These are my complete notes for the Introduction and Basics of Classical Astronomy.
I color-coded my notes according to their meaning - for a complete reference for each type of note, see here (also available in the sidebar). All of the knowledge present in these notes has been filtered through my personal explanations for them, the result of my attempts to understand and study them from my classes and online courses. In the unlikely event there are any egregious errors, contact me at jdlacabe@gmail.com.
Ancient Astronomy
II. Ancient Astronomy.
# Cosmology: The Science of the Cosmos, the human conceptualization and understanding thereof. The Mayans and Ancient Britons used calendar systems to record astronomical observations, such as the Mayan calendar and Stonehenge. Numerous cultures (like the Egyptians and Chinese) developed 365 day calendars, needed for things like the timing of the flooding of the Nile River (which follows a yearly pattern).
# A. Rule 13. Aristotle is the earliest significant scholar in Astronomy, working from 384-322 B.C. - he wrote of how the Moon's phases are caused by differing portions being lit by the sun as the months go by. He knew the Sun was farther away due to the solar eclipses, and that the Earth was spherical due to the shadow on the moon never being indicative of a disk (which, at least during some points of the movement, would be a straight line across).
He, and everyone else at the time, were also convinced that the Universe was revolving around the Earth, because the closer objects in the sky (the 'fixed stars') seem to move on a still distant backdrop, compared to the wandering stars.
This apparent fixed-ness is the result of the Parallax, the shift in direction of an object due to the perspective and motion of the observer. The perceived shift in direction of stars due to the Earth's orbital motion is called stellar parallax, and because it is practically imperceptible in more distance objects (believed to be closer due to their brightness), only two possibilities were immediately clear to the Greeks:
1. The Earth is not moving.
2. The stars are so far away that the Parallax shift is extremely small, much like how far-away mountains do not seem to move when you are moving far away from them.
Of course, they went with the former.
# A. Rule 14. 'Flat Earth Theory' is an ignoramus ideology for the following reasons:
Firstly, if you were to view a ship sailing across the horizon, you would find that it does not shrink and get smaller. The curvature of the Earth will interfere with your view of the ship, and the hull of the ship will disappear over the horizon before the mast does.
Secondly, if the Earth were to be assumed to be a disk, it would not be possible for it to be simultaneously night and day on different parts of the planet. The nature of a disk makes it so when the Sun is facing it, even if the disk is turned a little, it will be lit by the Sun, and thus be day.
# A. Rule 15. Geometric determination of the size of the Earth:
Because the sun is so far away, if everyone on Earth were to point at the Sun while it is visible (the entire enlightened side of the Earth), each arm would be parallel to one another:
Eratosthenes found that the bottom of a well in Syrene, Egypt, would be perfectly enlightened by the sun at noon on the first day of Summer. At noon of the first of Summer in Alexandria, he found that the sun was not directly overhead, but in fact off by a few degrees (based on the shadow of a column). Therefore, he deduced that in Alexandria the sun was roughly 7° off what it was in Syrene. This being 1/50th of the 360° of the assumed sphere, he multiplied the distance from Syrene to Alexandria by 50 and found a rough estimate of the circumference fo the Earth.
# A. Rule 16. Hipparchus was even more hardcore in his devotion to Astronomical truth than Eratosthenes. He catalogued the sky into a system based on coordinates and brightness - the position of objects in the sky were catalogued in a primitive celestial latitude and longitude system, and he divided the sky into different apparent magnitudes according to their brightness. The brightest stars were of the first magnitude, for example.
He then made a horrifying discovery.
The position of the sky of the north celestial pole, as it had appeared in the centuries prior and then, had shifted in position somewhat. The rate with which the entire sky dome rotates is slow, continuous, and constant. The North Celestial Pole is just the projection of Earth's North Pole into the sky - if the north celestial pole is wobbling around, then it is the Earth that is doing the wobbling. The motion of Earth's axis points is called precession. Precession, the Earth's wobble cycle, can be illustrated by the following conical pattern:
# A. Rule 17. The last ancient Astronomer who wrote hugely consequential astronomical treatises was Ptolemy, who wrote a compendium of astronomical knowledge known as the Almagest in 140-ish A.D.. This book also discusses many works from the past, saving knowlege of those works for future generations.
Ptolemy benefited from the mass of astronomical knowledge hoarded over the prior centuries, especially that of Hipparchus, and produced the Ptolemaic Model of the Universe, which no one expanded upon for 1000+ years. See Rule 18 for a full description.
# A. Rule 18. One of the most infamous of the baffling interpretations to result from the Earth rotating around the sun is the phenomenon of Retrograde Motion:
Most of the year, planets will appear to move eastward as they orbit the sun, but as shown in the image, from position B to D, a planet will appear to drift backward as it is overtaken by the faster Earth, moving westward, even though it is still moving to the East. As the Earth rounds it orbit toward point E, the planet will again seem to move westward - this is Retrograde Motion. Now that the Earth is known to also be a moving planet, this is much easier to understand than the astronomical concoction of intersecting circles cooked up by Ptolemy:
Ptolemy's theory, the Ptolemaic Model, declared that each planet revolved in an orbit of its own, called an epicycle, while it itself is revolving around the Earth in an orbit called the Deferant. The deferant isn't even exactly centered around the Earth, however, it is slightly off-centered on something called the Equant point.
Ptolemy invented this fantastical system by carefully calculating the speeds and distances for the movements of the celestial bodies so that 1. the movements of the of the planets matched his model, no matter how complex they were, and more importantly that 2. the model conforms to the ancient assumptions that Earth was stationary and that it is the center of the Universe.
While this made the model functionally correct, generally providing the positions of the stars based on its algorithmic system, it is all wrong and built off of false assumptions.
