Home-made vertical dial

I have always been interested in how time may be told from the shadows cast by the Sun. For my Astronomy ‘O’ Level project when I was 15 I made a shadow-stick and marked the hours with stones on the ground; then made a sundial using matches to indicate the hours in roman numerals.

Above is a fairly crude vertical south-facing dial I made in 1999 while living in Ireland. I was interested more in showing the Sun’s declination through the year than the time of day (though it shows both of course). In this type of dial it is the tip of the gnomon’s shadow that shows the time, in contrast to a horizontal dial where the gnomon’s shadow lies parallel to each hour line in turn through the day. The method used to lay out the dial follows that of Jean Meeus in Astronomical Algorithms (Willmann-Bell, 1991).

Although the markings are difficult to read in the photograph, the Sun casts a shadow along one line of dots from left to right depending on its declination. These lines are concave to the gnomon when the Sun is south of the equator, and convex to it when the Sun’s declination is north. The equinox line is straight (the tip of any stick always traces out a straight line on the equinoxes).

The dial markings are drawn for the longitude of Greystones, Ireland: 6° 03’W (= 24 minutes). I also added a rough graph of the equation of time on the dial.

Bronze Sundial at Kitt Peak

Sundials can be very sophisticated. This bronze equatorial dial at Kitt Peak observatory uses the shadow of the ball on the rod to show the time, as well as the declination of the Sun and the times of sunrise and sunset.

Equatorial Dial for a Specific Location

This is an equatorial dial, made in 2008 for a place called Hillcrest whose latitude and longitude are inscribed.

Equatorial dials are set up parallel to Earth’s equator, hence the disc is tilted to the co-latitude of the site and unlike horizontal dials the hour lines radiate from the centre and are equally spaced.

Making a dial for use in a particular place means that the correction for longitude (11 minutes in this case) can be built into the design, and as the Sun can only cast a shadow on the dial when north of the celestial equator‡, the hour lines can be marked out in Daylight Savings Time as this will be in effect for most of the time the dial is used. The only correction needed to the time read off the dial is therefore the Equation of Time.

The length of the style is chosen so that the Sun’s declination can also be read off the dial at the point where the ball casts its shadow, hence we can see that the photo was taken near midsummer. When the Sun’s declination is below about 14° (before about 28th April and after about 16th August) the tip of the shadow falls beyond the dial plate, but the time can still be read by the gnomon’s shadow.

‡ Some equatorial models include markings on the underside of the dial,  for use in winter.

I am indebted for this and several other sundial designs to Albert Waugh’s splendid book “Sundials: Their Theory and Construction” (Dover, 1973).