To begin with, twenty-four hours in a day and 360 degrees in a circle are arbitrary numbers. But they lead us to the question: What time is it?
People like patterns. Recognizing patterns allows planning. Planning has led to a lot of things, and arguably our success as a species is one of them. So folks noticed that the sun came up every day, set every day, and came up again the next morning, pretty much in the same place it came up the day before.
Higher and higher every day
Till over the mast at noon…
Noon is when the sun makes its meridian passage — when it’s due south (if you’re above the Tropic of Cancer) or due north (if you’re below the Tropic of Capricorn). That’s what we navigators call “LAN” or “Local Apparent Noon.” (Meridian comes from the Latin medius, middle; dies, day — the middle of the day.)
If you’re between the Tropic of Capricorn and the Tropic of Cancer, this is a bit trickier. The sun could be directly overhead. The Tropic of Cancer (in addition to being a book by Henry Miller) is 23.5 degrees north of the equator. The sun is directly overhead there when it’s in Cancer. The Tropic of Capricorn (aside from being another book by Henry Miller) is 23.5 degrees south of the equator. The sun is directly overhead there when it’s in Capricorn. (And you thought that those hours reading the newspaper horoscopes would never come in handy.) The area between the two Tropics is called “the tropics,” and a hot, nasty place to live it is, to be sure.
Anyway … back a long time ago, perhaps in Babylonia (hah! An Iraq reference! This post wouldn’t be complete without one!) someone decided that there were twelve hours in a day. Twelve is easily divisible by one, two, three, four, six, and itself … so it’s a good number. A natural day runs from sunrise to sunset. That’s the “natural day” that Dr. Faustus is carrying on about:
Now hast thou but one bare hour to live,
And then thou must be damn’d perpetually!
Stand still, you ever-moving spheres of heaven,
That time may cease, and midnight never come;
Fair Nature’s eye, rise, rise again, and make
Perpetual day; or let this hour be but
A year, a month, a week, a natural day,
That Faustus may repent and save his soul!
The natural day was divided into natural hours, twelve from sunrise to sunset, another twelve from sunset to sunrise. In northern lands that meant that a night hour in winter was a lot longer than a day hour. Chaucer, in his Treatise on the Astrolabe (the 14th century’s answer to Asimov’s An Easy Introduction to the Slide Rule) goes into quite some detail on how to figure the natural hours of day and night.
This worked great up until the invention and widespread use of mechanical clocks. Clocks didn’t change the length of the hours from day to day. (If someone asks you what time it is, betcha you say “o’clock” (of the clock) rather than giving the natural hour, dontcha?) Early clocks were only accurate to within a quarter hour. Due to the Equation of Time sundials varied over the course of a year up to around a quarter hour each way. Everything worked out fine.
Every day at meridian passage you’d reset your clock to noon, and let it run for another twenty-four hours, then reset. From meridian passage to meridian passage is one solar day. The clock time varies from the natural time by up to around sixteen minutes (depending on the day of the year). You figure the difference with the Equation of Time.
Look, up in the sky!
It’s a bird! It’s a plane! It’s an analemma!
Yes, this mild-mannered figure-eight of light, strange vistor from the Copernican solar system, is the true sun. If you put a stick in the ground and put a pebble on the tip of that stick’s shadow every day at noon, the pebbles, over the course of a year, would trace out that figure.
One of the mysteries of Chartres Cathedral (proving beyond the shadow of a doubt that it was built by the Templars to hide mystic knowlege) is that at one spot on the floor there’s a brass nail which is only illuminated at noon on Midsummer Day. Was Chartres built just so the light would hit that one spot on just one day? I kinda doubt it. I suspect that what happened is that someone waited until noon on midsummer day, saw where a random spot of light was landing, and banged a nail on in.
Not that the sun following a unique path and shadows casting a unique pattern would be a big secret or much of a surprise by then — the clever buggers who built Stonehenge had the fact that the sun rises and sets at different places on different days, and stands at a different place in the sky at mid-day, figured out.
The true sun is that big yellow thing up in the sky that you can see with your own two eyes if you don’t mind going blind. The mean sun is the average of the true sun over the course of a year. The Mean Sun is what Mean Time is based on, as in Greenwich Mean Time, beloved of navigators everywhere.
The analemma’s shape comes from the fact that the earth’s path around the sun is an elispe, and the axis of the earth is tilted at 23.5 degrees (actually 23 degrees 44 minutes, but who’s counting?) away from the plane of its orbit. By a weird coincidence, the analemma is 23.5 degrees of arc from top to bottom, and is the loops are thirty-two minutes of time (eight minutes of arc) across at their widest.
Local time, setting the clock to noon each day, worked great for centuries. It worked right up to the point people had to start comparing times in different towns. At the same time (so to speak), clocks themselves were moving from crew-served tower-mounted devices to man-portable pocket instruments.
First came the railways. The sun moves across the sky, and it moves pretty fast (about fifteen degrees an hour), but even so railway speeds made the differences in times noticeable.
At my latitude (around 45 North), say there was a train pulled by a John Bull engine, running due west at thirty miles an hour from Appleton to Beeville, thirty miles due west. If that train pulled out of Appleton straight up on noon by the courthouse clock and ran west at 30 MPH it would arrive in Beeville at 12:56 by the Beeville town hall clock. If the same engine running at the same speed left Beeville straight up on noon by the Beeville clock, though, it would arrive in Appleton at 1:04 pm by the Appleton clock. This made scheduling difficult.
If you ever hear the expression “railroad time,” it means the master clock kept by one railroad company or another to which all its conductors’ watches and all its station clocks were set.
The New York Central ran on New York time. The Pennsylvania Railroad ran on Philadelphia time. The B&O ran on Baltimore time. Stations served by more than one railroad had clocks set for several times, including that town’s local time. The situation was … amusing.
Within towns (and cities were growing to the point where the eastern part could be a minute or more ahead of the western edge), setting your watch could be confusing. Individual jewelers would determine the time at their shops and set regulators in their windows. (In case you were wondering what a Regulator was.) Their customers would come by and set their watches to the jeweler’s time. Eventually many towns adopted the Time Ball solution, whereby a ball on a mast on top of the town hall would be lowered at noon town hall time, so folks at a distance could see it all at once and set their watches accordingly. The last remnant of this is seen at New Year’s Eve in Times Square. Other communities rang a bell, blew a whistle, or fired a gun at noon — and you can find examples of those methods still.
The ultimate answer turned out to be time zones — adopted in the UK in 1848, in the USA in 1883, and in Germany in 1894 — in which the earth was divided into twenty-four time zones, each fifteen degrees across, where by common agreement it was the same time at all points inside of that zone. Zone time matched local time only at one meridian in each zone — along the Reference Meridian, mid-way between the zone’s boundaries. The whole country ran on Railroad Time.
Daylight Saving Time is just moving a little farther away from reality, by using the reference meridian for the time zone one zone to the west of your current location to set your clocks. It was introduced in the USA during WWI. I see that President Bush has recently announced the centerpiece of his new energy policy — extending the length of Daylight Saving Time by a couple of weeks. He’s turning the corner in the War on Darkness, I guess.
Now is the time to declare loyalty to True Time, based on the True Sun. That time is told with sundials.
Shall we tell time by the True Sun? We shall!
Naming the parts: This is your nodus point, which in your case you have not got.
You wanted to know the declination of your wall, didn’t you? The shape of a vertical sundial’s dial depends on the wall’s declination — noon is always vertical.
This is a calculator for creating your dial, given your latitude and the wall’s declination.
This is why number 12 is always vertical.
I got your equitorial sundials. The style is parallel to the earth’s axis, the dial plate is parallel to the plane of the earth’s equator: The dial has a northern and southern face; the northern face is read in summer, the southern face in winter.
I got your polar sundials. The dial plate is parallel to the earth’s axis. The style is also parallel to the earth’s axis and the hour lines are parallel to the style and to each other. The distance between the hour lines is not dependent on the latitude but only on the height of the style.
Some other fun and useful things:
How to find latitude and longitude by Local Apparent Noon.
More on Latitude.
A Solar Noon calendar.
In conclusion: Sundials. They aren’t just for earth any more.