Setting Sundials

Clocks are usually set to agree with other clocks. Ultimately all time keeping is set by reference to the position of the sun in the sky. This was done in ancient times using sundials.

Reference Clocks

In modern times we normally set clocks against other clocks that we trust. These days we usually trust the time from a cell phone. So we use that to set mechanical clocks.

After the invention of the telephone, local telephone companies maintained phone numbers that anyone could call to learn the time. National government run standards bodies also ran radio stations that constantly told the time. These were similar in nature to using cell phones for the time, just using older voice technology.

Before the invention of the phone and radio, clocks were also set against other reference clocks. This meant setting clocks against the clock from a local authoritative source, say the clock on City Hall or in a church bell tower.

Big Ben functioned this way for the city of London. Nearly every other developed town by the early 1800s had a similar reference clock.

These clocks often had bells that chimed at the hour so anyone within ear shot would know the time and would be able to set their own clocks in synchronization with the local master clock.

Reference clocks let the public schedule their lives around the time of day. Students could use time to know when their teacher was going to be in class. Law courts scheduled trials according to these clocks. Modern life itself is marked by everyone being able to understand the current time.

Bible Clocks Using A Reference Clock

Since most people have access to a reference clock, most people never need to know how to set their own clock using the sun.

Most people also have no reason for setting a clock against the sun because the purpose of most clocks is to coordinate with other people using time.

Bible Clocks are potentially different.

It is possible to setup a Bible Clock against a local reference clock. In this case the Bible Clock is being used to indicate time O'Day instead of time O'Clock.

This is useful by itself for general understanding of time references in scripture. Those references do not tell time the same way as modern clocks. But they would still be synchronized with local reference clocks.

In this configuration the hour and minute hands on the Bible Clock are in the same place as the hour and minute hands on any other mechanical analog clock. Reading the time from such a Bible Clock can done in either time format.

Alternatively, Bible Clocks can be setup without regard to a local reference clock. In this use of Bible Clocks the intent is to read the local time using the scriptural O'Day system of telling true local time.

Setting up a Bible Clock in this way is the same as how all clocks were setup into the 1800s. Most towns held a reference clock, say in the church bell tower, or city hall clock tower. Such public clocks marked the arrival of civilization itself.

These local reference clocks were always set against the sun. A local timekeeper, usually called a Horologist, would keep that reference clock working and accurate by measuring the movement of the sun.

For ranchers and anyone else living remotely, they had to set the time on their own mechanical clocks by themselves.

Without A Reference Clock

So it is important to understand how clocks work without any other reference clock. This is the fundamental skill that is needed for telling time by first principles.

Bible Clocks, especially if configured for various prophetic uses, are not normally set against a reference clock. They should be setup independently. Setting them up against the sun at their location is one of those prophetic uses.

It is possible to accurately setup a Bible Clock to track the local sun based time using cell phone data, but that is beyond our scope here. Instead we look at how to do this using ancient methods.

Learning how to correctly set a clock involves learning how to determine the point in time during the calendar day when it is high noon. The word "noon" is a scriptural term. It survives as an important word in English because it is so important compared to all the other possible times in a calendar day.

High Noon

The original meaning of the word "noon" meant the time of day when the sun is at its highest point overhead. Ahead of noon the sun is rising. After noon the sun is setting.

This definition changed some after the invention of time zones, which will be covered later. So these days noon means 12:00 Noon on the clock, not noon in the sky.

To find actual noon in the sky requires the use of some device that can track the location of the sun. That device must be able to accurately indicate when the sun is at its highest point. Ignoring clouds, this is also the point in the day when the sun is brightest.

Noon is important and special because it is the only trustworthy time when the sun can be known to be in a trustworthy east-west location.

Noon has another important feature. The sun is always directly overhead at noon, and that location it does not vary east-west due to season nor to terrain. Seasons and terrain impacts nearly every other aspect of the sun's location in the sky.

Sun Tracking Devices

Tracking the sun is not very easy. It is hard to look at because of its brightness. It can easily blind anyone who looks directly at it. So tracking the location of the sun is usually done through some device that tracks a shadow cast by the sun.

Anything that casts a shadow can be used to find high noon. This includes a simple stick in the ground. The taller the stick the more accurate the measurement that is possible. So tall trees and even structures can be used to find high noon.

Though any of these things will work, the fundamental device for tracking sun shadows in this way is known as a sundial.

Sundials are usually setup in a specific location and anchored to the ground. This fixed setup is done because adjusting the sundial for a particular location can be hard to do. The base of the sundial is locked into location using stone, or brick or modern concrete. Once fixed to the ground it remains accurate for years after original installation.


Various parts of common sundials have various technical names. Most of those names do not matter to our discussion here.

The part that does matter is the gnomon. This is the pointy part of all sundials that points up into the sky.

The edge of the gnomon is the part of the sundial that will cast a shadow onto some sort of marked dial. To understand this better, we turn to a big example.

Morehead Sundial

In the following 2 videos are of the Morehead Sundial. This is located outside of the Morehead Planetarium. That planetarium is part of the University of North Carolina's Chapel Hill campus.

The distinctive feature of this sundial is the huge brass gnomon.

Also note the large numbered dial around the outside edge of the sundial. That dial is very similar to the marks around the face of most modern mechanical clocks. Both numbering systems are the same. This particular sundial is using Roman Numerals.

This first video is from a news helicopter flying over that sundial.

The following is of some fellow walking up to the sundial. You can get a good sense of the size of the gnomon. Note he checks is cell phone and the time on his phone is over an hour different from the sundial. It surprises him, but this is not uncommon nor unexpected.

How could there be such divergence from time on a cell phone to time told using a sundial?

The problem is not with the sundial, but with the phone. The phone is telling time in a way ultimately derived from a political process. Politics is not a truth based system, as the phone in the previous video so testifies.

It is possible to calculate the error from the phone's sense of time back to the accurate local time. That is beyond our scope here. First, we need to understand how sundials are installed. That process determines how sundials tell time long after initial installation.

Installing a Sundial

So at noon each day, the sun is crossing over the location of the sundial from the east side of the sundial to the west side of the sundial.

The line on the ground that is being crossed stretches all the way from the north pole, through the sundial, and then on to the south pole. On maps, lines like this are called meridians. All sundials on the same meridian line tell the same time.

Meridian lines are fundamental to map making. So keeping time is fundamentally related to making maps. The relation between clocks and maps has mattered at several times in modern clock making history. We will return to stories stories later.

The gnomon of the sundial is set to align exactly with the meridian line that goes through the sundial. This is so when the sun crosses that line, the sun also crosses from the east side of the gnomon to the west side.

Setting up a sundial requires determining the exact direction of that meridian line on the ground at the sundial.

For sundials in the northern hemisphere, the gnomon points due north. In the southern hemisphere the gnomon points south.

At places on the ground that are at or or near the equator, sundial gnomons are usually suspended in a frame with both ends lifted above the ground.

Finding the North/South Meridian Line

Finding that north south line can be done without any special instruments, just be tracking the sun itself. Shadows cast by the sun are shortest at noon. So anything that casts a shadow can be used to find the meridian line needed to set a sundial. The taller the object, the longer the shadow and thus a more accurate meridian line.

More commonly, sundials are setup using a compass. Compasses also point along a line from north to south, so they point at the meridian line. The builders of the Morehead sundial probably used a compass to install their sundial.

Note that compasses are pointing at the magnetic north pole of Earth. This is not exactly the same location as the north pole. For most uses this difference does not matter. But, in locations near the poles, this approximation does not hold. So sundials cannot be setup using compasses near the Earth's poles. This was not a problem in ancient times.

Combining sundials and compasses into a single device has been done since ancient times. This makes it possible to setup a sundials in temporary locations, making sundials portable.

These combined sundial/compass devices can be small enough to strap on a wrist, or else they would be kept in a small case. Modern reproductions of these ancient devices combining sundials and compasses are readily available on Amazon.

Here is an example of such a device and how it is setup. This is showing how a compass is used to correctly orient the gnomon of the sundial itself.

Obviously, the builders of the Morehead sundial did not need to leave their compass at the sundial. Once that sundial was set into the ground, they could take their smaller compass with them.

Note the Morehead sundial is outside of a planetarium. Sundials have accompanied astronomy since ancient times. All astronomical observations depend on an accurate and calibrated clock.

Astronomers also depend on telling time at night, which a sundial cannot do. This is why astronomers were the first serious builders and purchasers of accurate mechanical clocks.

Mean Time

All sundials tell time in what is called "Mean Time." This is the time of day measured against the sun in that exact location.

Mean Time at any given place is almost never the same time of day as indicated on a modern cell phone at the same location, as the fellow in the video above found out.

Mean Time is continuously varying around the world. All locations east and west of any given sundial will tell time differently.

This did not matter in ancient times, nor did it matter to early astronomers working alone.

Most people did not travel nor did they have high speed communications with other places. This would become a serious problem only with the invention of the Railroads. We will return to that problem later.

Astronomers not only wanted mechanical clocks for telling time at night. They also wanted very accurate clocks. A famous astronomer eventually realized a pendulum could be used to regulate very accurate clocks. We turn there next.