Galileo (16th century A.D.)

Galileo is perhaps most famous for the invention of the telescope. This is too bad, since he did not invent a telescope. What he did do was build and use a telescope for astronomy, perhaps for the first time. What he saw with his telescope would forever change our concept of the solar system. The observations of Galileo, would forever seal the geocentric solar system in the coffin of dead theories. What did Galileo see?

Galileo saw many things with his telescope, but two observations are vital to the heliocentric solar system. First was his observation of Jupiter. Second was his observation of Venus.

When Galileo viewed Jupiter, he not only saw the planet Jupiter but he also saw four "stars" nearby. The "stars" lined up along the equator of Jupiter. In addition, as Jupiter moved from one day to the next, these "stars" moved with it. This told Galileo that the "stars" were actually moons orbiting Jupiter. That meant that not everything orbits the Earth. This was in contradiction to the doctrine of the time.

When Galileo viewed Venus, he noted that Venus underwent phases like the Moon. This means little, since the Moon also orbits the Earth. However, the position of Venus and the phases of Venus did not make sense. When Venus was gibbous, it should be on the far side of the Earth compared to the sun. This was not the case however. During the gibbous phase, Venus was in the same direction as the sun and smaller than during the crescent phase. In addition, Venus was never seen on the opposite side of the Earth than the sun. This was strong evidence that Venus orbits the sun and not the Earth.

In addition to viewing Venus and Jupiter, Galileo saw craters and mountains upon the Moon, the Milky Way as a myriad of many stars, and sunspots on the sun. All of these observations made the celestial objects imperfect. This also was in contradiction to the doctrine of the time.

By observing sun spots on the sun, Galileo was able to determine that the sun underwent differential rotation. That is to say, the equatorial regions of the sun rotate faster than the polar regions of the sun.

Never view the sun directly.

With these observations by Galileo, the geocentric system was put to rest and the heliocentric system rose to the top. However, Galileo was placed under house arrest for the remainder of his life. But he had forever changed how we view the solar system.

Newton (17th century AD)

Newton made numerous contributions to physics and mathematics, but we shall concentrate on two contributions particularly important to astronomy.

The first of these contributions were Newton's three laws of motion. The first law of motion states "a body in motion shall remain in motion moving with a constant speed and direction, unless acted on by an outside force". The second law of motion states "the magnitude of the force is proportional to the mass of the object being moved and the product of its change in velocity with respect to time". The third law of motion states "for every force there is an equal opposite force".

Here we must introduce the concept of acceleration. Acceleration is defined as the change in an object's speed and/or direction with respect to time. This is particularly important in the motion of planets. Recall from Kepler's first law that planets move in elliptical paths. Since their direction is changing, there must be an outside force acting upon them. We should also recall that the second law of Kepler states that a planet's speed changes as it orbits the sun. Since the speed is changing, we must again have an outside force acting upon the planet. The question then becomes "What is this outside force?"

This brings about the second contribution of Newton, the universal law of gravity. Newton's law of gravity states "two objects with mass exert a force proportional to the product of their masses and inversely proportional to the square of their separation, upon each other". That is to say, the more massive the objects the stronger the gravitational attraction, and the farther apart the objects the weaker the gravitational attraction.

As it pertains to planets, this indicates that when the planet is far from the sun the gravitational force is weak. Conversely, when the planet is near the sun the gravitational force is strong. Since the force is strong near the sun, the acceleration is large. This is seen when a planet (or comet) is making the turn around the sun at high speed. Consider turning a corner in an automobile. If you turn the corner at high speeds, it's difficult to hold the turn. Turning at slow speeds is a much easier task. That is to say a larger force is required to turn the corner at high speeds vs. slow speeds. The same is true for planets moving around the sun.

By utilizing Newton's three laws of motion we are able to mathematically derive Kepler's three laws of motion. We find that Kepler's laws apply to any object in orbit around a second primary body. In addition, two massive objects, as in the case of two stars, orbit about a center referred to as the center of mass. We shall discuss this in more detail at a later time.
We shall discuss other contributions of Newton as they fit in the subject matter of the course.

This page was last updated on 06/06/01.