After watching the Nova on Galileo (again) a question occurred to me -
Back when it was believed that the Earth was at the center of the universe what exactly did people think a year was a measure of? What cycle did they think was repeating after the sun went round the Earth 365 and ¼ times?
The seasons and equinoxes, I think - apparently there is evidence that many ancient monuments are (or were) aligned in such a way as to make it possible to astronomically determine key seasonal points.
In the pre-Copernican cosmos, the entire heavens revolved around the Earth once each day. The Sun by itself additionally revolved around the Earth once per year, moving against the background stars and moving from North to South and back causing the cycle of the seasons.
Nowadays we describe these separate motions as rotation about an axis and revolution around the Sun. Which is another way of saying, we on the surface revolve around the center of the Earth once per day and around the Sun once per year.
The math works equally well either way. It doesn’t matter whether the earth revolves around the sun in a year or the sun revolves around the earth in a year. For all daily purposes it came out the same, and the math for predicting future events, like eclipses, worked quite well too.
It’s only when you have to account for the future movements of all the other bodies in the sky does it become more sensible to make the assumption that the earth revolves around the sun.
So people thought a year was the length of time for the sun to return to its place against the rest of the sky. A perfectly good and mathematically justifiable assumption.
Right, but, nobody did it like that. At least, I don’t think so. If you assume that the celestial sphere is fixed and the Earth rotates once per sidereal day, then what you say is true. However, I think most pre-Copernican models had the Earth non-rotating, and the celestial sphere rotating once per sidereal day. In this system, as the OP rightly notes, the Sun exhibits two different periodicities - the solar day, which must be its orbital period, and the solar year, which is left unexplained.
Well, there are a number of astonomical phenomena that measure a year. The “sidereal year”, for example, is the amount time that passes before a repetition of the pattern of background stars (assuming geocentricity). So when the constellation Aries appears at a certain place in the sky, and then reappears at the same place, a sideral year (only slightly longer than a solar year) has passed.
That’s a sidereal day, which is slightly shorter than a solar day. A sidereal year is the time it takes for the sun to return to the same place with respect to the stars. It’s different from a solar year because of the Earth’s precession, and you’re right, it is slightly longer. It results in a third periodicity, of 26000 years, before the sun and the constellations synch up again.
Looking at the heavens, we observe two basic sorts of motion, 1) the rotation of the whole sky and everything in it, from east to west, once a day, and 2) the much slower motion of the sun, moon, and the five naked-eye planets against the background of the fixed stars, in the opposite direction (most of the time–see comments about retrograde motion below), over longer periods that are different for each. (Ptolemy referred to the first as the “motion of the same” and the second as the “motion of the other.”) Needless to say, we now know that the motion of the same is caused by the earth’s rotation on its axis.
The moon gives the most obvious indication of the motion of the other, because it is fastest. Every night it rises about 45 minutes later than the night before, meaning it has moved east some distance. If you watch it over the course of the evening, you can observe it moving west to east in relation to the stars nearby. (All the time, of course, it is also being carried east to west with the rest of the sky. Think of it like walking backwards in a train that is moving forward.) In the course of a month (by definition), it will have completed a complete circuit of the sky and be back to (about) where it started.
The planets do this, too, but it takes quite a bit more patience to observe their eastward motions. But the ancients had lots of time on their hands, no TVs or Web messge boards, and a much better view of the night sky than we do today. So they noticed.
As I said, the sun exhibits the motion of the other, too. Of course, unlike the moon and planets, you can’t directly plot the position of the sun against the background of the stars because YOU CAN’T SEE THE STARS DURING THE DAY! But once you have accurate star maps of the whole sky you can infer the sun’s position by noting the stars directly opposite it. The sun’s path through the stars is called the ecliptic, and over the course of exactly one year (by definition), it will return to its starting point. This is the cycle the OP was asking about. It is, of course, the result of the earth’s annual revolution about the sun.
The ecliptic is NOT the same as the celestial equator (a projection of the earth’s equator onto the sky). It is angled at 23.5 degrees from the equator. If you think of two concentric rings joined at two opposite points and angled (i.e., having one shared diameter), you will see they create four natural dividing points: the two places where they meet, and the two points 90 degrees away, where they are farthest apart. The meeting points are the spring and fall equinoxes, and the other two are the winter and summer solstices. As Mangetout mentioned, these key points also served to mark the period of the year.
Now, even though you didn’t ask, as for retrograde motion, if you carefully note the positions of the planets every night over many years, you will see something strange. Some of the planets (specifically Mars, Jupiter, and Saturn) will occasionally appear to stop in their slow eastward “motion of the other,” and go “backwards,” to the west for a while, then stop and go eastward again. Over the course of several weeks or months they trace out a little loop against the stars.
Under the geocentric world view, this was very difficult to explain and quantify, and required elaborate mathematical mechanisms like epicycles, eccentrics, and equants (don’t ask). Under the heliocentric view it is much more simply explained as the earth “passing” the planet, like one car passing another on the inside of a turn. Although both are moving forward, to the people in the inside car, the outer one appears to be going backwards.
Mercury and Venus do not exhibit retrograde motion because they are between us and the sun, hence we never “pass” them.
I hope this is helpful.
IIRC, the concept of a year was based on the lunar cycle rather than the solar. Twelve full moons and you’re back to the season you started at. Very useful for agricultural civilizations.
That’s a good explanation of a major problem with geocentricity, commasense, but one quick thing:
Mercury and Venus do have retrograde loops when they pass us. If we’re moving retrograde with respect to them, they’re moving retrograde with respect to us. The problem is that these loops take place while passing in front of the sun.
I remember seeing a special photograph that would be relevant to this discussion. A photographer had placed a camera facing south through one of his windows and exposed a film for a few seconds at high noon every 30 days. The imaged showed twelve suns in a rough figure-eight pattern, caused by the seasonal shift.
Even in pre-Copernican times, observers could have noted that the noontime sun progressed in a regular, predictable pattern, and thus could have judged the span of a solar year.
Surely you mean thirteen full moons.
Ironically, the people who though the sun went around the earth were not, in fact, incorrect. Its entirely accurate, if confusing, to place the earth at the center of the cosmos. Strange but true.
(365.25 days/year) / (29.5 days/full moon) = 12.4 full moons per year
What was more obvious (and more important) to the ancients were the annual cycles of nature. If you didn’t plant your crops at the right time, they didn’t produce. So it was crucial to know what time of year it was. Noticing correlations with the celestial events made it possible to plan your agriculture more accurately. Many early calendars, like the Jewish one, were based on the Moon’s cycles rather than the Sun’s. Then you had to adjust periodically to get back in sync with the solar year. The Jews did this by inserting an extra month as necessary. There is a document from the 1st or 2nd century AD which declares the need for an extra month, since the lambs were not yet big enough to celebrate Passover. This shows how closely the calendar was tied to agriculture, even at a relatively late date.
Are you sure about that, Achenar? The apparent motions of the inner planets are different from outer ones because they never leave the vicinity of the sun. The inner planets reach “greatest elongations” on either side of the sun, (points tangent to their orbit from our point of view) and start heading back in toward the sun. But I don’t see why the parity of retrograde motion you’re suggesting would occur.
To use my car analogy, when a faster car passes a slower one, the slower one looks like it’s going backwards to the faster one, but it is not the case that the faster one looks like it’s going backwards to the slower one.
I’m not saying you’re wrong, I just don’t see how it would happen.
Yeah, I’m sure, though it took me a while to realize it, and I had to check it with some simulation software to be sure. I tried to find a planet chart to verify it, and this (from here) was the best I could do. If you look closely you can see Mercury go retrograde through most of February (and again in June and October), and Venus go retrograde from early March through late April.
The Jewish calendar is properly known as lunisolar – the months synch with the lunar cycle, and the years synch with the solar year by using leap months.
Contrast this with the Islamic calendar, which is pure lunar and has no leap years. Its months keep shifting with respect to the solar year, and thus with respect to the seasons.
In terms of the OP, it’s important to note that the ancients did not consider Venus and Mercury to have retrograde motions, in the way that they saw it in Mars’ orbit.
In the dulcet tones of Isaac Asimov (from “The Seventh Planet” in The Solar System and Back)
Mercury has the same problem, except more so.
It depends what you mean by “in the way that they saw it in Mars’ orbit”. Retrograde motion is simply motion from east to west against the background stars. Mercury and Venus do exhibit such motion when they are racing through inferior conjunction, and the ancients knew this and used “retrograde motion” to describe it.
To be sure, this isn’t exactly the same as the retro motion exhibited by the outer planets. The “outers” go retro when opposite the Sun; the “inners” when in conjunction with the Sun. And the retrograde and prograde motion of the “inners” combine to keep pace with the Sun so that they never drift to the opposite part of the sky.
But the principle is the same, and Ptolemy accounted for both types of retrograde motion in the same way, with epicycles and deferents.
