Observatory · the sundial
Sun time and clock time
A sundial tells the time by the Sun, and the Sun is a poor clock: some days it reaches its highest point a quarter of an hour early, other days a quarter of an hour late. This instrument knows that. It is a heliochronometer, a sundial built to show clock time, to the minute. Turn its arm to the clock and the light through the little hole falls on a figure eight engraved on the screen, at today's date. Here it stands in Eindhoven, under the real Sun, now.
The heliochronometer
Fig. I. The heliochronometer, set up in Eindhoven (51.44° N, 5.47° E), polar axis to the north; right, its screen.
- Clock in Eindhoven
- Sun time (a plain sundial)
- The Sun
- The spot of light
The instrument stands with its axis pointing at the north celestial pole, tilted 51.4 degrees, Eindhoven's latitude, so the dial lies parallel to the equator and the Sun goes round it evenly, 15 degrees an hour. The arm turns over the dial. At one end is a ring with a small hole, at the other a screen with a figure eight and the twelve months. Turn the arm to the clock time and the spot of light through the hole lands on the figure eight, at the day of the year: high on the screen in winter, when the Sun is low, low in summer. If the spot misses the eight, the arm is not at the right time. Try turn it yourself: hunt for the eight, and read the clock off the arm.
Sun time is what an ordinary sundial shows: noon is when the Sun is due south. In Eindhoven that is never at twelve. Our clocks run on Central European Time, the time of the meridian 15 degrees east, through eastern Germany, and Eindhoven lies 9.5 degrees west of it, so the Sun gets here 38 minutes later; in summer time an hour more. On top of that comes the equation of time, which the figure eight takes care of.
A year of early and late
Fig. II. The equation of time through the year, and the figure eight it makes against the Sun's height.
Two things make the Sun an uneven clock. The Earth's orbit is an ellipse, and Kepler found that a planet goes fastest when it is nearest the Sun; we are nearest in early January. Then the Earth has to turn a little further each day to bring the Sun back to the south, and the Sun falls behind: the blue, dashed curve. The second cause is the tilt of the axis. The Sun moves evenly along its yearly path, but that path is tilted against the equator, and only the part of its motion along the equator counts for the clock; that part is smaller near the equinoxes and bigger near the solstices: the red, dotted curve. Added up they make the brass line, the equation of time: the Sun up to 14 minutes behind the clock in February, up to 16 ahead in early November.
Put those minutes across and the Sun's height up, day by day, and you get the figure eight on the right. It is the eight on the instrument's screen, upside down: there the low winter Sun throws its spot high, and the high summer Sun low. Photograph the Sun at the same clock time every few days through a year and it draws the same eight in the sky: the analemma.
Who worked it out
Where it already matters
Amsterdam time. Until the railways, every town kept its own sun time. From 1 May 1909 the whole of the Netherlands, railways included, ran by law on the mean sun time of the Westertoren in Amsterdam, 19 minutes 32.13 seconds ahead of Greenwich. In 1937 it was rounded to twenty minutes. On 16 May 1940 the German occupier put the country on German summer time, moving the clocks 1 hour 40 minutes forward, and the Netherlands has kept Central European Time since: that is why our noon comes so late.
Noon in Eindhoven. The Sun is due south here at about 12:38 CET in winter and 13:38 in summer time, give or take the equation of time: earliest in early November, about 12:22, latest in February, about 12:52.
Every sundial on a church wall shows sun time. The tables that went with them, to turn sun time into clock time, were the equation of time, printed in almanacs once pendulum clocks became good enough to notice the difference.