Article of the month February

Azimuth sundial 2

Oughtred sundial

This is another azimuth sundial, but it is constructed completely different from the dial described last month.
The celestial sphere with hour lines and date lines is projected onto the horizontal plane, using a stereographic projection.
Circles on the celestial sphere are mapped into circles on the sundial.
Oughtred (1574-1660) designed this sundial centuries ago.

The figures show date lines according to the zodiac.
From the inside out we see: Cancer, Leo, Virgo, Libra, Scorpio and Sagittarius.
Going back, we see: Capricorn, Aquarius, Pisces, Aries, Taurus and Gemini.
The twelve ‘months’ are so represented by six bands.

In the centre of the sundial is a vertical gnomon, the shadow of which indicates the azimuth.
The time is read at the intersection of this shadow and the actual date line.

The figures below show several time systems.

hour lines for local apparent time

hour lines corrected for longitude and equation-of-time
red: shortening days 21 June - 22 Dec
blue: lengthening days 22 Dec - 21 June

hour lines for:
Italian hours (red)
Babylonian hours (blue)

The figures also show a scale reading the altitude of the sun, on a ruler that is turned about the gnomon foot to the readout point.

An Oughtred sundial can show lots of information even if the sun is not out. It will show azimuth and altitude of the sun for any date and time.
You can also find the day length for a specific date, as well as the times of sunrise and sunset.
The instrument is thus much more than just a sundial.