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History of Astronomy


A. Stonehenge, on the Salisbury Plain in southern England, was built in stages from about 2800 BC to about 1075 BC to observe the sun and the moon, and thus bring regularity to the builder's calendar.

B. Big Horn Medicine Wheel, an arrangement of rocks resembling a 28-spoke wheel in the Big Horn Mountains of Wyoming, was used as a calendar by the Plains Indians from about 1500-1700 A.D.

C. The Caracol Temple on the Yucatan peninsula is a 1000-year-old astronomical observatory.


Greek, astronomy was based on the astronomy of Babylon and Egypt, which was heavily influenced by astrology.

A. Plato (478-347 BC) argued that the reality we see is only a distorted shadow of the perfect ideal form. Further, he taught that the most perfect form was the circle.

B. Aristotle (384-322 BC) suggested two reasons to believe the Earth was round. First, when a ship came over the horizon, the mast was initially visible, then the deck, and then the entire ship. Second, he observed that the Earth's shadow on the moon during a solar eclipse was curved. Only an Earth which was curved could produce this. He also proposed a geocentric (Earth-centered) solar system.

C. Aristarchus (c. 200 BC) proposed a theory that the Earth rotated on its axis and orbited about the sun.

D. Eratosthenes (c. 200 BC) devised a method for determining the Earth's circumference to within 5 percent of the currently accepted value.

E. Hipparchus (c. 150 BC) discovered precession and made the first catalog of stellar magnitudes.

F. Ptolemy

  1. Claudius Ptolemacus (Ptolemy) (c. 100 AD) lived and worked in the Greek settlement of Alexandria (now Egypt). He ensured survival of Aristotle’s geocentric universe theory by fitting it to a sophisticated mathematical model.
  2. Ptolemy found that simple spheres were not enough to account for the motions of the planets. Planets sometimes move faster, sometimes slower, and occasionally appear to slow to a stop and move backward over a period of days or months. This is called retrograde motion. He accounted for this motion by placing the planets on small circles (epicycles) that moved along larger circles (deferents).
  3. His work was published in 140 AD in what is now known as the Almagest.



A. Nicolas Copernicus (1473-1543) lived and worked in what is now Poland. Because of his long and abiding relationship with the Christian Church, he hesitated to publish his revolutionary ideas in astronomy, so he distributed an unsigned pamphlet in 1507, which outlined his hypothesis of a heliocentric (sun-centered) solar system.

B. Copernicus worked on his book, De Revolutionibus, over a period of many years. It was published in 1543, when he realized he was dying.

C. The Copernican system explained retrograde motion without epicycles, and was elegant and simple compared to the Ptolemaic system.


A. Tycho Brahe (1546-1601) was a Danish nobleman. He developed new and better instruments for viewing the stars, sun, moon, and planets. (Telescopes had not yet been invented.)

B. Brahe published his results in what are now called the Rudolphine Tables (after his patron, Holy Roman Emperor Rudolph II). To assist him, he hired other mathematicians and astronomers, including Johann Kepler.


A. Johann Kepler (I 571-1630) was born in what is now southern Germany. Ten days before Brahe died, in 1601, he asked that Kepler be made imperial mathematician. Upon Brahe's death, Kepler inherited his records.

B. Using Brahe's tables of the positions of the planets, Kepler was able to deduce his three laws of planetary motion:

  1. The orbits of the planets are ellipses with the sun at one focus.
  2. A line from the planet to the sun sweeps over equal area in equal time.
  3. A planet's orbital period squared, is proportional to its average distance from the sun cubed, where P is the period in years, and A is the distance in AU. One AU is the distance from the earth to the sun, and is equal to 93 million miles. A2 =P3

C. Kepler's laws are empirical (based on observations). They do not describe the causes of the motion; they only predict where the planets will be in the future.


A. Galileo Galilei (1564-1642) was born in Pisa, Italy. Galileo was the first scientist to make systematic use of the telescope in looking at the heavens.

B. Galileo's discoveries with the telescope include -

  1. The moon was not smooth; it had valleys and craters. This conflicted with the notion that all heavenly bodies were perfect spheres.
  2. The Milky Way was made up of thousands of stars too dim to be seen with the unaided eye.
  3. The discovery of Jupiter's moons lent credence to the Copernican model, as it was now recognized that objects other than the earth could have moons orbiting about them. These Galilean moons are Io, Europa, Ganymede, and Callisto, all satellites of Jupiter.
  4. Galileo later observed that the sun had sunspots and rotated with a period of 25 days.
  5. Galileo saw that Venus passed through phases similar to those of the moon. That meant it orbited around the sun, not the Earth.

C. Galileo published two major works, Sidereus Nuncius and Dialogue Concerning the Two Chief World Systems. The publication of the second of these created a storm of controversy. He was interrogated four times by the Inquisition, and in 1633 he was forced to recant his views of the heavens. Upon recanting, Galileo was put under house arrest until his death in 1642.


A. Isaac Newton (1642-1727) was born in the English village of Woolsthorpe. In 1665-1666, when Black Plague closed Cambridge where he was studying, he returned to Woolsthorpe, and there derived his famous three laws of motion. These laws of motion were found to work for objects in the heavens, as well as objects on Earth, thereby making Newton the first astrophysicist. His laws of motion are:

  1. A body continues in motion in a straight line at constant speed, or remains at rest, unless it is acted upon by some external force.
  2. A body's change of motion is proportional to the force on it and the direction of the force, F=ma, where F=Force, m=mass, and a=acceleration.
  3. When one body exerts a force on a second, the second body exerts an equal and opposite force upon the first.

B. Newton distinguished between an object's mass, which is how much matter it contains, and its weight. A person who is on the moon is attracted by the moon's gravity less than that same person will be attracted to the Earth by Earth's gravity. That person's mass is the same in both places, but the weight is different. Weight is a force, mass is the amount of matter.

C. Newton determined that for the planets to orbit the sun in elliptical trajectories, they must be subject to a force that decreases proportional to the square of their distance from the sun. In addition, the force must be proportional to the masses of the sun and the planet. In equation form, this is stated by F=GMm/r(2)

D. In the above formula, F is the mutual force of attraction between the planets, G is the universal gravitational constant, 6.67X10-11m2/kg s2, r is the distance between the sun and the planet, M is the mass of the sun, and m is the mass of the planet. This is the Law of Universal Gravitation because we can extend this equation to any two objects, in the universe. For example 'M' could be the mass of Jupiter, while 'm’ could be the mass of its satellite Europa. Therefore all massive objects are gravitationally attracted to all other massive objects in the universe.

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