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•: Force of Gravity (in N): Universal Gravitational Constant = : Masses of the 2 objects (in kg): Distance between the 2 objects (in m)

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.