EARLY PLAYERS IN ASTRONOMY

The Really Early Days

Astronomy began the first time a Cro Magnon (early homo sapien) walked out at night, looked up at the sky, and said “Wow, Dude.”

That person was promptly eaten by a nocturnal predator because he should have been looking around and not up.

The ancient Greeks were the first to make scientific observations of the sky.

Aristarchus

Greek astronomer from 3rd century BCE.

Estimated the sizes of the Sun and Moon relative to the Earth.

First proposed the Sun as the center of the universe, since it was larger than the Earth.

Heliocentric (Sun centered) theory did not gain acceptance until Copernicus about 1800 years later.

Aristotle

Greek philosopher lived from 384 - 322 BCE

Devised a system where the Earth was the immoveable center of the universe.

The heavens were perfect and unchanging.

Aristotle’s geocentric system was used for centuries.

Hipparchus

Lived during 2nd century BCE Made a catalog of 850 stars

that was still in use by astronomers during the Middle Ages.

Divided these stars into 6 brightness categories called magnitudes.

First magnitude stars are the brightest and sixth magnitude stars are the dimmest.

The current magnitude system is based on this one.

Claudius Ptolemy

Lived from c.100 - c.178 CE Wrote a major astronomical

encyclopedia called the Almagest - an attempt to summarize all previous Greek astronomy

orbits of the Moon and planets were circles whose centers were offset from the Earth.

embraced the idea that the heavens were perfect, only shape sufficient for motions in the heavens was a circle or a sphere.

The Dark Ages

For nearly 1500 years after Ptolemy, Europe was in the Dark Ages. No advances were made and earlier knowledge was lost.

Greek knowledge passed into Arab hands, who preserved it and passed it back into Europe when the Moors conquered Spain.

As a result of astronomy advancing via Arab cultures, many stars have Arabic names, like Aldebaran, Altair, and Algol.

Nicolaus Copernicus

A Polish monk who lived from 1473 - 1543, during the Renaissance.

By this time errors in predictions of the planets’ positions based on Ptolemy’s model were obvious.

Copernicus returned to the heliocentric (sun-centered) model developed by Aristarchus.

The heliocentric model allows a much simpler description of the motions of the celestial bodies.

Copernicus on the Heliocentric Model

Sun is the center of the universe. A planet’s speed of movement through the

sky depends on its distance from the Sun. Mercury and Venus are never far from the

Sun in the sky. This can be explained if they orbit closer to the Sun than Earth does.

Retrograde motion of some of the planets (the reason for some of the complexity in Aristotle’s model) could be explained by them orbiting farther out and Earth periodically catching up to, and passing them.

Retrograde Motion of Mars

Retrograde Motion

Copernicus

Copernicus published his theory in the year he died (1543) in a book called On the Revolutions of the Celestial Spheres.

The model had a problem. It still assumed the orbits were perfect circles.

Tycho Brahe

Danish astronomer who lived from 1546 - 1601.

He realized that all previous theories of planetary motion were inaccurate. He decided to make very precise measurements of the motions of the planets first, then to try to develop a theory of celestial motion based on the data.

In doing so, he proved that many of the long-held ideas about the heavens were incorrect.

He proved that a new star that appeared in Cassiopeia in 1572 was far off in space, contradicting the idea that the heavens were unchanging and any transient events were actually in the Earth’s atmosphere.

The remains of Tycho’s Supernova are still detectable.

He showed that the comet of 1577 moved among the orbits of the planets, contradicting the notion of crystalline spheres governing the motions of celestial bodies.

Tycho developed his own theory of celestial motion in which the planets orbited the Sun, but the Sun and the stars orbited the Earth.

Tycho took a German mathematician named Johannes Kepler as his assistant. He hoped Kepler would take his observations and use them to refine and confirm his model of the heavens.

Kepler ended up reviving the Copernican model of celestial motion.

Tycho Brahe

Johannes Kepler Lived from 1571 - 1630. Inherited Tycho’s vast collection of

astronomical observations. Kepler determined the planets travel

around the sun along simple elliptical curves, not the complex combinations of circular motions Tycho and others devised.

He later discovered a formula that links a planet’s distance from the sun with the time it takes to complete its orbit.

Kepler published his model in 1609.

Johannes Kepler

Kepler's Laws Explained

In the mean time:

In 1609 the Italian scientist Galileo Galilei (1564 - 1642) heard about the telescope and decided to build one.

A Dutch optician named Hans Lippershey (c.1570 - 1619) is credited with inventing the telescope.

Galileo’s refracting telescope had an objective lens 1.75 in. in diameter and magnified 33x.

Galileo Galilei

What Galileo Saw:

The surface of the moon was pockmarked with craters, further proof that the heavens were not perfect.

The Milky Way, the band of light that crosses the sky, was made up of countless stars that individually were too faint to be seen with the naked eye. Since the stars could not be seen as disks, they must be very far away, as Copernicus had theorized.

More Discoveries

The planet Venus went through phases similar to those of the Moon. The only was this was possible was if Venus orbited the Sun.

The planet Jupiter had four moons that orbited it.

Both of these observations put the final nails in the coffin of the geocentric (Earth-centered) model of the universe.

Sir Isaac Newton

English, lived from 1642 - 1727 Published his theory of gravity in 1687,

which explained why the planets orbit the sun.

This was the final evidence that the heliocentric theory was the correct one.

Using the 1663 idea of Scotsman James Gregory, in 1668 Newton built a telescope that used mirrors instead of lenses to collect and focus light.

Isaac Newton

2.2 The Geocentric UniverseSun, Moon, and stars all have simple movements in the sky

Planets:

• Move with respect to fixed stars

• Change in brightness

• Change speed

• Undergo retrograde motion

2.2 The Geocentric Universe

• Inferior planets: Mercury, Venus

• Superior planets: Mars, Jupiter, Saturn

Now know:

Inferior planets have orbits closer to Sun than Earth’s

Superior planets’ orbits are farther away

2.2 The Geocentric Universe

Early observations:

• Inferior planets never too far from Sun

• Superior planets not tied to Sun; exhibit retrograde motion

• Superior planets brightest at opposition

• Inferior planets brightest near inferior conjunction

2.2 The Geocentric Universe

Earliest models had Earth at center of solar system

Needed lots of complications to accurately track planetary motions

2.3 The Heliocentric Model of the Solar System

This figure shows retrograde motion of Mars.

Sun is at center of solar system. Only Moon orbits around Earth; planets orbit around Sun.

Discovery 2-1: The Foundations of the Copernican Revolution

1. Earth is not at the center of everything.

2. Center of earth is the center of moon’s orbit.

3. All planets revolve around the Sun.

4. The stars are very much farther away than the Sun.

5. The apparent movement of the stars around the Earth is due to the Earth’s rotation.

6. The apparent movement of the Sun around the Earth is due to the Earth’s rotation.

7. Retrograde motion of planets is due to Earth’s motion around the Sun.

2.4 The Birth of Modern Astronomy

Telescope invented around 1600

Galileo built his own, made observations:

• Moon has mountains and valleys

• Sun has sunspots, and rotates

• Jupiter has moons (shown):

• Venus has phases

2.4 The Birth of Modern Astronomy

Phases of Venus cannot be explained by geocentric model

2.5 The Laws of Planetary Motion

Kepler’s laws were derived using observations made by Tycho Brahe

2.5 The Laws of Planetary Motion

1. Planetary orbits are ellipses, Sun at one focus

2.5 The Laws of Planetary Motion

2. Imaginary line connecting Sun and planet sweeps out equal areas in equal times

2.5 The Laws of Planetary Motion

3. Square of period of planet’s orbital motion is proportional to cube of semimajor axis

More Precisely 2-1: Some Properties of Planetary Orbits

Semimajor axis and eccentricity of orbit completely describe it

Perihelion: closest approach to Sun

Aphelion: farthest distance from Sun

2.6 The Dimensions of the Solar System

Astronomical unit: mean distance from Earth to Sun

First measured during transits of Mercury and Venus, using triangulation

2.6 The Dimensions of the Solar System

Now measured using radar:

Ratio of mean radius of Venus’s orbit to that of Earth very well known

2.7 Newton’s Laws

Newton’s laws of motion explain how objects interact with the world and with each other.

2.7 Newton’s Laws

Newton’s First Law:

An object at rest will remain at rest, and an object moving in a straight line at constant speed will not change its motion, unless an external force acts on it.

2.7 Newton’s Laws

Newton’s second law:

When a force is exerted on an object, its acceleration is inversely proportional to its mass:

a = F/m

Newton’s third law:

When object A exerts a force on object B, object B exerts an equal and opposite force on object A.

2.7 Newton’s LawsGravity

On the Earth’s surface, acceleration of gravity is approximately constant, and directed toward the center of Earth

2.7 Newton’s LawsGravity

For two massive objects, gravitational force is proportional to the product of their masses divided by the square of the distance between them

2.7 Newton’s LawsGravity

The constant G is called the gravitational constant; it is measured experimentally and found to be:

G = 6.67 x 10-11 N m2/kg2

More Precisely 2-2: The Moon is Falling!

Newton’s insight: same force causes apple to fall and keeps Moon in orbit; decreases as square of distance, as does centripetal acceleration: a = v2/r

2.8 Newtonian Mechanics

Kepler’s laws are a consequence of Newton’s laws; first law needs to be modified: The orbit of a planet around the Sun is an ellipse, with the center of mass of the planet–Sun system at one focus.

More Precisely 2-3: Weighing the Sun

Newtonian mechanics tells us that the force keeping the planets in orbit around the Sun is the gravitational force due to the masses of the planet and Sun.

This allows us to calculate the mass of the Sun, knowing the orbit of the Earth:

M = rv2/G

The result is M = 2.0 x 1030 kg (!)

2.8 Newtonian Mechanics

Escape speed: the speed necessary for a projectile to completely escape a planet’s gravitational field. With a lesser speed, the projectile either returns to the planet or stays in orbit.

Summary of Chapter 2

First models of solar system were geocentric but couldn't easily explain retrograde motion

Heliocentric model does; also explains brightness variations

Galileo's observations supported heliocentric model

Kepler found three empirical laws of planetary motion from observations

Summary of Chapter 2, continued

Laws of Newtonian mechanics explained Kepler’s observations.

Gravitational force between two masses is proportional to the product of the masses, divided by the square of the distance between them.