Ancient Astronomy

Ancient Astronomy explores the roots of the study of the stars, as far back as they are documented in the historical record. From the early seafarers, to the Sumerians who first documented the positions of the stars, the Babylonians and Egyptians, and ultimately to the Greeks and Romans who took the knowledge of their predecessors, and did not waste it. Instead, they refined it, accumulating their own ideas, and elevating it to new heights.

This is the story of ancient astronomy.

Contents

• Introduction
• Astronomy of the Ancient World
• Early Greek and Roman Cosmology
• The Writings of Aristotle
• Aristarchus of Samos
• Measurement of the Earth by Eratosthenes
• Hipparchus and Precession
• Ptolemy and the Ptolemaic Model of the Cosmos

Introduction

Before we delve fully into Modern Astronomy, let us delve briefly into history, and explore the Ancient Astronomy of the peoples of the world.

In the discipline of astronomy in particular, the history is long, and the foundations were laid over thousands of years.

(This is also true with biology, medicine, physics, and engineering to a degree, but I believe it to be especially relevant in Astronomy.)

Much of modern Western Civilization derived from the ideas of the Greeks and Romans. However, it is important to note that their excellence in early science and ideas was itself the product of earlier Babylonian, Egyptian, and Sumerian work to a significant degree.

This is true in astronomy as well. Many ancient cultures developed sophisticated systems for observing and understanding the sky.

Astronomy Of The Ancient World

Ancient Babylonian, Assyrian, and Egyptian astronomers knew the approximate length of the year. They saw — and documented — the constellation that the Sun rose (or set) in at one point of the year.

Every day, they watched it move through a band of constellation slowly. Counting the days until it returned to the position against the background of stars in which it began.

The Egyptians of 3000 years ago adopted a calendar based on a 365-day year. [1] They also kept track of the rising time of the bright star Sirius in the predawn sky (a star that can be found by following the line of Orion’s Belt) since this star was especially important to them because it’s yearly cycle corresponded with the flooding of the Nile. [1]

The Chinese also developed a calendar, and determined the length of the year at around the time of the Egyptians. [1] Later, they recorded comets, bright meteors, and sunspots (dark spots on the surface of the Sun). [1]

Later still the Chinese documented the existence of “guest stars” — stars that are normally too faint for the eye to see (generally because of how far away they are) but which suddenly flare up for weeks or months, becoming visible to the eye. [1] Today we know these as supernova — the fiery explosion the marks the death of giant stars. Ancient Chinese records are still used today on occasion to study stars that died long ago.

The Mayans (perhaps beginning with the Olmecs before them) developed a sophisticated calendar based on the motions of Venus — the somewhat infamous Mayan Calendar. Their culture located in modern Mexico and Central America built astronomical observatories for this purpose of studying the sky.

Cultures like the Polynesians sailed across the Pacific Ocean — not only settling the islands dotting this great expanse, but navigating to them, and recording their location — all based on their sophisticated understanding of the stars in the sky. Its very possible that they often travelled at night for the purpose of using the sky, since that is the only way they could tell their current latitude and match it with the known latitude of the island they were trying to find.

In Ancient Britain — long before writing — great megalithic (“large-stone”) monuments were used to keep track of the motions of Sun and Moon. Many stone circles still exist that were built for this purpose, dating back to — and before — about 2800 BCE. Stonehenge is one of the best known examples.

It should be noted that the motivation of the ancients had a very specific intention for studying the skies — to learn the will of the gods, divination. Today, we are interested in understanding the universe. But back then, while they were actually motivated by this same interest, they didn’t have the ability to conceive what the galaxy and universe is on the level that we do today. Nonetheless, their work made our breakthroughs possible.

Early Greek and Roman Cosmology

Our concept of the basic structure and origin of the cosmos is called cosmology. Before telescopes we could only rely on our senses for understanding the universe — literally what we can see. Ancient peoples deduced as much as they could about the universe through their observations, developing their cosmologies, which they infused heavily with elements from their superstitions, mythologies, philosophies, and religious ideas.

At least 2000 years before Columbus, the educated of Ancient Greece, Egypt, and Babylon knew that the Earth was a sphere. [1] This probably stemmed from Babylon and Egypt — transmitted from the Egyptians to Pythagorus, who lived c.500 BCE.

Pythagorus was a philosopher and mathematician, who lived and studied in Egypt for a time (which was one of the major centers of scholarship in the world back then). He believed that circles and spheres were “perfect forms” suggesting that the Earth was therefore a sphere. [1] That the gods favoured spheres (as a shape to be used in creation) was explained by the Greeks with evidence such that the Moon is a sphere. [1]

The Writings of Aristotle

The writings of Aristotle (384-322 BCE) summarized many of the ideas of his day. [1] The progression of the Moon’s phases — it’s apparent changing shape due to Earth-Sun-Moon Geometry over the course of a month — was (correctly) explained as us seeing different portions of the spherical Moon’s sunlit hemisphere over the course of a month.

Aristotle also knew that the Sun must be further away from the Earth than the Sun — due to the phenomenon of a solar eclipse — when the Moon periodically blocks the Sun from view.

Aristotle cited many convincing arguments for a spherical Earth (which are relatively impressive deductions, especially for his time) such as:

1. The Moon enters and emerges from the Earth’s shadow during a lunar eclipse — only spherical objects produce a round shadow, and the Earth’s shadow can be clearly seen on the Moon during a lunar eclipse. If the Earth were a disc, there would be times when the shadow became a line or oval, which has never been witnessed.
2. When travellers go south from Greece, new stars become visible that are below the horizon — this can only be explained by a curved surface on the Earth, and the full dynamics of the celestial sphere (like the celestial poles and equator) if the Earth is a full sphere. In addition, this is the only explanation for why the height of the North Star changes when you move south.

There are a number of other clues that help us to prove that the Earth is round, which can be found in How Do We Know The Earth Is Round?

Aristarchus of Samos

Aristarchus of Samos (310-230 BCE) who, in hindsight, was a truly great thinker of Ancient Greece, suggested that the Earth is moving around the Sun. (As explained in Understanding The Night Sky this is an equivalent explanation for the observed movement of the Sun’s position against the fixed stars each night.) However, Greek scholars of his day, including Aristotle, rejected this notion.

Their arguments against Aristarchus’ were logically sound for the time. They believed that if the Earth was revolving around the Sun, that the patterns in the stars would change. If you walk around a town square looking outwards, the building positions with respect to the trees and landscape in the background change as well. Therefore, if the Earth was moving, then the patterns in the stars would shift slightly with the motion with respect to one another — but they don’t.

Aristarchus was right, while the great scholars of the time were wrong — even though their reasoning was sound! The issue is just that they had no understanding of the true scale of the universe. Just how far away those stars are.

The apparent shift of the direction of an object resulting from the motion/change in position of an observer is called parallax. The shift in the apparent direction of a star due to Earth’s orbital motion is called stellar parallax.

The Greeks made dedicated efforts to observe stellar parallax, enlisting the aid of Greek soldiers with the clearest vision — yet were not able to detect this shift. [1] The brighter (and presumably nearer) stars did not appear to shift when they observed them in the spring (when the Earth is on one side of the Sun) and again in the fall (when Earth is on the opposite side of the Sun).

This meant one of two things — that the Earth is not moving, or that the stars are unfathomably distant meaning that parallax would be minute as to not be detectable with the eyes. Even today the scale of the galaxy, interstellar distances, intergalactic distances, and that of the universe itself is literally beyond comprehension. We have units for these distances, so we can become familiar with them relative to numbers. However, the minds ability to truly comprehend these sizes is (in my opinion) limited.

Ancient Greek scholars and philosophers were not ready to take this leap of faith and imagination, and so remained with the geocentric (Earth-centered) view, which dominated Western thinking for another 2000 years.

Measurement of the Earth by Eratosthenes

In addition to theorizing that the Earth moved around the Sun (and testing the hypothesis) they also knew the Earth to be round, and were able to measure its size.

Eratosthenes was the first human ever to estimate the Earth’s diameter, with a surprising degree of accuracy, in 200 BCE. Eratosthenes (276–194 BCE) was a Greek scholar, who lived and worked in Alexandria, Egypt (and would have had regular access to the famed Library of Alexandria).

He used geometry, along with observations of the Sun. Specifically he used the fact that the Sun is directly overhead at noon in a town called Syene at a certain time of year, but not directly overhead at the same time in locations north or south of Alexandria. This allowed him to use trigonometry.

The Sun is so distant from us — and massive relative to the Earth — that all light rays that approach us are essentially parallel lines. In the diagram below, you can see how light source at position A, B, and C have paths that diverge more the closer the source gets to the Earth. The more distant the source, the smaller the angle between the rays. An infinitely distant source will have rays that travel along parallel lines.

While the Sun is not infinitely far away, it’s distance of 150 million km (1 AU) is far enough that they diverge from one another with an angle as to be negligible with the unaided eye. If every person on Earth who could see the Sun at a given time were to point at it, every finger would, essentially, be pointing in the same direction parallel to one another. [1] The same is true for the planets and stars. [1]

Eratosthenes had heard that on the first day of summer at Syene, Egypt (near modern Aswan) sunlight struck the bottom of a vertical well at noon (when it was at zenith) without leaving a shadow. This meant that the Sun was in a direct line from the center of the Earth through the tube of the well to the Sun. This also means that people, towers, an obelisk, or sticks erected vertically in the ground do not cast shadows on this day at Syene (when the Sun is at zenith).

However, at this same date and time in Alexandria, Eratosthenes noticed that the Sun was not directly overhead (at zenith) but slightly south of zenith — he was able to witness a shadow made by a column/ [1]

Investigating he realized that the angle of the shadow cast — the angle of the sun’s rays with respect to the vertical — was equal to about 1/50 of a circle (7°). [1]

Since the Sun’s rays striking the Earth are essentially parallel, then why would two rays on cities quite close to one another (Alexandria being about 5000 stadia north of Syene by ancient measurements) not make the same angle with the Earth’s surface?

In other words, why would a shadow be produced in Alexandia but not Syene just to the south?

Eratosthenes correctly reasoned that the only physical conditions that could explain this phenomenon was if the surface of the Earth had a curvature to it — that the Earth was round. Only this could explain how “vertical” did not point in the same direction in two different locations.

He also realized that he could use the angle he measured to calculate the circumference of the sphere of the Earth (and from there, its diameter also). The earliest estimate of the true size of the Earth.

From his measurement of the angle of the Sun’s rays with respect to the vertical (which could have been found using trigonometry by the length of the shadow and height of the column) he realized that this represented the angular distance between the two cities around the circumference. He determined it to be 7 degrees (1/50 of a circle) which means that the 5000 stadia between Syene and Alexandria is 1/50th of the way around the circumference of the Earth.

Therefore

\[ 50 \times 5000 = 250,000 stadia \]

He estimated the circumference of the Earth to be 250,000 stadia. Now, we cannot know exactly how accurate this estimate was because we don’t know how long the stadia was that he used.

If it was the common Olympic stadium, then his result is 20% too large. [1] If it was another measure for the stadium (about 1/6th of a modern kilometer) then his value was within 1% of the correct value for the circumference of the Earth: 40,000 km. [1]

Regardless of his accuracy, his method was sound. If he had the technology at the time to accurately measure the distance between Syene and Alexandria, he would have been incredibly precise.

His success using only shadows, sunlight, and trigonometry might be on of the greatest intellectual achievements of history; displaying the power of his keen mind, and only outdone by a man who came after him, Hipparchus.

[Full Article: Eratosthenes Measurement of the Radius and Circumference of the Earth]

Hipparchus and the Precession

Hipparchus of Nicaea — one of the greatest scholars and astronomers of antiquity (and in my opinion) in contention for greatest scientist of all-time with Einstein, Newton, Galileo, Copernicus, Maxwell, and probably Born, Dirac, and Marie Curie — known for his pioneering work developing star catalogues, organizing the stars by magnitude, creating a coordinate system, determining the true length of the year (365.25 days), and discovering precession.

His list of accomplishments is exceptional and, considering the times, nothing short of extraordinary.

Hipparchus was born in the city of Nicaea (located in modern Turkey). [1] He erected an observatory on the island of Rhodes c.150 BCE. Thus was during the time when the Roman Republic was expanding influence around the Mediterranean. [1]

At his observatory in Rhodes he measured the positions of the stars, incorporating around 850 entries into his star catalogue, including specifying their position with his newly-developed celestial coordinate system . [1]

When dividing the stars into apparent magnitudes according to their apparent brightness (how bright the star appears to be from earth) he divided the brightest stars into “stars of the first magnitude”, then “stars of the second magnitude” and so on in descending order of brightness. [1]

This system remains in use today, in a modified form, even though it is less and less useful for professional astronomers. [1]

Through his precise observations, and comparing that data with older observational data, Hipparchus was able to make a truly remarkable discovery:

The position of the north celestial pole had shifted its position in the sky.

The current pole star, Polaris, has not always been the pole star. Meaning that the axis of the Earth’s rotation has not always pointed in that specific direction (as we touched on in Understanding The Night Sky).

Hipparchus correctly deduced that this wobble of the Earth’s axis is a continuous motion; not just occurring during the period of observation covered by the records he had access to, but before — and after too.

If the North Celestial Pole is changing its position in time (tracing a continuous circle through the fixed stars near to the celestial pole) it must be the Earth itself that is doing the wobbling in a continuous, periodic motion that occurs concurrently with the Earth’s rotation on its axis and orbit around the Sun. [1]

This wobble occurs over a very, very long period of time — calculations estimate a full wobble to take around 26,000 years — which is why Hipparchus’ discovery is so remarkable. The change is so small, it is almost like noticing a single grain of sand out of place at the beach. Incredible.

We call this motion precession, more formally, precession of the equinoxes. If you have ever watched a gyroscope spin, while it spins with great stability at first, it gradually begins to wobble, the axis of the gyroscope tracing the path of a cone in the air as gravity is bringing it down. The is the same type of motion as precession.

This wobble occurs because the Earth is not an exact sphere. It bulges slightly at the equator, causing the gravitational pull of the Sun and Moon to create this wobble.

As a result, the changing direction of the Earth’s axis traces a great circle in the sky above the North (and South) poles. Today Polaris is the closest star to the north celestial pole (and will reach its most exact alignment near the year 2100). However, 14,000 years from now, Vega in the constellation Lyra will be the new North Star.

Ptolemy and the Ptolemaic Model of the Cosmos

Claudius Ptolemy (Ptolemaeus) was the last great astronomer of Roman times. He is especially important in the story of ancient astronomy because his great work, the Almagest played a founding role in the Renaissance and the birth of Modern Astronomy.

Ptolemy’s great work Almagest (meaning “The Greatest” in Arabic, which is what it was known as in the Islamic world of the Middle Ages) was an enormous compilation of astronomical knowledge. Ptolemy compiled this work around the year 140, which included lengthy discussion of past astronomical achievements — the work of Hipparchus in particular.

The Almagest remains today the principle source for the works of ancient astronomers (Hipparchus and other Greeks in particular) which have largely been lost to time otherwise.

Of Ptolemy’s own work, his most significant contribution was a geometric representation of the solar system that allowed the positions of the planets to be predicted for a given date and time. [1] Hipparchus was limited by the amount of data available at his time, so he was not able to solve this himself. He did, however, devote considerable energies to amassing a trove of positional data for the stars and planetae essential for the work of future astronomers.

Ptolemy used this material to produce his own cosmological model — which was wrong, but it did work reasonably well — and added his own new observations to the foundations laid by Hipparchus. It took another thousand years from Ptolemy before the great Copernicus was able to achieve the breakthrough, and attain the correct Heliocentric (Sun-centered) model of the solar system.

The reason why the motions of the planets in our solar system took so long to understand is because numerous compounding motions make the motions of some planets strangely complex. Furthermore, the ancients were operating on three incorrect assumptions (and a further factor which they couldn’t have even conceived of since it took Relativity of Einstein to solve) which meant their perspective could never resolve itself to truth. Their assumptions were:

• The Earth is stationary — the Earth orbits the Sun, and these orbital dynamics impact our perceived apparent motion of the planets.
• The Celestial Sphere turns around Earth — the Earth is revolving on its axis, not the celestial sphere, which is wrong, but, interestingly, does not impact their observations.
• The Wanderers trace circular orbits — the Greeks considered the circle was perfect, believing that the gods would not use a lesser shape for the motions of celestial objects.

The complicating factor is that we are watching the planets move, while the Earth is moving too, and revolving on its axis. Consequently, some planets appear to move forwards and backwards. This is like when you pass a car on the highway going 80 mph, while the car beside you is going 70 mph. Probably every child has had the dysphoric experience of looking up and thinking that they are stationary while the car they are passing is actually moving in reverse! (Neither is true, but for a second, that’s exactly what it looks like).

The image above shows how the orbits of two planets — one on a closer orbit and another further out — interact with one another. In this case, the orbit of the Earth and Mars. They are both travelling in the same direction, on nearly the same plane, but the Earth orbits faster, taking less time to complete a single orbit (since it is a shorter path). You can see clearly how the way the planets line up in their orbital position changes their relative geometry, and where Mars appears to be located against the stars behind.

Even though both Earth and Mars travelling in the same direction at relatively consistent speeds, in our sky, it appears that Mars temporarily changes direction! This was a major problem for astronomers for thousands of years, since we know of no natural physical forces — aside from the magic of gods, angels, demons, or the fae — that could explain this.

Planets normally move eastward across the sky for weeks and months in their orbit. However, when in positions B to D, the Earth begins to “overtake” Mars in orbit, passing between Mars and the Sun. Like a car passing another on the highway, it appears to temporarily be moving in reverse — westward in the sky rather than its normal eastward motion. Then as the Earth moves toward point E Mars begins to continue its normal eastward motion.

This temporary westward motion in the sky is called retrograde motion. Today this makes a great deal more sense because we know what the solar system looks like, along with the distances, speeds (and size of planets) involved.

Hipparchus, Ptolemy and others did not have this advantage. Moreover they were operating from false assumptions, and were trying to explain these phenomena with the wrong pieces for the puzzle. Ptolemy actually faced a far more complex problem trying to explain these motions from the standpoint of the Earth being stationary. [1] Nonetheless, it is because of their efforts that Copernicus, Galileo, and Kepler were able to find the solution.

In addition, the Greeks believed in the perfection of circles, so Ptolemy had to use circles alone to construct his model. [1] Using circles, and assuming that the Earth was stationary with the Sun and planets orbiting around it, . From a modern perspective, this makes zero scientific sense. It is gloriously inaccurate. Yet, with that being said, his model was truly brilliant, and an outstanding scientific achievement.

Given the facts, technologies, and mathematics available, in conjunction with the cosmology of the time, I find Ptolemy’s solution to be rather elegant. Wrong, necessarily complex — but elegant nonetheless.

Ptolemy solved the problem of explaining this retrograde motion using a small orbit called an epicycle — essentially small periodic circular orbits on a greater circular orbit around the Earth. As if the planets got caught periodically in little whirlpools in space, that caused them to orbit around a point in their orbit (called a deferent) before continuing along their normal path.

With the right combination of speeds and circumference for the epicycle, Ptolemy actually succeed fairly accurately to replicate retrograde motion with his model.

As it turns out, the planets orbit the Sun inscribing paths that form ellipses — flattened-circles, or ovals — not perfect circles. No matter what Ptolemy did, he would never have been able to perfectly represent their actual behaviour using perfect circles.

To get as close as he did, he had to center the deferent circles on pints some distance from Earth, and further introduced uniform circular motion around yet another axis called the equant point. It is famously reported that when Alfonso X, King of Castile, had the Ptolemaic Model explained to him, he remarked:

“If the Lord Almighty had consulted me before embarking upon Creation, I should have recommended something simpler.” [1]

It is well known that one of the main driving forces of ancient astronomy was divination. Knowing the positions of the planets and sun on a given date and time is not that helpful for anyone — except for navigators who might need these positions to arrive at a destination, or, far more importantly, for astrologers to write accurate birth charts for understanding personality, vocation, life, and predict the future. We simply don’t know if Ptolemy was trying to solve this problem as a truth-driven scientist simply to understand nature, or if there were other motives (including that of powerful nobility and public desire).

Whatever the case was, Ptolemy’s mathematical and scientific genius is on full display with this impressive scientific achievement. The Ptolemaic Model of the cosmos was accepted (with some modification) as authoritative in the Muslim world and later Christian Europe where it dominated for more than a thousand years.

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