What Astronomy Taught Me About Stewardship
Episode II – The Heliocentric Model
Kopernikus (or Copernicus) was a Polish mathematician, astronomer and Catholic canon who published his model of the Universe, De revolutionibus orbium coelstium (On the Revolutions of the Celestial Spheres) just before his death in 1543. Kopernikus adopted the heliocentric model of the Universe, Sun at the center with all the planets orbiting the Sun. He got the planets in the correct order and correct distances from the Sun. This model simply explains retrograde motion of the planets (Ocam’s Razor). Unfortunately, he kept the idea of celestial bodies being perfect, and therefore moving in perfect orbits, i.e., circular orbits. His model was no better than Ptolemy’s model of predicting the future positions of the planets.
Now comes Tycho Brahe, a Danish nobleman and astronomer. He has been described as “the first competent mind in modern astronomy to feel ardently the passion for exact empirical facts.” He built the most advanced observatory of its time, Uraniborg on the island of Hven in Denmark, and developed precision instruments and timekeeping devices for his observations. Tycho’s model combined the geocentric and heliocentric models of the Universe: the Moon orbited the Earth, the rest of the planets orbited the Sun, and the Sun orbited the Earth. After all, Tycho was the “greatest astronomer” ever, and if he couldn’t see stellar parallax, then it didn’t exist, so Earth could not orbit the Sun! He worked with Johannes Kepler in his later years, and gave Kepler all of his recorded observations.
Johannes Kepler was a German astronomer, mathematician and astrologer. Years of study of Tycho’s data allowed Kepler to publish Astonomia nova, Harmonices Mundi, and Epitome Astronomiae Copernicanae. Here Kepler laid out his three laws of planetary motion:
All the planets orbit the Sun. The orbits are not circular, but ellipses with the Sun at one focus of the ellipse.
Planets sweep out equal areas in equal times. This is the same as saying planets move faster in their orbit when closer to the Sun (near perihelion) and move slower when farther away from the Sun (aphelion). You already know this. Count the number of days in each season (for the northern hemisphere). Which is the shortest season? That is when Earth is closer to sun. Does the answer surprise you?
The cube of the semi-major axis (a) of a planet’s ellipse is proportional to the square of its period (p) (sorry, a little math here). This is also known as the harmonic law. This is most easily seen if the semi-major axis is measured in astronomical units (1 AU is the average distance between Sun and Earth) and period is measured in years, For Earth, a = 1AU, a3 = 1 = p2 = 1year. Try it for another planet, say Mars. The semi-major axis for Mars is about 1.5AU. What is the period? You should get 1.84 years, almost exactly the time it takes Mars to orbit the Sun.
What about the stellar parallax? Could Kepler see it when no one else could? Of course not! The reason stellar parallax could not be seen is because the apparent shift of the nearby star is so small that you must use a telescope to observe it. This of course means that the stars, even the closest stars to Earth are very, very far away from Earth.
Next week, Galileo and Newton.
Dr. Mike Fisher
Stewardship Chair