In today’s world of telescopes, man-made satellites, and animated videos of flying among the planets, we take for granted the three-dimensional visual space of our solar system and beyond. But this understanding does not come from the senses, and is possible only with a physical hypothesis of the underlying motion. Let’s take a trip back in time, to the world of the astronomers of ancient Egypt and Greece. What did their observations show them?
It is difficult today to have the opportunity to truly see the splendor of the night sky, free from urban light pollution. In the darkness of a moonless night, the human eye can become sensitive to even very faint light, and the starry sky is wonderfully full. Let your eyes adjust to the sky of 2000 years ago.
Now, let’s have a seat, and watch the stars as we talk. During the night, this is what we see:
The stars move &emdash; some more, some less, and one hardly at all. It appears just as though we are watching the rotation of a very large sphere around us. We’ll watch through the night again, this time with markings to indicate the circles the stars seem to travel upon. Based only on this observation, the simplest explanation is that we are surrounded by a sphere of stars, which moves around us just as the sun does.
Now, we’ll turn around and face away from that star that didn’t move, and watch the night several times in a row. Each night looks almost the same, but there are differences. See what you can spot.
I bet you noticed that the moon changed. The first night, it was a quarter moon, and was already halfway through the sky when dusk made it visible. The side of moon that is bright, is the side facing the sun. One week later, it is full, and comes into view nearly opposite the sun, rising as it sets. During each night, the whole sky moves to the right as we face south, but the moon appears to be moving more slowly. The moon doesn’t keep up with the stars in its neighborhood, and lags behind them. Even during one night, it moves slowly to the left, although this is hard to see. The moon cycles through its phases again and again, once every 29-1/2 days, creating the period of time we call the month. It’s not a coincidence that the words moon and month are similar: they have the same origin!
Now let’s look for another difference, one that’s harder to spot. We’ll watch sunset for the first night, and then one week later. Did you notice any difference?
If we consider the sun as our point of reference, the stars seem to move to the right! Or, if the stars are your reference, you could say that the sun moves to the left, that the sun does not move as quickly as the stars every night, and lags behind them, like the moon, but much more slowly.
Now, this is a time-lapse sequence of a month of sunsets. The sun is moving against the stars along a particular path. Obviously, unlike the moon, we can’t directly see which stars the sun is near, since we can’t see any stars when the sun is out. But, if we had a star map, created by our minds, and recorded or devoted to memory by some means, we could map out the path of the sun by relating it to the stars we do see.
Hipparchus famously created the first star map, recording the position of 1000 stars in the second century BC. With such a map, it was possible to map out the position of the sun against the stars. The sun’s course along the stars forms a circle, and returns to its beginning over the course of one year. This path is called the ecliptic, and there are twelve special constellations of stars along this path &emdash; the twelve constellations (or signs) of the zodiac. In case you’ve wondered what your “sign” is, it means the constellation the sun is in when you were born. When such work was first done in the Mediterranean, the sun was in Aries on the day of the spring equinox, the start of the year. This name has stuck, and the position on the ecliptic where the sun is located on the first day of spring continues to be called the first star of Aries.
The spring equinox actually moves along the ecliptic, in a 26,000 year cycle, such that it now occurs in the neighboring constellation Pisces, rather than Aries, and is nearing Aquarius. So if you look up what “sign” you are, and compare that to where the sun actually appears on your birthday, it is one sign off! The 26,000 year cycle was discovered by Hipparchus.
So, back to the motions we observed, the overall motion of the entire heavens (going to the right here) is called the first motion, while the relative motion of the sun and moon to the left is called the second motion. This second motion, from night to night, can only be seen by the mind, not the eyes. Actually, even the first motion of the stars is so slow that you don’t really see the stars move during the night (unless you’re using a telescope). Instead, you notice that their position with respect to the horizon has changed over time.
Now, we’ll look for another change in the sky from night to night. We direct our attention on a very particular, bright star in the sky. Now, we’ll look at this exact patch of sky the next night, and the next and the next. Over the ten days shown here, this star is wandering from place to place in its second motion. It is from the Greek word for “wanderers” that we have the word “planets.” In this case, the wandering star you see here is known as the planet Mars.
This slow motion could only be studied in a thorough way by a civilization making frequent and systemic observations of the sky over many years. The bewildering motions of these few wanderers, only five visible to the naked eye among a sea of fixed stars, were (and continue to be) the cause of wonder, curiosity, research, discovery, and not infrequently, exasperation.
How were they studied? Earlier, I mentioned Hipparchus’s catalog of 1000 stars. How do you think he measured and mapped the locations? How do you draw a sphere of stars on a flat piece of paper or stone? Or if you are to record positions as numbers rather than a drawing, what are your reference points? How would you keep track of the motion of one star among many, over a prolonged period of time?
Let’s now watch Mars over a number of years, as it traces out its path. In this video, I have cheated by removing the earth and its atmosphere, so we can see stars during the daytime without the sun washing out the entire sky with its light. In reality, we wouldn’t be able to see Mars when it is very near the sun, because we can’t see any stars during the day.
As you saw, the speed of Mars changed dramatically -- it sped up, slowed down, and even went backwards. It stayed near the ecliptic, the course traced out by the yearly motion of the sun.
What are some of the basic observations to make? First, we can measure how often Mars makes its loop. We could also see, on average, how long it takes Mars to return to the same position against the stars. These observations themselves, do not tell us what is physically happening out there. What would you think, if you were living two millenia ago? Are the stars moving on a sphere? Is Mars? Is it closer or farther than the fixed stars we measure it against? How far away are they? From these observations, how would you suspect that the moving stars (the planets) are bodies like our earth, or that the earth spins, or that the planets move around the sun?
In the next video, we’ll jump ahead to Kepler’s day, and compare three hypotheses for the motion of the heavens.