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Stonehenge astrolabe
We meet many hundred guests each year on our Stonehenge tours and many come back on repeat visits. One such group is led by Dr John Wallin, Professor of Physics and Astronomy at the Middle Tennessee State University who has brought his undergraduate classes on tours for several years. Last year he presented me with a thank you gift of an astrolabe calibrated for Stonehenge. He included a note which contained the words ‘Any astronomy friends you have will almost certainly be completely useless in helping you figure it out.’ Little did he know that one of my colleagues is a well known local astronomer Simon Banton who is an acknowledged expert on the archeoastronomy of Stonehenge. Simon has very kindly written some explanatory notes on astrolabes below. So a big thank you to Dr Wallin for making such a wonderful gift and to Simon for writing the explanation. “Astrolabe” derives from the Greek works astron and lambanien usually translated as meaning “star taker” and the first examples date to over 2000 years ago. It is a mechanical device that creates a two dimensional representation of the celestial sphere, allowing its user to calculate the positions and times of events in the sky. The sophistication of astrolabes increased over the centuries, reaching its zenith with Arabian scholars and astrologers in the 10th century when Abd al-Rahman al-Sufi described over 1000 uses. Throughout the middle ages in Europe and beyond, learning to use an astrolabe was regarded as a vital skill for any educated person. Most astrolabes are made of metal, comprising a circular disc around 6” to 8” in diameter called a mater which is usually engraved around the edge with the hours of time. The back of the mater carries useful tables as well as a year calendar around the edge divided up into the constellations of the zodiac and a degrees of arc scale.Also on this side of the astrolabe is the alidade - a straight bar often with sighting notches that can be used to target objects in the sky. It is free to rotate about the central pivot of the device. On the front of the astrolabe, the mater supports a disc called a climate or tympan for a particular degree of latitude, on which is engraved a stereographic projection of lines of altitude and azimuth representing the celestial sphere above the local horizon. Mounted on this combination of discs is a framework called a rete. This is free to rotate over the tympan and carries pointers and labels for a couple of dozen bright stars as well as a projection of the ecliptic – the apparent path through the stars that the Sun takes over the course of a year. Finally, a linear rule marked with degrees of declination is provided, also free to rotate about the central pivot that holds the entire assembly together. These days it's possible to create a working astrolabe from acrylic plastic and it's one of these that we'll use to try and discover the time of sunset at the winter solstice at Stonehenge.
The first step in the process is to work out whereabouts the Sun is against the background stars at the winter solstice. We do this by using the alidade on the back of the astrolabe to indicate the date on the calendar scale and then read off the Sun's position along the zodiac scale. The date of winter solstice is the 21st December. By setting the alidade to point to that date, we can see that the Sun is moving out of Sagittarius and into Capricorn, at a position of “0° of Capricorn” on the scale. Step 2 makes use of the other side of the astrolabe. We're interested in the time when the position“0° of Capricorn” just reaches the western horizon, so we rotate the rete until the ecliptic scale for that position intersects with the horizon line on the tympan. Unfortunately the etching on this astrolabe is missing the labels for the constellations in this part of the rete, so you'll just have to take my word for it that it's lined up correctly in the following picture. I've added some annotations to make it clearer. The horizon line is the curved etched line on the right hand side of the spider-web of lines that you can see through the rete. The straight rule has been spun round until it lines up with the intersection point, and it lets us read off the time on the scale around the edge – 3.50pm, give or take.
Now this all seems quite simple, we have an answer and it feels about right. However, there are a few other factors we need to take into account. This first thing is that our astrolabe gives us “sundial time” or “local solar time” (LST), where 12 noon LST is the moment when the Sun is directly south of wherever we are. We don't use local solar time in our day-to-day lives, we use Greenwich Mean Time (at Stonehenge, in winter) and that's different. The next thing is that the difference between sundial time and clock time varies day by day throughout the year because the Earth's orbit is an ellipse rather than a circle and also because the apparent path of the Sun is tilted with respect to the equator by 23.4°. The varying difference in minutes between sundial time and clock time is described by the Equation of Time formula and there are tables that give the precise values for each day of the year (see http://www.ppowers.com/EoT.htm).
Daylight savings time also needs to be corrected for, if it's in effect, but for winter solstice it isn't so we can ignore it. To convert from our local solar time given by the astrolabe to GMT at the Greenwich Meridian is easy. Stonehenge is 1.8262° west of Greenwich, and the Earth spins at a rate of 1° every 4 minutes so local solar time at Stonehenge is 4 * 1.8262 = 7 minutes and 18 seconds behind GMT. We have to take the time given by the astrolabe – 3.50pm – and add that correction on to it, giving 3.57pm and 18 seconds. To take account of the effect of the Equation of Time, we check the tables for the correction on the 21st December and find that local solar time is ahead of clock time by 1 minute and 49 seconds. We therefore need to subtract this from 3.57pm and 18 seconds and, having done so, we find that sunset on the winter solstice at Stonehenge happens at 3.55pm and 29 seconds GMT. This is the instant that the centre of the Sun crosses the 0° altitude horizon, the geometric sunset. It's not the same as the time when the last gleam of the Sun disappears from view. That depends on other things, like the height of any hills, trees or other obstructions on the horizon and the light-bending effects of atmospheric refraction. To check we've not got our calculations horribly wrong, we can use an astronomy program to show us when the centre of the Sun crosses the horizon – a luxury that medieval astrologers didn't have!
Many thanks to Pat Shelley for lending me the astrolabe to play with and to Richard Wymark of www.astrolabeproject.com for sparing the time to clarify some of the more technical details of astrolabes for me.
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The back, showing the alidade bar and the zodiac degree and calendar scales marked around the edge of the mater