History of Astronomy

Try this link to information on the history of astronomy.

Tycho Brahe (1546-1601)

In trying to decide between the geocentric and heliocentric models, Tycho was disappointed with the predictive capability of both models. Much of the reason was the crudeness of the available instruments. Tycho felt that he needed better instruments to make more accurate measurements. He hoped that better observations would allow him to choose between the models.

Contributions to Astronomy:

Tycho was the first to suggest a non-circular orb it for a celestial body (a comet).
Used calibrated and bigger instruments, new techniques to measure angles (similar to a sextant).
Built an observatory (remember - no telescopes yet) and made accurate and continuous measurements for 20 years. His measurements helped to prove that planets orbited the sun.
Accurate map of the stars with 777 stars.
Measured length of the year to within 1 second.
Was still unable to choose between the geocentric and heliocentric model. He had his own model with the Earth at the center, orbited by the sun and the moon, with planets orbiting the sun. Never worked out the mathematical details, and his model was never accepted.
Using Tycho's data, a German astronomer (Kepler) was able to refute the geocentric model.

Johannes Kepler (1571-1630

Was interested in two questions:

Why are there only 6 planets?
How are their orbital periods related to their distance from the sun?

To answer the first question, Kepler hypothesized that the spheres supporting the planets were separated by invisible regular solids (only 5 are possible - cube, tetrahedron, octahedron, etc.). This hypothesis had nothing to do with the fact that there were only 6 planets known in the 16th Century.

Astronomers knew that the outer planets took longer to orbit the sun, and moved slower in their orbits.

Kepler hypothesized that a physical force moved the planets, and that the force diminished with distance.

Planets closer to the sun feel a stronger force and move faster.

The concept of a physical force was a monumental step. Kepler was on the verge of assigning physical causes to celestial motions.
In 1600, Kepler began working as Tycho's assistant.

They recognized that neither the Ptolemaic (geocentric) or Copernican (heliocentric) models could predict positions of Mars as accurately as they could measure them. Kepler tried to calculate an orbit that would fit the data.

Tycho died in 1601; Kepler then had full access to Tycho's data, and analyzed the data for 8 years! He tried to calculate an orbit that would fit the data, but was unable to do so.

Kepler later determined that the orbits were not circular but elliptical.

Kepler's Laws of Planetary Motion

The orbits of the planets are elliptical.
An imaginary line connecting a planet and the sun sweeps out equal areas during equal time intervals.
The Earth's orbital speed varies at different times of the year.
Terms to know:
1. PERIHELION = where a planet is closest to the sun
2. APHELION = where a planet is farthest from the sun
Kepler's Second Law of Planetary Motion was calculated for Earth, then the hypothesis was tested using data for Mars, and it worked!
A planet's speed changes with its distance from the sun.
Kepler's Third Law of Planetary Motion showed the relationship between the size of a planet's orbit and its orbital period, P.
a cubed = p squared
a cubed / p squared is the same for all planets. Constancy of ratio.
The distance of a planet from the sun varies. Its orbital size is defined as a = 1/2 of the major axis A. This is the seim-major axis, a. (a = 1/2 A)
IF THE PERIOD OF A PLANET IS KNOWN IN EARTH YEARS, ITS SEMI-MAJOR AXIS CAN BE CALCULATED IN A.U. (and vice versa) .

Kepler's 3 laws replaced the cumbersome epicycles to explain planetary motion with three mathematical laws that allowed the positions of the planets to be predicted with accuracies ten times better than Ptolemaic or Copernican models.

Galileo Galilei (1564-1642)

Was the first to report using the telescope to view the heavens.

Telescope invented in 1604 by Hans Lippershay.

Galileo used the telescope in 1609. Built his own. Two lenses in a metal tube about 4 feet long, diameter = 4 cm (1.6 inches). Magnification 3X to 33X.

His observations between 1609 and 1612 changed our ideas about the universe.

What did he see?

New stars (Milky Way made up of stars)
Mountains and valleys on the moon
Four moons orbiting Jupiter (now called Galilean moons)
Phases of Venus
Sunspots (rotating around the sun about once a month)
The rings of Saturn (sketches. was puzzling; not identified as rings until about 50 years later.)
Planets are disks, not pinpoints of light like the stars

The significance of what he saw:

Cast doubt on the view of the "perfection of the heavens" (of Aristotle and Plato)
Showed deficiencies of the geocentric (Ptolemaic) model
Rotation of sunspots around sun suggested that if the sun could rotate, perhaps the Earth could too.
Phases of Venus would be a natural consequence of the heliocentric model.
Jupiter's moons showed that centers of motion other than Earth existed.

Galileo published in Italian, not Latin. Widely read. Language of the people, rather than language of the scholars.

Arguments against the geocentric model were so forceful that he came under fire from the Catholic Church and was forced to give a public denial of the heliocentric/Copernican system, and was placed under house arrest for the last 10 years of his life.

Was not pardoned by the Church until 1992.

Science in Italy was dealt a severe blow. The center of scientific investigation shifted to northern Europe.

Many scholars refused to believe his ideas and a few even refused to look through the telescope. Many clung to old ideas.

Isaac Newton (1642-1727)

At this time, much scientific activity in Europe (physics, chemistry, mathematics, biology, medicine)

When Newton entered Cambridge University in 1661, the scientific world had accepted the heliocentric model.

Kepler had supplied the mathematical theory that agreed with planetary observations.
Galileo had supplied the observational evidence.

Many questions remained that Newton answered. In 1665, a plague in London caused Cambridge Univ. to be closed; everyone was sent home. Newton had a long vacation and set out to study the force that holds the planets captive to the sun.

Newton's Three Laws of Motion:

Every object remains at rest or moves at a constant speed in a straight line unless an unbalanced external force acts on it.
The acceleration "a" produced by an applied force "F" on an object of mass "m" is
a = F/m
or F=ma
If an object exerts a force on another object, the second object exerts an equal and opposite force on the first; action = reaction. (Every action has an equal and opposite reaction.)

Newton invented calculus to prove his ideas.

Planets orbit the sun in curved paths. Newton deduced that a force was acting onthe planets. If no force was present, planets would travel in straight lines.
The force is directed toward the sun (like swinging a ball on a string.)

Mass = a quantity of matter.
The more mass an object has, the harder it is to move.
More force is needed to give the same acceleration to a more massive object. Twice the mass requires twice the force.

The sun is much more massive than the planets. Both feel the same force, but the planets have greater accelerations.

The force of gravity is proportional to the mass of the sun times the mass of the planet

The force diminishes with distance - The force is inversely proportional to the square of the planet's distance from the sun (R).

Force of gravity "F" is proportional to 1/R squared.

For a large spherical mass (like sun, planet), the force can be considered as concentrated at their centers.

Mathematical expression for the force acting between planets and the sun:

G is a gravitational constant.

Newton calculated the gravitational acceleration at the distance of the moon and compared it to the moon's actual orbital acceleration. The actual value = the predicted value!

This proved that gravity maintains the moon in its orbit.

Hence, the force of gravity explains the motions of the planets. This force is described by a simple mathematical formula.

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This page created by Pamela J. W. Gore
DeKalb College, Clarkston, GA
pgore@gpc.edu
February 4, 1996