Chapter 3: Early History

Question 1

What is a light year? Parsec? Look-back time?

Answer 1

• Light year: 3x10⁸ m/s
• Parsec: 3.26 light-years
• Look-back time: the more distant the object, the further back in time we are gazing

Question 2

Copernicus’ greatest accomplishment?

Answer 2

Nicolaus Copernicus (1473–1543) is famous for placing the Sun rather than Earth at the center of the Solar System. He was not the first person to consider the idea that Earth orbited the Sun, but he was the first to develop a mathematical model that made predictions that later astronomers would be able to test.

Question 3

What did Tycho offer Kepler? How did this help Kepler abandon circular orbits in favour of ellipses?

Answer 3

• When Kepler used Copernicus’s model to calculate where
  in the sky a planet should be at a particular time, he expected
  Tycho’s data to confirm the circular shape of orbits, but instead he found disagreement between his predictions and the observations.
• Rather than discarding Copernicus’s ideas, however, Kepler played with Copernicus’s heliocentric model until it matched Tycho’s observations.

Question 4

Kepler’s 3 Laws of Planetary Motion

Answer 4

1. Planets follow elliptical paths in their orbits around the Sun
2. Planets orbital speed changes with its distance from the Sun (closer is faster, farther is slower)
3. Square of the Period of the revolution is proportional to the cube of the semi-major axis of the Orbit (R)

Question 5

The relationship between how “squashed” an ellipse is and how fast a planet moves on various parts of the ellipse:

Answer 5

• The more elliptical an orbit is, the less time the planet spends near the sun
• How elliptical an orbit is given by the difference of the semi-major and minor axes divided by the sum of these axes