An equatorial sundial in the Forbidden City, Beijing.

An equatorial sundial in the Forbidden City, Beijing.

It’s a rainy day here in Paradelle and summer is winding down – so it’s a good time to think about sunnier times – and sundials.

Once, people were not so driven by the clock. All they had was the sun and a shadow to mark the passing of the day.

If you drove a stake into the ground (as the Phoenicians, Egyptians, Etruscans and other ancient civilizations did), you could mark the shadow and create a dial to tell the “time” of day. Of course, they thought about time differently too. Minutes and seconds didn’t carry any weight.

The remnant of all that is the sundial which probably some readers still have in their backyard, though they might treat it as more of an ornament than a time-telling device.

The summer sundial is one type so named because its accuracy is limited to just the summer season. I think it’s interesting to think that those ancient peoples began to realize that “mid-day” moved as the season moved. So begins astronomy and other sciences.

That summer sundial is remarkably accurate. Why? The gnomon (the upright shaft that casts the shadow) is parallel to the earth’s axis and the sundial face is parallel to the equator. Imagine a real pole stick in the ground at the North Pole with a circle drawn on the ground (snow?) of twenty-four hour numbers around it. The shadow would do a nice job of marking the hours.

I’m guessing that you don’t live at the North pole (if you do, comment below – I’d love to hear from you!),  so you will have to adjust the angle of your sundial accordingly.

Though we have come a long way from the astronomy of the ancients, their model of the Sun’s motion helps explain why the sundial works. For the purposes of our sundial study, it is a good approximation to assume that the Sun revolves around a stationary Earth on the celestial sphere, which rotates every 23 hours and 56 minutes about its celestial axis, the line connecting the celestial poles.

Since the celestial axis is aligned with the axis about which the Earth rotates, its angle with the local horizontal equals the local geographical latitude. The stars are fixed up there, but the Sun changes its position on the celestial sphere. That angle that you need to adjust on your sundial is because of the positive declination in summer. (If you are a map and compass person, you know about that too – because the North pole “moves” too. I’ll leave that for another post.)  Anyway, there is a negative declination in winter and zero declination (on the celestial equator) at the equinoxes.

The path of the Sun on the celestial sphere is known as the ecliptic. If you’re an astrology fan (laugh if you will, but the ancients learned a lot about the heavens that way), you may know that the ecliptic passes through the twelve constellations of the zodiac in the course of a year.

If you really would like to get in the math of this, try

Precision sundial, Belgium. (Precision = ±30 seconds)

Precision sundial, Belgium. (Precision = ±30 seconds)

If you watch the sundial throughout the summer, you will notice that it continues to gain a few minutes a week for a while, and then it starts to lose time at about the same rate for a while.

I thought someone was fooling around with the dial. But it is  because the earth alternately does slow down and then speed up in its orbit and the the change is reflected in the sundial’s readings.

By the end of  September, my summer sundial will have stopped working for the year. That’s because the sun has gone below the equator. The sundial face is a parallel to the equator, so the sun is below the sundial face.

And that’s what autumn is all about. It begins when the sun moves goes south of the equator.

You can buy a sundial, but building a sundial is pretty easy and a great DIY project. And if you have kids, it’s a great project that can teach a whole range of things.