Modern Astronomy Notes

Recall of Ptolemy's geocentric model.
Planets orbit in a small circle called an epicycle.
Each epicycle orbits in a large circle called the deferent.
The Earth is not at the center of the deferent but is slightly to the side.
The equant is the point opposite the Earth, concerning which the epicycles move at a constant speed.

Ptolemy’s model was accepted for 1,400 years.
Ptolemaic geocentrism: ~150 CE
Copernican heliocentrism: ~1550 CE
During the Middle Ages in Europe (5th to 15th century) there wasn't much progress made in astronomy, because people were too busy with plagues, wars, and crusades to worry about scientific advancement.

Islamic and Persian astronomers and mathematicians improved astronomical measurements and developed new mathematical methods.
They also built the first observatories.
The first Islamic observatory was built in what is now Iran around 1074.
The great observatory of Maragheh was built around 1260, also in Iran and contained instruments of very high quality for the time and a huge library of 400,000 manuscripts on astronomy.
Similar observatories were built in Uzbekistan in 1420 and Istanbul in 1575.

The Renaissance period started in the 15th century in Europe.
Many new developments in mathematics, physics, and astronomy began to appear.
These developments led to the scientific revolution, which forever changed humanity's understanding of nature and the universe.

In the 16th century, Nicolaus Copernicus, a Polish astronomer, developed a new heliocentric model of the solar system.
In this model, the Earth was just one of several planets orbiting the Sun, and not the center of the universe.
In 1543, the year of Copernicus's death, his model was published in his book "On the Revolution of the Celestial Spheres."
This publication is considered the beginning of the scientific revolution.

Copernicus's model correctly placed the Sun at the center, with the Earth and other planets orbiting it.
It also correctly stated that the celestial sphere stays fixed, and only appears to rotate due to the Earth’s rotation around its axis.
However, this model wasn't yet fully accurate and still assumed that the planets were moving in epicycles, the deferents were circles, the epicycles were moving at a constant speed, and the celestial bodies were embedded on actual celestial spheres.
Today we know the orbits of the planets are actually ellipses, there are no epicycles or celestial spheres, and their speed varies along the orbit.

Heliocentrism wasn't a new idea, it was first suggested in ancient Greece by Aristarchus of Samos.
The idea that the Earth rotates around its axis was proposed by Heraclides Ponticus a century before Aristarchus.
However, these ideas were never taken seriously until Copernicus's time, 1,800 years later.

Copernicus's work sparked more than a century of scientific progress, known as the Copernican revolution.
Practical outcome: Humanity finally had a correct understanding of the structure of the solar system.
Philosophical outcome: The idea that humans and the Earth are not special and not the center of the universe, a.k.a. the Copernican principle.
We're just one species, on one planet, in one solar system, in one galaxy, among trillions of other galaxies, each of which may contain other forms of life that are no more or less special than we are.

The heliocentric model raised some objections.
Some objections were religious because the model contradicts a literal interpretation of the Bible, but religious texts are not a reliable source of scientific facts.
Even in Copernicus's time, most people didn't take these religious objections too seriously.
Copernicus himself wrote in response that the people who claimed his model contradicts the Bible are merely choosing to interpret the Bible in a way that benefits their argument.
Scientific objections to Copernicus's model were taken seriously.
One was the absence of stellar parallax, an issue that was already raised by the ancient Greeks in response to Aristarchus.
The most important objection was that the claim that the Earth is moving seems to contradict our experience from daily life.
Why don't we feel that motion?
Why do things that are not bolted down not fly away?
Why does a ball dropped from a tall building hit the ground directly below it, instead of in an angle?

The Earth rotates around its axis at a speed of around $0.46 \text{ km/s}$ at the equator and rotates around the Sun at around $30 \text{ km/s}$ .
The entire solar system, including the Earth, is moving at $220 \text{ km/s}$ around the center of the Milky Way galaxy.
The galaxy itself moves at around $1,000 \text{ km/s}$ towards the Great Attractor, a mass at the center of the Laniakea Supercluster.