Sundial
The oldest clock in the world has no moving parts at all. It is the Earth that moves; the shadow simply writes it down.
- ≈ 3,500 yearsthe age of the oldest Egyptian shadow clocks
- 15° an hourthe speed of the Sun across the sky
- 38.97°the latitude of Gandia, which the drawing is set for
A stick, the Sun and patience
Egyptian scribes set a bar with a crosspiece on the ground and counted how many marks its shadow crossed. They split the day into twelve parts, so a summer "hour" was longer than a winter one.
Move the pointer over the drawing on the right: the Sun will travel across the sky and the gnomon's shadow across the dial.
Tilted by the latitude
The triangle on the dial is called the gnomon. Its sloping edge rises above the horizon by exactly the latitude of the place, 38.97° for Gandia, and points at the Pole Star.
That makes the edge parallel to the Earth's axis. The Sun goes round that axis once a day, so the edge's shadow turns evenly at any time of year.
Why the spacing is uneven
The Sun crosses the sky at exactly 15° an hour, but a horizontal dial catches the shadow at a slant. The lines crowd together around noon and spread apart toward morning and evening.
angle = arctan(sin φ · tan 15°t)That is why you can't move a sundial to another town: at a different latitude it would tell the wrong time.
A long day, a short shadow
In June the Sun climbs to 74.5° over Gandia. The shadow is short and the day lasts nearly 15 hours, so the dial needs lines from five in the morning to seven in the evening.
The same hour, a different shadow
In December the Sun never rises above 27.6°. The shadow is longer and the day shorter, but the gnomon's edge still lands on the same hour line. That is the whole point of the tilt.
Why noon comes at two
Gandia sits almost on the Greenwich meridian, yet since 1940 Spain has kept Central European Time. Add an hour for summer time and solar noon arrives at about 14:00 by the clock.
There is a second correction too, the equation of time: the Earth's orbit is an ellipse, so solar days are sometimes longer and sometimes shorter. The difference reaches 16 minutes.