How the Ancients measured the Earth (Megalithic)

[DPCrisp]

"No answer," came the stern reply. OK, this is what this thread made me think of, not necessarily what Keimpe/everybody has in mind.

Let's say the Dawn is the edge of the shadow that 'retreats' across the country as the Sun rises. If we choose a north-south line, parallel with the Dawn, to make observations along, then the Sun will rise simultaneously for all our observers and we won't be able to deduce anything.

What we need is a line at right-angles to the Dawn so we have the maximum difference in Sun-up times and we stand a chance of measuring it.

Took a bit of rolling this around in my head to conclude the obvious: if the Sun rises along the Michael Line {Dragon Line or Drawn Line [G=U], per the Walkabout[?] thread.} on May Day, then the Michael Line is perpendicular to the Dawn on that day.

Now, we know the Earth or Sun turns 360° in 24 hours. So time translates to angle 'straightforwardly'. We need to equate a time (angle) to a distance so we can calculate the size of Earth.

So let's set up beacons along the Michael Line, what, every 30 miles? 16 of 'em?

Light the beacon the instant you see the Sun from the Norfolk coast. Supposing it takes 30 seconds for each beacon to be seen by the next, the "Sunrise in Norfolk" message will arrive at St. Michael's Mount in 8 minutes.

Sunrise itself arrives in Cornwall just over half an hour later. 40 minutes altogether (say) means the Michael Line is... forty sixtieths of a twenty-fourth... 1/36th of a circle.

Now, since we've done all the grunt work setting out the beacons and measuring their distances, we know the Michael Line is 500 miles long, so said circle is 36 x 500 = 18000 miles around.

That's pretty good as an order of magnitude... but if we've done this much, we've also worked out that we're at 50°N, so the total circumference of Earth must be... divide by the cosine of 50°... 28000 miles. Even better.

Now just substitute some actual measurements.

[Mick Harper]

This is similar to but rather better than my idea ("my" idea) which was that the Michael Line can be used by measuring how far the Norfolk coast was north of St Michael's Mount and then using a Pythagorean triangle. But clearly the perpendicularity of May 1st sunrise obviates this.

However, I am not happy with the lighting-of-the-bonfires. My recollection of for example the Armada bonfires is that the time-lag is a) much greater and b) unpredictable. However, this is when nobody is particularly looking. If everybody is keen-eyed on Mayday morn, then perhaps eight minutes is realistic. I would like to hear other techniques though that could be done by simple measurement other than time, cosines etc.

On the other hand Dan's explanation accounts for two things independently: why May 1st is so important (it isn't in itself, it just happened to be when the Sun shines up the longest available line); and bonfires on hilltops (on ordinary megalithic grounds this is a no-no on the grounds of expense). Coincidentally here is an extract from a book I was working on today, listing the Top Twenty megalithic markers

[DPCrisp]

You don't quite know what is being suggested in this thread then.

The actual time it takes to light the beacons is pretty immaterial as long as it can be guestimated and added to the interval between the signal and the sunrise. Piece of piss, really.

And so are cosines for anyone anywhere near competent to undertake measuring the Earth.

I can't see any way around measuring as-long-a-distance-as-possible in order to infer the size from an angle/time, by the way. (I think there was a suggestion somewhere here that this wasn't necessary. I'm prepared to be convinced.)

[Mick Harper]

[DPCrisp]

My exposition isn't clear enough for you to spin into a paragraph or book of any required size? Hmph.

[Mick Harper]

Well, look, this is the kind of thing I was referring to in the other thread. Anybody who can put together a coherent account of how the Megalithics measured the circumference of the earth would be earning themselves slightly more than a footnote-in-history. But I know from long experience that nobody will and that in the end I will put together some sketchy account which will be lost among the welter of even better ideas in my next book (with or without acknowledging other people's contribution to the final product).

It's up to you (all). If you want to spend a bit of time actually doing the work then I will stick it in the book under your byline. If not, you'll just have to take your (rather dim) chances.

[DPCrisp]

A little train of thought:

Could they have signaled with a mirror? That's as instantaneous as you can get. But to relay the reflection of the Sun forward into a area where the Sun itself can't be seen yet means a shallow grazing angle: the Sun and its reflection would be coming from almost the same point, so it would be hard to see the reflection against the brightening sky. Plus, there's not much to reflect as the limb clears the horizon. For a strong reflection, you need the Sun to have risen a long way, maybe all the way. But you don't want to see it directly if you're looking for the delay between the reflection and the Sun itself.

We've talked about hilltops raised or razed so as to be visible from one another... but what about hills being erected or shaped to deliberately mask and reveal the Sun in a controlled way?

[Hatty]

Staring you in the face! Whenever you read about a megalithic site, you come across someone who says "that hill there marks the midsummer solstice" or similar. Ties in with Mick's 'they went to great lengths to set up the system and then let it run for the next few millenia'. Only thing is, you wouldn't want those hills getting themselves eroded and spoiling the show; not just an incalculable amount of animal dung (though good building material) but a deliberate policy?:

[Hatty]

It would be necessary for hill-tenders (hermits) to carry out an annual test with a stick which came to be known as the May pole. Possibly. Would the erection of a long thin pole (not especially hefty, with flags on top) be enough of a signal to other hilltop surveyors in the chain?

[DPCrisp]

I wondered about that, but i) only in daylight and ii) not when they're typically on village greens.

[Ishtar]

[DPCrisp]

Hmm. What we've been talking about here is more like breathing fire into the Dragon (Line)...

[Keimpe]

I've been absent (busy) for a while and only now see your discussion on how to measure the earth's circumference.

I have described a very practical way of measuring the earth's polar circumference, but I hear people did not quite get it.

So I'll put it up again as a summary in the next post.

[Keimpe]

We know the sun rises on the St. Michael's Line, May 1st every year.
We are megalithic people and want to use that knowledge to measure the earth.
We are in the bronze age.
We are strong and clever, and have all the time in the world.
We have no machines, and certainly no watches (which makes the let's-calculate-the-time-difference-of-sunrise-across-the-St-Michael's-Line method nearly impossible)

What you need in order to calculate the polar circumference of the earth is two points anywhere on earth north-south of each other with a known distance between them. This distance is a part of the circle that is the earth (assuming the earth is round).
All you need to know now is exactly what part of the circle you have measured, and you have the total circumference.

You can calculate the part of the circle by measuring the difference between the angles of point N (North) and S (South) towards one specific point. This will give you a certain number of degrees, and since we know a circle is 360 degrees and we know the distance between points N and S, we can now calculate the circumference of the entire circle.

To make this method reliable, the distance between points N and S must be as far apart as possible. Preferably one half to one full degree of the circle. But then we have a problem, because now we can't see one specific point from both places because they're too far apart. Enter the Sun. We can easily see the sun from points N and S.
But now we have a new problem: the sun won't stay put. So we have our specific point, but it's moving about all the time! So we need one more thing: one specific moment to measure our angle towards the sun.

Enter St. Michael's Line. We know the sun rises across that line on May 1st.
We're getting close, but not quite close enough.
Where shall we put our points N and S? Somewhere along the east coast of Britain so they can both see the sun rise at exactly the same moment on May 1st?
Unfortunately, on the east coast you cannot put points N and S very far apart (north-south of each other).

Right in the middle of Britain would be much more practical. This will give us a north-south distance all the way up to Scotland if we want to.
Only from there (in the middle of England) we can't see the sun rise on the eastern horizon properly. Bugger.

But, wait a minute! Almost exactly in the middle of the St. Michael's Line (at Avebury) there's a very high ridge (called the Ridgeway) with a clear view towards the north.

Let's do the following: we build an artificial hill, that is of precisely such a height, that together with two other hills in the vicinity, all three hills are shined upon by the sunlight on May 1st at exactly the same moment. So if you stand upon the artificial hill, which is Silbury Hill, there is a moment on May 1st, where the sun touches (from east to west): the Ridgeway, Waden Hill, and Silbury Hill all three at the same moment. It's like looking through the barrel of a gun. Let's make THIS our moment.

From the northern part of the top of the Ridgeway we can see both the sun (to the south-east) and Silbury Hill (to the south-west), so as soon as we see the sun touch Silbury Hill, we put up a big (bronze?) mirror and deflect the sunlight towards the north. From south to north, we can repeat this process as many times as we want.

Now we can measure the angle towards the sun from points N and S at almost exactly the same moment in time, and with that information calculate the polar circumference of the earth.

[Hatty]