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The Astrolabe.

on March 24, 2013

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Measuring the elevation angle of the sun.

After much frustration, using wobbly paper and uneven surfaces, pens that don't work and and old plastic protractor, I found that the safest and most accurate way of measuring the elevation angle of the sun is the dioptra. A long tube.. Plastic Cardboard Anything! Just so long as it is fairly long, light enough to hold with one hand and you can fix a protractor to it. Method of use. Stand with your back to the sun Hold the tube with one hand Hold your other hand close to the lower end of the tube so that the sun disc will be projected onto your palm when the tube is tipped upwards at the same angle as the sun. Tilt the tube so that a bright disc of light is projected onto your palm. Or onto the ground. Read the angle from the protractor. 'Clear disk' on the ground shows that the tube is pointed directly at the sun. Why this is a good method! Shadows are excellent, but...the post must be vertical, not leaning to one side. The ground should

"Caput Draconis in Virgo"

The term "Caput Draconis in Virgo" now belongs to Harry Potter books and Dan Brown (The Lost Symbol) but if I take it seriously I think I may be able to unravel the mystery. My guess is it refers to minor lunistice. Caput Draconis is a medieval term for the ascending lunar node. A lunar node is the point of intersection between the ecliptic and the plane of the lunar orbit. The term translated is literally 'the dragon's head' and refers to the moon ascending from the South to the North.... The moon crosses the ecliptic twice every month and each crossing is called a node, but when people refer to the nodes , they generally mean the nodes that occur when the moon crosses the ecliptic as it crosses in front of the sun . This happens twice each year, and a solar eclipse will be visible somewhere -often over the sea - about twice each year. In this diagram the moon is on the ecliptic (the same path as the sun) as it crosses in front of the sun, it is

Stonehenge, Thuban and the Celestial North Pole.

There is no such thing as the pole star (even though we draw it, talk about it, and believe in it). The pole is the earth's axis of rotation. As if the earth were a ball spinning around an invisible spindle-pole pushed through the centre of the earth. A pole star is a position occupied by a star relative to earth. A pole star happens to be any star in the right place at the right time as the top of the invisible, non-existent spindle pole drifts one degree per 70 years, around a massive circle 26,000 years wide. Precession. The numbers on the yellow circle are dates. The earth spins daily. 23 hours and 56 minutes in relation to the distant stars. 24 hours in relation to the sun. Rotates yearly. 365 and a quarter day to travel 360 degrees around the sun. Around about 2870 BC the pole star was Thuban. Using Stonehenge as a reference point. 3000 BC. At that time, Stonehenge is a circular earthwork about 110 meters in diameter.

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The Astrolabe.

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