Lecture 2 - Astronomy as a Science

Definition of science according to Wikipedia

A rigorous systematic endeavour that builds and organizes knowledge in the form of testable explanations and predictions about the world

Where do today’s ideas of astronomy and modern scientific method come from?

Greece

Explain how Greek civilizations were able to measure the circumference of the Earth in 240 BC

They estimated the Earth's size using the angles of shadows at different locations, notably by comparing the noon sun's angle in Alexandria and Syene. They were able to assume that the sun rays were parallel because the sun is so far away.

Explain the Ancient Greek model of the universe

The Ancient Greek model of the universe is a geocentric view that positioned Earth as the center of the universe, with the sun, moon, planets, and stars revolving around it in perfect circular orbits.

How was the Ancient Greek model of the universe based on evidence and reason?

Their evidence was that the sun and stars appear to move around the Earth in a spherical pattern, and that there was no perceptible motion on Earth. Their reason was that planets must move on perfect spheres because they assumed the universe must be mathematically perfect.

What led to the refusal of new models of the universe despite new evidence?

Insistence on perfection

Why was retrograde motion hard to explain using a geocentric model of the universe?

Retrograde motion is when planets appear to move backwards and form a loop. This contradicted the geocentric view because it suggested that planets do not always follow a simple circular path around Earth, complicating the understanding of their motion.

How are we able to explain retrograde motion today using a heliocentric (everything moves around the sun) model?

Retrograde motion is an optical illusion that occurs when Earth overtakes another planet.

How did Ptolemy adapt the geocentric model to explain retrograde motion? Why was it flawed?

In Ptolemy’s model, he suggested that planets moved in little orbits called epicycles along their big orbit, which would result in a retrograde loop occurring. It was flawed because it did not match the experimental evidence they had, no matter how many epicycles he added to the model.

4 key figures, in order of appearance, in the evolution of our model of the universe

Copernicus, Brahe, Kepler, Galileo

How did Copernicus change the model of the universe?

He suggested that the Sun is at the center of the universe, and that the planets, including Earth, revolve around it in circular orbits. His calculations were not any better than Ptolemy's because he still insisted on perfect spheres, but it was a simpler explanation.

What was Brahe’s role in astronomical studies?

He spent his life improving astronomical measurements by making observations with the naked eye. His observations were very accurate.

Why do people call Kepler the “first modern theorist?” How did he change our model of the universe?

Kepler is referred to as the first modern theorist because he was the first to propose elliptical orbits instead of perfect spheres. At first he insisted on perfect spheres but couldn’t make the data fit, and decided to not ignore the imperfections and include them in the model.

Kepler’s first law of planetary motion

The planets orbit the Sun following an elliptical path with the Sun at one focus (not at the center)

Kepler’s second law of motion (and what it implies)

A planet sweeps out equal areas of the ellipse in equal time as it orbits the Sun. This implies that planets move faster when closer to the Sun and slower when farther away, thus they do not travel at a constant speed.

Kepler’s third law (and what it implies)

P² = a³. This law relates the orbital periods of planets to their average distance from the sun. It implies that planets closer to the sun move faster.

What were the 3 objections to the heliocentric model that followed Kepler’s laws?

1) If the Earth is moving, why don’t we feel it? 2) Ellipses are not mathematically perfect, which bothered people. 3) The lack of observable stellar parallax, which is the apparent shift in position of stars due to Earth's movement.

What law was Galileo the first to show experimentally?

Newton’s First Law of Motion, which says that things in motion stay in motion unless acted on by an external force

How does Newton’s First Law of Motion explain one of the objections to Kepler’s heliocentric model?

It explains that if the Earth is in motion, we do not feel it because there is no external force acting to change our motion

How did Galileo shut down the objection to elliptical orbits which were not mathematically perfect?

His observations with telescopes showed imperfections in the planets, so there was no need for perfect spheres

How did Galileo explain the lack of observable stellar parallax?

Galileo showed that the Milky Way was made up of many stars and argued they were much further away than we thought, so the parallax effect was happening on too small of a scale to see.

Explain how Occam’s Razor is a guiding principle in science

Occam’s Razor says that if you have two competing ideas to explain the same phenomenon, you should prefer the simpler one. You can always come up with a complicated theory to explain a set of data, but the more complicated it is, the more unlikely it is to be true.