Astronomy: EARTH-051Historical Highlights in Astronomy

Discover the highlights of historical astronomy, including the theories of the Greeks, the discoveries of important historical astronomers, Copernicus' heliocentric model, and the Kepler model.

Greek Astronomy

This video walks through some of the important theories of early Greek astronomers.

Video Transcript

0:00 Let's talk a little about how we came to know what we know today. Now, to begin the story of astronomy, we could easily take it as far back as the earliest cavemen, who looked up to the sky and wondered what was up there. But for the sake of brevity, we're going to just limit our discussion for the time being to the ancient Greeks.

0:19 It turns out there are a lot of ideas the ancient Greeks popularized that still hold to this day, and many, of course, that have since been discarded.

0:28 We begin our story with Aristotle, who was a major proponent of the idea of the so-called "geocentric universe." That is, the Moon, Mercury, Sun, and all the planets all were revolving around the Earth, with the Earth at the center of the universe.

0:46 This was a very easy idea to come to. After all, everything in the sky appears to be moving around us, so it just seemed to make sense that so did the planets, and perhaps the stars themselves were just fixed upon the sky.

1:00 Now, one thing that Aristotle and indeed most of the ancient Greeks believed was that the Earth was, in fact, round. The reason why is because they observed lunar eclipses. If you notice the way the Earth's shadow passes along the face of the Moon, you'll see that the shadow is curved. It turns out that the only object capable of casting a circular shadow is a spherical object.

1:27 So for this reason, and also for some other lines of evidence, the Greeks well understood that the Earth was round. And today, thanks to the internet, more people believe the Earth is flat than at any time since the 1400s. Yay, progress.

1:42 Anyway, it turns out that the idea that the Earth was at the center of the universe wasn't 100% shared among all Greek thinkers. Aristarchus of Samos, for example, gave us the first known heliocentric model. In other words, he proposed that the Sun was the center of the cosmos and everything revolved around the Sun.

2:04 He published his ideas in something called On the Sizes and Distances, and he used lunar eclipses to work out the relative distances and sizes of the Earth, Sun, and Moon. He also proposed that the stars were distant suns and that the universe was vast. However, his idea that the Sun might be at the center of the universe was largely rejected.

2:28 The reasoning went something like this: If the stars were at varying distances from the Sun, and if the Sun were at the center of what we now know today to be our Solar System, and Earth were to be traveling around in an orbit, from one location we should be able to look at a relatively nearby star and see its apparent image projected onto the background stars.

2:53 That means six months later, we should be able to make a similar observation and see that same star, this time with its position shifted with respect to the background stars.

3:04 This phenomenon is called parallax, and it's something you and I are familiar with all the time. We have two eyes, and our brains constantly take the information from each eye and infer a distance to some object, which is useful for not running into everything all the time.

3:24 Well, what happened was that the Greeks decided to test this idea at the time by enlisting their very best visually acute individuals, even pulling soldiers from their ranks who specialized in scouting. They looked at the brightest stars, hoping to detect a parallax. Unfortunately, no parallax could be detected, and therefore the Greeks largely gave up on the idea of heliocentrism in favor of geocentrism.

3:55 Another Greek thinker was Eratosthenes of Cyrene. He actually made the first calculation of the Earth's circumference. His calculation went something like this: There was a column in the city of Alexandria in what is now modern-day Egypt, and there was also a well at Syene. The distance between the two was about 5,000 stadia.

4:22 Eratosthenes learned that the incoming sunlight on the day of the summer solstice, the Sun would be directly overhead at Syene, meaning that from the bottom of the well, the Sun would be directly visible. At the same time, Eratosthenes learned there was a shadow being cast at the column in Alexandria. With a little geometry, he reasoned that the shadow cast made a 7-degree angle, meaning the sunlight was 7 degrees tilted from the vertical at Alexandria.

4:58 Therefore, the opposite angle must also be 7 degrees. If we were to draw an imaginary line from both these locations to the center of the Earth, that angle should be 7 degrees as well. Now, 7 degrees is equal to 1/50th of a circle, and since the distance between the two cities is 5,000 stadia, then 50 times 5,000 stadia gives you 250,000 stadia.

5:26 Depending on what the actual value of a stadia was relative to today's units, it turns out Eratosthenes may have come very close to measuring the actual circumference—at worst, within about 20%, but he may have done as well as 1%. So it's a remarkably accurate result using just some simple geometry and reasoning.

5:49 This brings us to Hipparchus of Nicaea. His work involved making a detailed catalog of stars and also measuring the first brightness system, something we call the Apparent Magnitude, which is a system we still use today, albeit with some modifications.

6:07 Hipparchus' magnitude system worked like this: as we see the Sun set toward the late afternoon and early evening, the first stars come out, and he assigned these stars magnitude 1. They were, after all, the brightest stars and therefore first magnitude, since they were the first stars seen after sunset.

6:28 A little while later, more stars would come out, so he assigned these magnitude 2 for the second brightest stars. As more stars emerged, they would be designated third magnitude, fourth, fifth, and so on until reaching about magnitude 6. Hipparchus not only catalogued the stars by their brightness, but he was very careful to measure their positions in the sky.

6:55 He also compared his measurements of the positions of stars to those of the ancient Babylonians and Mesopotamians. He discovered something remarkable. Instead of viewing things from the Earth, let's imagine a viewpoint south of the Earth's south pole, looking at a wide-angle view of the sky.

7:22 We have the north celestial pole at the top, and the modern-day Ursa Minor with Polaris, our present-day north star, at the end of the handle. In the lower left, we see the intersection of the celestial equator (in red) and the Sun's path through the sky, the ecliptic (in green). To make things more visible, let's choose a date when the Sun happened to be located at the Vernal Equinox.

7:52 It looks like it’s close to Pisces, but believe it or not, in Hipparchus' era, the Vernal Equinox was actually considered to be in Aries. Hipparchus made very careful measurements of the positions of these stars, and when he compared his measurements to those of the ancient Babylonians and Mesopotamians, he determined that their positions were a little bit different. In fact, every star in the sky was systematically shifted by a few degrees.

8:26 Hipparchus was able to intuit that it wasn't the sky moving around, but rather that the Earth itself must be wobbling like a top. This wobbling is called "precession." We now know that the Earth has a 26,000-year precession cycle, and its axis points to different locations in the sky over time.

8:54 This has interesting implications. If we could wind the clock back to 3,000 BCE, the north celestial pole would be near the star Thuban in the constellation Draco the dragon, not Polaris. Likewise, the Vernal Equinox would be located in the constellation Taurus.

9:17 Fast forward 1,000 years, and the north celestial pole now precesses away from Thuban, and the Vernal Equinox makes its way toward Aries. By 1,000 BCE, we're farther from Draco in the north and farther from Taurus at the Vernal Equinox; we are certainly in the constellation of Aries.

9:38 To this day, the Vernal Equinox is still sometimes known as the "first point of Aries," representing the location of the Vernal Equinox when Hipparchus was doing his work.

9:54 Around the turn of the millennium, the Vernal Equinox had moved firmly toward Pisces, and by 2000 or so, the north celestial pole was almost exactly where Polaris is today.

10:13 As the Earth continues to wobble along its precession, over 26,000 years we will no longer have Polaris as the north pole star. Instead, we'll have Vega in a few thousand years before eventually coming back again toward Thuban. We'll return to an orientation of the north celestial pole similar to when the ancient Egyptians built their pyramids.

10:47 At the center of the pyramid are the King's burial chambers, with two air vents—one facing south and one facing north. The position and angle of the air vent was carefully chosen so that looking through it from inside the burial chamber would reveal the star Thuban, the north star at the time the pyramids were built.

11:19 This way, the king could gaze upon the circumpolar stars and watch them revolve around the north star, Thuban, for all eternity. This was a very sacred idea to the ancient Egyptians.

11:38 Thuban was the north star of its day, about 3,000 years before the common era, and the stars circumpolar surrounded Thuban, which was almost at the location of the north celestial pole at the time.

11:58 Most of the works of the ancient Greeks, Babylonians, and Mesopotamians were lost in the great fire of the Library of Alexandria. However, Claudius Ptolemy was careful to compile much of that work and published his ideas in something called the Almagest, or "the Greatest," paying tribute to the greats who came before him.

12:21 Ptolemy was able to popularize the idea of the geocentric model. This was the prevailing idea of the ancient Greeks, as it seemed to make the most sense despite a few dissenters. Ptolemy also gave the first explanation for something called "retrograde motion," and this idea would dominate for over 1,500 years, well into the next millennium.

12:50 Retrograde motion is simply the apparent shift in the position of the planets. Normally, planets move eastward, or prograde, but once in a while, they appear to backtrack—this is retrograde motion—before resuming their prograde motion again.

13:16 For everything to rise in the east and set in the west, it would be reasonable to conclude that the Earth was at the center of the cosmos. But retrograde motion was an anomaly that didn't make sense. To explain it, Ptolemy introduced the concept of the Epicycle—an invisible circle that carried Mars, and the Epicycle itself revolved around something called a Deferent.

13:56 It was this Epicycle moving about the Deferent that would create the apparent retrograde motion. You can imagine yourself on Earth looking at Mars. As you watch Mars, it appears to move in prograde motion before executing a slight zig-zag back and forth in the sky, giving us retrograde motion.

14:16 This was a good first-order approximation, but the problem was that the Epicycle model alone was not accurate enough to predict exactly when the next retrograde motions would be. To solve this, Ptolemy introduced a modification: the Equant. The Earth was still at the center, but now the Epicycle, the Deferent, and so forth were centered on an offset point called an Equant.

14:46 This helped to make the retrograde motion of Mars and all the planets more timely and accurate—do a slightly better job of predicting when these retrogrades would occur-- but even then, sometimes additional modifications were required, such as adding an additional Epicycle to the Epicycle. Over time, things got quite complex, and while the Epicycle model did a good job of predicting retrograde motions for about 1,500 years, it was also a little messy.

15:31 This complexity allowed some people to begin to think that maybe there were alternatives, and maybe some of the ideas of the geocentric model should be revisited.

Attribution

Launch Pad Astronomy. (2017, Oct 11). Ancient Greek Astronomy [Video]. YouTube. https://www.youtube.com/watch?v=BMPaq8sf8Ko

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