Heavens' Henge

[Iona]

Fairy pools in Scotland are uncannily like mill pools. You get a fast swirling one upriver and below a quiet still one.

[hvered]

There are lots of tidal pools in Cornwall, many of which are in Perran locations, i.e. Piran (patron saint of Cornwall and of tin). This site http://www.tessabunney.co.uk/projects/t ... ls-201011/ mentions Wild Swim, which gives a brief description of most of these pools.

Re Perranporth, the pool is hidden in Chapel Rock and "without a tip-off you wouldn't know it was there". According to 'Wild Swim' the Chapel Rock pool can generate rip tides in the surrounding sea on a falling tide "making it unwise to swim to it at this time (Wait a little longer and you can walk or wade there.)"

[jon]

Was the discussion about the moon and how to find longitude any use in your discussions (with the prof) Harriet? I wasn't sure that it was something worth developing. Neil had some good ideas but personally I think the easiest way is event timing:

If you know that a planet will cross a boundary line between two stars, you can line up a rod so that the exact time it passes through the line is known. This point in time is the same over the world. You can then point a pole at a second 'reference' star (an east star such as Betelgeuse would be the best) to find a reference angle which can be compared to a plumb-bub set perpendicular to North (found by finding the North Star, which was Thuban in 2500BC).

Compare this angle to the same angle found at your reference location (eg Greenwich) and bingo.. you know your longitude. There's a few other ways to do it and before clocks they probably had a way that is even easier than this (they had centuries to think about it). It's very difficult at sea, which is why so many resources were put to developing clocks (and why we don't even bother to think about it now).

(http://heavenshenge.blogspot.co.uk/2012 ... -moon.html)

[Penny]

There are lots of tidal pools in Cornwall, many of which are in Perran locations, i.e. Piran (patron saint of Cornwall and of tin). This site http://www.tessabunney.co.uk/projects/t ... ls-201011/ mentions Wild Swim, which gives a brief description of most of these pools.

Salt water like mineral-laden water is good for you, as Boreades said somewhere miners would relish a relaxing bath. Here the tide can renew and replenish the bath water.

Re Perranporth, the pool is hidden in Chapel Rock and "without a tip-off you wouldn't know it was there". According to 'Wild Swim' the Chapel Rock pool can generate rip tides in the surrounding sea on a falling tide "making it unwise to swim to it at this time (Wait a little longer and you can walk or wade there.)"

Presumably Chapel Rock was intended as a landmark and/or warning beacon rather than a lido but it's significant that holy and healing are etymologically related.

[Maribel]

Compare this angle to the same angle found at your reference location (eg Greenwich) and bingo.. you know your longitude. There's a few other ways to do it and before clocks they probably had a way that is even easier than this (they had centuries to think about it). It's very difficult at sea, which is why so many resources were put to developing clocks (and why we don't even bother to think about it now).

(http://heavenshenge.blogspot.co.uk/2012 ... -moon.html)

Why the moon and not the sun? Greenwich is after all a meridian.

[jon]

Why the moon and not the sun? Greenwich is after all a meridian.

Planets are probably better than the moon. Greenwich is an arbitrary location: The sun rises at different times of day relative to the time in Greenwich depending on where you are in the World. If you have a clock, it is a great method of finding what your longitude is, but if you do not, it doesn't help you to find out where you are relative to somewhere else.

But it could be used: You could use the same reference star at the moment of sunrise (after sunrise you won't be able to see most stars), but you would have to compensate for the rotation of the celestial sphere relative to the reference location. This adds a bit of difficulty to the exercise so my guess is that it would not be the first choice.

[Martin]

There's a few other ways to do it and before clocks they probably had a way that is even easier than this (they had centuries to think about it). It's very difficult at sea,

How can measurements be accurate without knowing the time of day (or night)? A rough guess wouldn't be adequate.

[Royston]

Aaarr. That'd be why the stones dance, Jon lad.

[jon]

How can measurements be accurate without knowing the time of day (or night)? A rough guess wouldn't be adequate.

You don't need a clock if you have a known simultaneous event: You could do this using the planets or Neil's way; using signalling. Each method has advantages and disadvantages. Depends how accurate you want the measurement to be (and how accurate your measurement equipment is); there are other ways.

[hvered]

Was the discussion about the moon and how to find longitude any use in your discussions (with the prof) Harriet?

There didn't seem any point in discussing the problem of longitude, he is after all a professor of archaeology and his interest in megaliths seems to be confined to sites rather than sights. There may be an academic archaeologist out there who takes archaeoastronomy seriously but I have yet to meet him/her!