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Natural Science - Year I

Unit 28: Galileo and the New Astronomy

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Science Weblecture for Unit 28


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Science Lecture for Unit 28: Proof for Copernicus

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Observational Proof Supporting a Heliocentric System

When Copernicus proposed his heliocentric system, he lacked observational proof for his theory. One of the more interesting questions in the history of science is why, in the face of this lack of supporting evidence, so many people chose to accept and use his theory. Inn the nineteenth century, the observations considered crucial to support the Copernical theory were finally carried out, but by then, Newton's theory of gravity had provide Copernican astronomers with sufficient theoretical support that the heliocentric system was already widely accepted.

The Revolution of the Earth Around the Sun and Stellar Parallax

Parallax is a common phenomenon that we use every day to determine the distance to objects we see. In the picture on the left below, we see the lamps on the bridge, the green steeple of a distant church (just below and to the right of the lamp), and the Eiffel Tower along the same line of sight. The photographer then backed up slightly and moved 15 paces to the left. This change in the observer's position resulted in a different set of positions for the lamp, the steeple, and the tower. Because the lamp is the closer object, it appeared to move further to the right compared to the background position of the Eiffel Tower and the church steeple.

Parallax Lined Up Parallax Wide

Our own eyes are far enough apart that the parallax they detect allows us to determine how far away something is, so that we can accurately judge how far to reach for the glass on the table, or whether or not we have time to get through an intersection before the light changes.

The astronomers in Galileo's time recognized that parallax should be observed if the Earth was in motion around the Sun. The fact that they could not observed parallax, and that stars maintains their relative positions to each other, seem to them observational proof that the Earth did not move. What they did not realize was how vast the universe actually is. Stars are so distant that we need telescopes to magnify their change in position relative to one another enough so that we can perceive it. We also need to identify stars which are closer and likely to have larger parallax movement so that we can measure them against the more distant stars. In 1838, Friedrich Bessel became the first astronomer to detect and measure the parallax of a nearby star, Cygnus 61.

Astronomers now use the change in position of nearby stars against the background sky due to the changing position of the Earth over the course of a year determine the distance to nearby stars. Over the course of six months, the Earth moves from one side of the Sun to the other. Its distance from the Sun (93,000,000 miles or 150,000,000 km) is more conveniently expressed as one astronomical unit, in this Unit is often used to express distances within the solar system. For example, the distance between the Sun and Venus is .73 AU, and the distance between the Sun and Jupiter is 5.2 AU -- or 5.2 times the distance between the Sun and Earth.

Stellar Parallax

We also measure angles in terms of degrees, minutes, and seconds. A degree has 60 arcminutes, and the arcminute has 60 arcseconds. (We use the prefix "arc" to indicate that these are minutes and seconds of angular measurement, and avoid confusion with the minutes and seconds of time measurement.) There are 360° in a full circle, so there are 21600 minutes in the circle and 1296000 arc seconds in the circle.

We can draw triangle between the Sun, the Earth, and the star. The Earth-Sun distance is of course 1 AU. If a star is far enough away that the angle between the Earth-star line and the Sun-star line is one arcsecond, the star is one "parsec" away. This gives us an easy way to measure distances to nearby stars.

The Rotation of the Earth and Foucault's Pendulum

It is also important to realize that there was at Galileo's day no direct observational evidence that the earth rotated on its axis, another important component of the Copernican solar system model. The philosopher Leon Foucault first demonstrated the Earth's rotation by using a pendulum suspended from the dome of the Pantheon in Paris. Foucault's apparatus has been reconstructed using pictures taken at the time of the demonstration in 1851. The freely-suspended pendulum swings back and forth constantly in the same plane, but because the Earth turns underneath it, it appears to change direction to an observer standing on the Earth nearby. If this is hard to conceptualize, imagine a pendulum swinging up of a plate as the plate spins below. To a pea on the plate, it will look like a pendulum is changing direction, although you as an independent observer who is not spending with the plate will see the pendulum moving back and forth in the same plane.

Pantheon Pendulum

We now accept the heliocentric model of the solar system because it's the easiest way to explain the phenomena we can observe-stellar parallax, the rotation of the pendulum, and the elliptical orbits of the planets. But in Galileo's time, this evidence did not exist, and those who chose to accept the Copernican model did so for other reasons than proof from direct observation.

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