UNDER THE MICROSCOPE:WHEN I WRITE articles explaining biological evolution or criticising attacks on evolution, such as intelligent design, I get letters taking me to task from people who interpret the Bible in a literal fashion. Science tells us that the Earth spins on its axis every 24 hours and revolves around the sun once every year, and that the other planets in our solar system also revolve around the sun, each with its own period of revolution (the heliocentric model).
On the other hand, the Bible indicates that the sun revolves around a stationary Earth (the geocentric model). I recently received two letters challenging me to prove that the Earth moves around the sun. I will briefly present some of the evidence in today's article.
The most direct evidence of the daily rotation of the Earth on its own axis is the Foucault pendulum. In 1851, the French physicist Jean Bernard Léon Foucault set a huge pendulum swinging from the dome of the Panthéon in Paris. Physics tells us the pendulum will swing in a fixed plane regardless of any rotation of the Earth. At the North Pole, the pendulum would remain swinging in plane while the Earth rotates under it counter-clockwise in 24 hours. The same would happen at the South Pole, except in the opposite direction.
Rotation at intermediate latitudes occurs
in longer and longer periods as one moves away from the poles, but no movement should be evident at the equator. Foucault's pendulum demonstrates all of this - you can see the Earth turn under the pendulum. Of course since you are carried along with the Earth you have no physical sensation of movement.
It was considerably more difficult to prove that the Earth goes around the sun. One simple test of whether the Earth moves failed to detect motion for a long time and provided key evidence to bolster the geocentric model of the universe. The test is based on the phenomenon of parallax. The position of an object against the background depends on the angle it is viewed from.
You can demonstrate parallax easily. Hold your thumb up at arm's length. Close one eye and look at your thumb with the other eye. Then close that eye, open the other and look again. The position of the thumb appears to change.
If the Earth moves around the sun, then the apparent position of the stars viewed from Earth should certainly change between January and June. But, although the effect of parallax was known since ancient times, no parallax was observed no matter how hard people looked at the stars.
It wasn't until 1838 that FW Bessel measured parallax when observing a nearby star and showed that the Earth was in a different place relative to the star in June than it was in January.
Geometry showed that the two places are on opposite sides of the sun. The parallax effect when observing the stars is too small to be detected by the naked eye. The stars are too far away and discovery of the parallax effect looking at stars had to await the invention of the telescope. You can check out the effects of distance on the parallax effect by repeating the experiment with your thumb held close to your face compared to the effect when you hold it at arm's length.
The Polish astronomer Nicolaus Copernicus (1473-1543) ushered in the age of modern science when he proposed that the sun and the not the Earth sits at the centre of the solar system and all the planets, including Earth, rotate around it.
Prior to Copernicus the geocentric model of the universe held sway, with the Earth sitting motionless in the centre and everything else rotating around Earth. In the geocentric model, as you moved out from Earth you would encounter, in the following order: moon, sun, Venus, Mercury, Mars, Jupiter, Saturn, the fixed stars.
Space probes sent from Earth into the solar system in recent times confirm the order of the planets and the fact that they all revolve around the central sun. If the geocentric model were correct we would need to pass
in succession the moon, sun, Venus and Mercury to reach Mars - but this is not what we find.
This is all very interesting, is it not? I only have space to discuss a small part of the evidence that the Earth spins on its own axis and rotates around the sun. For your homework I want you to look up how the Coriolis effect also shows that the Earth spins on its axis and how the Doppler effect on the wavelength of light we receive from stars shows that the Earth is moving relative to the stars.
William Reville is associate professor of biochemistry and public awareness of science officer at UCC - http://understandingscience.ucc.ie
