I was just reading about the sun dagger in Chaco Canyon. It’s a pretty cool tool for determining seasons which I assume was used for planting crops and such. But without such a device, what was their method for calibrating the seasons in the first place? And why not use that method instead of carving rocks on the top of a butte?
Link to the sun dagger site: Chaco canyon, Mexico
Typically, some sort of astronomical observation was used. If you think about it, it’s not very difficult to notice a correlation between the sun, the moon or the stars and seasonal regularities.
I don’t really understand the question. Seasons aren’t a fixed thing like solstices and equinoxes they come and go at different times.
Plants manage to recognise them pretty well without benefit of a brain, so I am sure that primitive man could stick his head out of the cave and see that spring was beginning or whatever. As the days begin to get shorter and the nights colder, he would know that winter was coming.
They might also have used rocks for weather forecasts:
If the rock is wet, it’s raining.
If the rock is swinging, the wind is blowing.
If the rock casts a shadow, the sun is shining.
If the rock does not cast a shadow and is not wet, the sky is cloudy.
If the rock is not visible, it is foggy.
If the rock is white, it is snowing.
If the rock is coated with ice, there is a frost.
If the ice is thick, it’s a heavy frost.
If the rock is bouncing, there is an earthquake.
If the rock is under water, there is a flood.
If the rock is warm, it is sunny.
If the rock is missing, there was a tornado.
If the rock is wet and swinging violently, there is a hurricane.
If the rock has white splats on it, watch out for birds.
Wheel of the Year. The year starts on Samhain (November 1st) and the solstices and equinoxes are the middle of the season.
Makes more sense to me.
Note that there are two different types of astronomical observations that can be used, those based on the Sun or those based on the stars. The Sun Dagger in Chaco Canyon is one example of a sun-based method, though you can go simpler with things like “When you stand in the middle of the village and the sun rises right over that mountain with the funny-shaped jagged bit, it’s time to plant”. By contrast, the Egyptians used a star-based method to predict the flooding of the Nile: When you could first see Sirius in the fall, the floods were soon to come.
In the short term, either method will work equally well. In the long term, however, star-based methods will fall out of synch with the actual seasons, at a rate of about two weeks per thousand years, while (absent climate change) the sun-based methods will stay in synch.
Traditionally the need for good calendars was driven by agriculture. Because there are a non-integer number of days in a year, any calendar with the same number of days in each year is going to drift. It has only been the last thousand years or so that people have worked out the required corrections.
It was very common for early calendars to be lunar based, which really screws up trying to track the seasons.
It is fairly important to work this out independent of weather observations, specifically because weather variability can often disguise the coming of the season. A warm winter may trick farmers into planting early only to have seedlings destroyed by frost when the weather turns typical. Similarly, a cool spring might cause them to delay planting and run out of growing season before the fruit is ready to harvest.
The required astronomical observations are not too hard to work out. A given constellation will be on the horizon at sunset every vernal equinox. Anyone who was keeping track would notice that Polaris doesn’t move. If you mark that direction, then you might easily notice that sunrise and sunset are always at equal angles (almost!) to true north. You might also notice that when the shadow of a vertical stick is the shortest, it is pointing the same direction as Polaris. Clearly something special about that direction.
Right angles seem to be almost universally used in architecture, so it wouldn’t be hard to make the leap that there was something special about the days when sunrise and sunset were at right angles to true north. If you build a square or rectangular building with one wall running N-S then you have two walls that will serve to mark the equinoxes.
Solstices are a bit harder to judge. The rate of change of the length of a noon shadow is very small near the solstices, and very careful measurements would be required. Similarly the extreme southward or northward position of the sunrise is pretty constant a few days before and after the solstice.
It is worth noting that it is fairly rare for claims of ancient observatories to include solstice markers. If they do, then the observatory claim is much more likely to be valid, because all that is needed for equinox sightings is a true east-west line, and there are plenty of other reasons for that to happen. A solstice marker will be in a direction peculiar to that latitude, and the change in sun position is so small that it needs to be usable in a pretty precise way to nail down the solstice to a particular day.
But they come and go within a fairly narrow range of variation, maybe a couple of weeks. If you’re an agriculturalist, you need to be able to predict when to plant. If you’re in the temperate zone, you may not be able to predict within less than a few weeks when the last freeze will come, but you may be able to predict with great accuracy the latest date that it’s possible for a freeze to occur in your location. The last snow may come in March or early April, but by late April there is close to zero chance of snow. So you calibrate your planting to when you have minimal risk of losing your crop. For that you have to know the date, and you can determine that by calculating from the equinox.
People back then were intimately in touch with the cycles of nature and weren’t any dumber than us today; also we are practically wired to notice patterns.
On top of that, being able to predict patterns like the seasons would had been seen as a demonstration of divine insight by ancient people; that would place a strong incentive in, shall we say entrepreneurial people, to put every effort into it.
So no wonder that some of them figured out the significance of the motion of heavenly bodies, perceived that as a sign of cosmological design and more often than not ended up ritualizing the whole concept.
Yea, anyone that spends a non-trivial amount of time at night outside will notice pretty quickly that different constellations are up during different seasons, and that they shift to rise earlier each day.
From there its a pretty short jump to systemizing the idea, and say that when star x starts to rise at or after sunset, its time to go plant crop y.
Its actually obvious enough that it seems puzzling that so many calandars used a lunar calendar for date keeping, since they presumably had to keep track of the seasonal year for agriculture anyways. But I guess the moon has the advantage that you can just glance at it and tell to an accuracy of a day or two where you are in the month.
The heliacal rising of stars was used in many cultures around the world to figure a relative precise date.
For a time, the Ancient Egyptians used the heliacal rising of Sirius to predict the Nile flood. No doubt other risings were tracked.
So-called medicine wheels were used by several peoples of North America. Some, not all, of these were aligned for the heliacal rising of key stars, esp. during the summer/early fall.
Polynesian people were also keen on the rising/setting positions of stars for navigation, but they also could be used to judge the right season for favorable winds.
But, in most cases, such an accurate date (down to a day or so) is not really needed. Just noting which constellations are visible where at sunrise/sunset is good enough for most purposes.
If the rock is wet on one side, a dog was here recently.
Several people in this thread have suggested a similar idea. I think I wasn’t clear enough in my OP. Let me try again using the above example. I understand that any two fairly stable objects can be aligned with the sun somehow and it will happen next year and the next and so on. And if that alignment is at a solstice or equinox then you have a reliable indicator of when to plant. But my question is, without that device how did they know when a solstice or equinox was? In other words without a solstice indicator how would you know when to stand in the middle of the village and look at the mountain with the funny-shaped jagged bit to take that bearing?
The time between planting and harvesting seems too long for guess work or trial and error.
ETA: In other words, how did the guy chipping the spiral at Chaco know he was chipping on the day of the solstice?
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Well, first of all, the solstices and equinoxes aren’t themselves the dates of interest, except possibly for ceremonial purposes. The spring solstice probably isn’t the ideal date to plant on, and the fall solstice probably isn’t the ideal date for harvest. I imagine that it started with planting when the sunrise was at that rounded hilltop, and discovering that that was a little too late, and then trying when the sunrise was at that outcropping that looks like a man’s nose, and that was too early, before settling on the funny jagged bit.
That said, suppose you do want to know the exact dates of the four-corner days for some ceremonial purpose. For the solstices, accuracy is easy, but precision is hard. The sunset point will move along the horizon from night to night, and will move quickly near the equinoxes, then slow down and come to a stop at the solstice, then turn around and go back again. When the sunset point is as far north or south as it ever gets, you can be sure that that’s the solstice: That’s what I meant by “good accuracy”. The problem is that, since it’s moving slowly around then, it’s hard to get it more precise than within about a week ago. The best way to get more precise is to pick some point on the horizon that’s a little less extreme, find the two dates when it crosses that point coming and going, and split the difference.
For the equinoxes, there are two methods you can use. You can either just use the points directly east and west (Kevbo explained how to do that), or you can count the days from winter solstice to summer solstice, and split that difference. Due to both the eccentricity of the Earth’s orbit and the irregularities of the horizon, these two methods will give you slightly different results (the Chaco peoples apparently used the day-counting method). On the other hand, while pinning down the equinoxes isn’t as accurate as the solstices are, it’s more precise: If you mark an equinox point, and throw a party one year on that date, and then party again the next year, you can be certain that your two parties are precisely a year apart, even if the party day isn’t actually the true equinox.
**Colibri **did a good job of answering this. It’s precisely the irregularity of climate that mandates the need for astronomical observations (or just day counting) for the predictions needed in agricultural.
It’s interesting how many very ancient structures (e.g. Stonehenge) seem designed to observe, predict, or celebrate events like the solstices.
Kevbo’s post was excellent, but this sentence is wrong. Eudoxus, who lived almost 2400 years ago, is credited with being first to notice that 365.25 is a better approximation than 365 for days/year. Julius Caesar introduced leap years more than 2000 years ago.
I’m not going to try to research which ancient astronomer first improved on Eudoxus’ 365.25 estimate of year length, but Omar Khayyam’s 900-years-ago estimate of 365.242198 days per year is almost unbelievably accurate.
Nitpick: Polaris has only been a decent pole star for about the last 500 or 600 years. This is due to the precession of the equinoxes. That is, the Earth’s pole moves in a big circle against the stars. Somewhere around 2700 BCE, Thuban was almost smack on the pole, so for serveral hundred years before and after that, it was the pole star. In between, there was no real good pole star, although Beta Ursae Minoris was sort of close for a while around 1000 BCE.
In the future, Polaris will remain the pole star for another 600 years, then we won’t have one until about 3600 CE, when Gamma Cephei starts getting close.
Thanks for the follow up, Chronos.
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Kevbo’s post was excellent, but this sentence is wrong. Eudoxus, who lived almost 2400 years ago, is credited with being first to notice that 365.25 is a better approximation than 365 for days/year. Julius Caesar introduced leap years more than 2000 years ago.
[/QUOTE]
Yes, I didn’t state that well. I was using the adoption of the Gregorian calendar as my reference point. Clearly there were people who understood the problems far earlier. Because the drift of the Julian calendar actually did become a problem, I deprecated it, even though it does (imperfectly) address the non-integer number of days in a year.