H-Astronomy-002.htm

Astronomy 1400-1700

Our Sun Centered Solar System

When Columbus was a boy they thought the Sun rotated about the Earth.

Copernicus (1473-1543) put forth the proposition the Earth rotated about the Sun, and expressed his beliefs as follows.

1. There is one center for all celestial speres.

2. All the spheres revolve around the sun, and the sun is, therefore the center of the universe.

3. The distance from the earth to the sphere of fixed stars is much greater than the distance from the earth to the Sun.

4. The daily motion of heavenly bodies results from the earth’s motion about it’s axis.

5. The apparent annual motion of the sun is caused by the revolution of the earth around the sun.

6. The planet’s retrograde motions occur because of the motion of the earth relative to the planets.

Astronomers of the day did not rush to accept the theory, though it did solve the retrograde problem, other problems remained. Copernicus assumed there was a fixed outer sphere of stars

Tycho Brahe of Denmark born into a family of nobility was sent off to study law but instead studied astronomy. While teaching astronomy and astrology he came to the attention of King Fredrick II. About this time there was a very bright star, an exploding super-nova, Tycho had measurements taken from various places. These proved the light source to be very far away, contrary to prior beliefs there was a fixed blanket of stars. King Fredrick gave Brahe use of an island plus funds to build and operate his own observatory. Brahe worked in grand style building & using nothing but the best available. From 1571-1630 accurate data was taken for the first time in astronomical history, Tycho wined and dine visitors in grand style. When Fredrick died, Tycho fell out of favor but obtained the support of Emperor Rudolph of Bohemia. One night, while at a party with the king, Tycho drank too much. As it was considered impolite to take leave of the king during the affair, Tycho could not get away to relieve himself and suffered a prostrate infection and died. A considerable price to pay for venture capital.

Kepler a was a bright student and excellent mathematician. He taught for a while then specialized in preparing almanacs. He took and interest in astronomy, striving to mathematically predict planetary motion. He was influenced by Gilbert who had published works on magnetism, Kepler wondering if there was a magnetic attraction between sun and planets. Gilbert trained as a Dr and was the Queen Elizabeth I’s physician, had proved the earth behaved as a magnet with north & south poles. Brahe, in need of someone capable of making sense of his huge accumulation of data, made contact with Kepler. They did not get along well at first but eventually Kepler worked at the observatory. Before Tycho died he appealed to Kepler to finish the work, after Tycho died Kepler was put in charge. Kepler was not without problems, his father a mercenary soldier went away and did not return, his mother a thin disagreeable woman was later killed as a witch.

Kepler spend eight years on a single task, the motions of mars. He might work for three days in pursuit of a given idea, only to find it too failed. It’s difficult to appreciate the lonely tedious calculations to be done by long hand – pouring over hand written data. It’s difficult enough for a civil engineer to spend all day making relatively simple slope stake calculations by hand, or to work all day using a slide rule to multiply and divide between manual additions and subtraction – often making mistakes placing the decimal point. Hand calculators and computers make what was once tedious a now breeze – time can be spent on setting up a problem rather than doing calculations. None the less Kepler managed to come up with the general laws of planetary motion using Brahe’s data.

Kepler’s Law of Ellipses: The orbit of each planet is an ellipse, the sun being located at one focus. This law states that the orbits are elliptical rather than circular with the sun at the focus. The other focus is located in space and not centered on any body.

Keplers Law of Equal Areas: The line drawn between the planet to the sun sweeps out equal areas in equal time. The orbital velocities are non-uniform, but vary in a regular fashion: the further a planet is from the sun, the more slowly it moves in its orbit. The speed is inversely proportional to the distance at aphelion and perihelion; if a planet is twice as far at aphelion as at perihelion, it moves with half the speed.

Keplers Harmonic Law: The square of the period of a planet equals the cube of the planets average distance from the sun. The planet moves more slowly the greater the distance from the sun.

Kinds of orbits

Inner and outer planets orbiting our Sun

Glass lenses had been in use for some 300 years before Galileo. Galileo was the first to develop and use a telescope, his refined telescope could make an image 100 times larger. With this Galileo proved the Copernicus model was right, contrary to church doctrine. Galileo observed that all velocities are relative. Galileo and Kepler lived at the same time, however each tended to ignore the other.

Galileo spent much time on the subject of gravity and inertia. Aristotle believed that heavy objects fell faster than light weight objects. This seemed obvious from dropping feathers vs rocks. Galileo found that in a vacuum all things dropped to earth at the same rate, independent of their weight. Galileo determined that falling object accelerate under the constant pull of gravity. These findings were applied to explaining and aiming artillery.

Galileo was placed in confinement for the last 18 years of his life for publishing findings in conflict with teachings of the Catholic Church. Galileo, who grew up in the church, thought he had approval of the church for publishing his beliefs – however detractors prevailed – while his ideas spread.

Newton used Kepler & Galileo’s findings plus his own experiments to arrive at the laws of motion & gravitation. Newton invented calculus, which was also independently invented by Leibnitz (1646-1716). Leibnitz calculus terminologies are the ones used today. Where Kepler had a problem with defining inertia, Newton stated the case this way: Mass may be considered that property of an object by virtue of which it possesses inertia. Where mass is the Weight of an object divided by the force of gravity on the object. M = W/g were g is the gravitational constant 32 ft per second per second determined by Galileo. Robert Hooke (1635-1703) known for Hooks Law regarding stress was Newton’s archrival in the British Royal Society; both died the same year.

Laws of motion #1: A body at rest remains at rest, and a body in motion remains in motion continues to move at a constant speed (velocity) along a straight line, unless the body is acted upon in either case by an unbalancing force.

Law of motion #2: An unbalanced force acting on a body causes the body to accelerate in the direction of the force , and the acceleration is directly proportional to the unbalanced force and inversely proportional to the mass of the body.

Expressed mathematically: Force = mass x acceleration. F = m a.

Law of motion #3: For every action there is an equal and opposite reaction.

Law of Gravitation: Each particle of matter attracts every other particle with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

Expressed mathematically: where P=pull, G=gravitational constant, m = mass of respective object & d = distance between the objects.

Further treatment of motion, velocity and acceleration are covered under Physics.

Newton discovered white light contained multiple colors of light. This in turn would open up further advances in astronomy – covered in Astronomy 5.

From his work on optics Newton designed and built the first reflective telescope, a considerable improvement over prior telescopes.

By combining the work of Kepler, Galileo & Newton it became possible to determine the mass and size of the planets.

The period from Copernicus birth 1473 to Newton death 1703, a new scientific era was opened. I’ve referred to prior time as before Columbus and that following after Columbus. Greek geometry had laid the foundation for later advances in mathematics. Things long known in a practical world such as distance, time, inertia, gravity, speed, velocity and acceleration were given specific definitions and meaning. Kepler defined planetary motion, Galileo define the effects of gravity, Newton and others who followed brought scientific method to bear on all phenomenon. The studies of motion, force, inertia, mass, shifted to the domain of physics for application to earth bound phenomenon. The learning race was on – man was leaping from mystical beliefs to proof by test. Yet much ignorance prevailed in the world.