Maggie: Let’s continue our discussion of astronomy. Maybe we could do a review of the history of astronomy?
John: I like that idea. Science is a lot more fun when we understand its history. Where do you want to start?
Maggie: Let’s start at the beginning and work forward to Hawking.
John: Okay, we will take a chronological approach.
Maggie: And let’s start with Stonehenge. It has always fascinated me. How did it get there? Why is it there? It’s weird.
John: I agree, it is weird. But let me make a few prefatory remarks.
Maggie: Oooh, big word.
John: I will restate: let me say a couple things before we talk about Stonehenge.
Maggie: All right.
John: The time period we are talking about in this conversation is sometimes called “Prehistory.” It is a time before history was actually recorded. So a lot of what we are going to say is not based on recorded documents or oral history. Some of it is based on reasoned conclusions about what we see and some of it is just speculation.
Maggie: So we are just guessing?
John: Pretty much. But they are good guesses.
Maggie: That works for me.
John: What do you think were the earliest uses of astronomy?
Maggie: Are you asking me what people first used the sun and the stars for?
Maggie: I think people first used them when they traveled, to help guide them.
John: That is exactly right!
One of the earliest uses of astronomy was for navigation. People traveling across bodies of water and across land used the stars, such as the North Star, to tell them which way to go. Probably the best at it were the early Polynesians. They were experts at traveling thousands of miles across the Pacific Ocean using only the stars and the sun to guide them. They were real adventurers.
Do you have any other ideas?
Maggie: How about for planting crops?
John: Right again. Others used the stars to help them determine when to plant. For example, the appearance of certain constellations at specific times of a year helped with this. And here is something exciting: this may be what Stonehenge was used for.
Maggie: I think Stonehenge is the most famous monument in ancient astronomy. It is so cool.
John: I agree with you. And since we are talking about ancient astronomical monuments and buildings, here is a new term to learn: archaeoastronomy. It refers to the study of ancient monoliths and structures having to do with astronomy. So when we are talking about Stonehenge, we are engaging in archaeoastronomy. And that, my young friend, is also pretty cool.
Maggie: I like that word! I am going to work it into conversation today. But why don’t you tell me how Stonehenge was first built.
John: Good idea. Stonehenge was built in three stages, over a period of several thousand years. The dates, however, are only approximate dates.
Maggie: I get it.
John: In the first stage, around 2950 BC, early Britons constructed a circular ditch and a bank on the site. They then dug a series of holes—about 56 such holes—inside the ditch. These are known as Aubrey Holes, named after John Aubrey, the person who discovered them. Some scientists now think that the Aubrey holes once held timber posts.
Then around 2550 BC, the second stage of construction took place. During this phase, between 60 and 80 “bluestones” were brought over from South Wales. This was quite an undertaking because these stones weighed several tons. They were placed close to the center of Stonehenge, either in a horseshoe or circular shape, and 43 of them remain standing today. Scientists guess that the important Heel Stone was also placed during this time.
The last phase took place about 100 years later, when sandstone slabs called “sarsens” were put in place, forming both the inner crescent and an outer ring. Each one of these weighted 40 tons or more and stand to about 24 feet. About 50 of these stones still stand today.
Maggie: How long did each stage take to complete?
John: Several hundred years for each of the second and third stages.
Maggie: I have heard some pretty weird theories about who built Stonehenge. Some say the wizard Merlin built it. You know, King Arthur’s Merlin.
John: Yes, that is one of the theories, and one of my favorite ones. The story is that hundreds of British nobles were slaughtered on the site of Stonehenge. To honor them, Merlin used magic to move the stones from Ireland, where they were previously arranged in a circle known as the Giant’s Ring.
Maggie: But according to what you said, Stonehenge was build thousands of years before Merlin.
John: That, my friend, is why it’s just a story.
Maggie: Tell me another theory.
John: Here is a fun one. John Aubrey, famous for discovering the Aubrey holes, believed that Celtic high priests built Stonehenge.
John: Yes, druids. He thought it was built as a sacred druid site.
Maggie: But that can’t be true because it was built many years before the Celts came to Briton.
John: And right you are again.
I think the best theory is that Stonehenge was built by several different people-groups over many years. That would account for the stages in which it was built.
Maggie: What was it built for?
John: There are a lot of theories. Some suggest it is an ancient burial site. Others think it is a religious site, especially for those who worship the sun.
Maggie: What do you think?
John: I think we cannot ignore the fact that the stones and Aubrey holes line up with the cycles of the sun and the moon in such a way that allows us to use Stonehenge to predict the seasons of the year and certain astronomical events. This is important for several reasons, such as the fact that it would help farmers know when to plant their crops.
Maggie: Can you give me an example of how Stonehenge lines up with the sun and the moon?
John: Sure. Look at the picture below.
Do you see the Heel Stone?
John: The midsummer sunrise arises almost directly over the Heel Stone, providing evidence of the exact time of midsummer. This is just one example of how Stonehenge can be used to track the seasons.
The cycle of the moon can also be tracked by moving a marker along the Aubrey holes. And by tracking the cycles of the sun and the moon, some astronomers suggest that Stonehenge could also be used to predict lunar and solar eclipses.
Maggie: Is this mostly just guessing about the purpose of Stonehenge?
John: Yeah, for the most part. But we need to remember something very important. It doesn’t matter if we are exactly correct on the all the purposes that Stonehenge was built for, or even how accurate it actually was in predicting astronomical events. What does matter for our purposes is that Stonehenge shows that early human beings were deeply interested in the stars, the sun, the moon, and the planets. They then put their curiosity to work, building significant monuments to help them understand more about these objects in the sky. And that is just cool.
Maggie: I never heard it put that way, but it makes perfect sense to me now. Stonehenge is not just an astronomical monument, but also a monument to human curiosity. Very cool!
John: What you just said is genius. I am going to borrow it.
Maggie: Go ahead. Are there any other prehistoric monuments we can talk about?
John: Yes, many. But let’s talk about just one more.
John: It’s called Newgrange and it’s a prehistoric monument in Ireland. I really like it. It is a large mound that is about 40 feet high and covers a little more than an acre of ground. Inside is a large chamber, with smaller chambers attached to it. It was built in about 3,200 to 3,100 B.C.
Maggie: Why is this significant for archaeoastronomy? Did you see what I did there?
John: Yes, nice use of the term.
It’s significant because of the 60-foot passage that leads to the central chamber. This passage was built so that it aligns with the rising sun on the winter solstice.
Maggie: You are going to need to explain that.
John: Okay. As you know, the winter solstice is the shortest day of the year. The passage was built so that on this day, as the sun rises, it shines through a roof-box into the passage, illuminating the passage and the inner chamber. The roof-box is right above the door. The light lasts for about 15 minutes. It is only at this time that a person can see the intricate carvings in the inner chamber without artificial light.
Maggie: So what is the purpose of having the sun enter the passageway and the inner chamber?
John: Scholars have speculated two key purposes.
The first was a religious purpose. Newgrange was a burial site, with bodies entombed in the smaller chambers. The sun was considered sacred in many early religions, and scholars have theorized that by shining the sun into the tomb on the shortest day of the year, the early humans were venerating the dead.
The second is an astronomical purpose. By marking the shortest day of the year, some scholars believe that early inhabitants of Ireland then knew that the days would soon be getting longer.
Maggie: So early humans put the sky and its objects to work for them.
John: That is a perfect way to summarize our discussion. But I want to mention one other thing.
We can’t forget about early cultures in the Americas. Both the Mayan civilization in Central American and the Hopi civilization in North American used celestial objects to develop extensive calendars. The Mayan calendar, for example, is incredibly complex.
Maggie: I bet this is true of many eastern cultures also.
John: Correct. The point of our discussion is that many civilizations and cultures throughout pre-history used the objects in the sky for many purposes: religious, agricultural, scientific, and for calendars. So astronomy has played a robust role in human life since the beginning of time.
Maggie: I like it! And thanks for filling me in on Stonehenge. We should go see it in person some day.