Why Sundial Time is Slightly Different than Normal Cock

The solar time was well defined in the equation of time presented by Ptolemy about two thousands year ago. The equation analyze the earths motion on its axis and around the sun in different period of a year to measure time. The equation was a subject matter of Astronomers until recently.

Ancient people used sundial to measure time until the last centuries invention of mechanical clock take over timekeeping from sundial. Although modern people do not use sundial to keep time but they use to buy or collect sundials as a matter of curiosity and antiquity. They observe the difference between the sundial time and their clock time and the issue of the Ptolemy's`s equation of time came in light again.  

The earth is rotating on its own axis and it is spinning around the sun as well. It takes approximately 365 days or a year for the earth to finish it`s full orbit around the sun , and this path is not circular it is elliptical. So, the earth come close to the sun for several months in a year and according to Kepler's law the spinning motion of earth on it`s axis become faster during this time. This phenomenon influence on a major measurement our clocks are based on, that is it counts the day with 24 hours. But, when  each spins very fast at that closer time to the sun, it actually spins around its axis less than 24 hours.

That make a lot of difference between time when we measure it upon solar basis and and measure it upon a regular clock basis. Scientists estimated that the difference become 14 minutes in Mid-February and just over 16 minutes in mid November. On the other hand, when earth gets further than the sun , it takes a bit more than 24 hours to finish a spin on axis or meridian. So, the clock gives us a average time all over year which is roughly 24 hours; but it is not totally accurate. But, it works as  the minor change does not give any  significant effect on our daily life.

Factors that Influence accuracy of a Sundial

Think you are flying from USA to Europe. Do you need to change the time of your clock? Yes you do. Because, time is different in various location of the world. So, like normal clock, does the time of a sundial depends upon the city where you living? Quick answer, yes. More specifically, it is important that on your location in the planet earth. Therefore, if you need tools to guide you when you deal with sundials.

First you must understand that magnetic north is not the same as True North, pointing to the globe at the North Pole. Sundials use true north as a reference point not the magnetic north. So, if the sundial is calibrated to align with magnetic North, it will not give the correct time display.

The difference between magnetic North and True North is that magnetic inclination is different in different places. Therefore, while you buy one, you need to position it with actual geographic north by compensating with the magnetic north of your location.

Also note that the position and length of the shadow cast by sundial gnomon depends not only on time of day, but also the time of the year. Different seasons mean that the position of the sun at a certain time of day is different in the sky and has significant impact on the shadow cast by gnomon. 

The shadows cast by the gnomon tell the time of a day, because the shadows change direction corresponding to the times of the day. A sundial uses the position of a shadow to tell time. This is not exactly what you want on your clock that shows the standard time, compared with the solar time that is the time from the sun itself. The two measures are not exactly the same time, of course, if the trend of telling time is in the same direction: with the direction of sun.

Before the invention of the mechanical clock, for example, before 1883, people used primarily sundial. Each neighborhood has maintained their own time that they followed, depending on the sundial, which of course is based on the position of the sun in the sky of that particular neighborhood. Of course, the time may significantly vary in closer neighborhoods; say between two cities located 25 miles to the east or west from each other. But it works for that time until the arrival of the mechanical clock, which is not really important that the time was wiped out in minutes.

The Science behind the Invention of Sundial

What could be more boring than walking up in the morning by listening the alarm clock on the table beside the bed? Time does not understand any fun and sometimes we just want to disappear the clock from our everyday life.

Ever wondered how to monitor the time when all the clocks in the world is no more? Long ago our ancestors were faced with the problem. Watch, were, as we know it today had not yet invented.

It is unknown to us how human first measure the time. The difference between the dark of night and light was probably the first division of the former recognition. They also noticed that the sun rises above the eastern horizon and climbed under the western horizon, to bring the dark world.

People observe and think of the shadow of trees, rock, and even own body for a long time in the early morning hours and were becoming shorter and shorter until it disappears when the sun was higher in the middle of the day. They also found that the shadow was still lying on the other side of the tree at dusk.

The length of shadow gives a approximate idea about how much passed in a day. Hours were probably from someone who put a stick in the ground, and made signs are visible on the ground, invented in the shadow of the rod that represents hour. This shadow of the stick is the oldest form of sundial.

People then judge the length of day from the position of the shadow of a rod. The technical term for the gnomon shadow stick translated from Greek and means one who knows the time.

Egyptians built obelisks; a sort of  stone towers. Everyone can say, in time to see the shadow of the obelisk. Cleopatra's Needle is also a known synonym of Egyptian Obelisks.

As the Earth rotates, we see the movement of the sun across the sky. The sun casts a shadow on the right of the objects in the northern hemisphere. When the sun rose, and at the same time and place on the horizon every day, it would be a shadow clock to adhere to accuracy. But the earth still turns like a top. It revolves around an imaginary line called its axis. Line between the center of the Earth North Pole to the South Pole. Earth's axis is always tilted in the same angle.

Earth makes a full circle or turn around it`s axis in every 24 hours.. This rotation on its axis is from west to east which eventually causes day and night. Since the Earth rotates, the night side is moving towards the sun, and the day moves towards the dark side.

The North Pole is tilted toward the sun for six months and gets away from the sun the rest six months because the axis of earth rotation makes around 30 degree angle. The shadow is different in different months of the year because of this reason.

Because the earth is round, or curved, the soil on the basis of the shadow of the stick will not be the same angle with the sun, like Ecuador. For this reason, the shadow of the stick is not moving at a constant rate throughout the day.

Finally human discovered that a aiming the gnomon in the north presents a more accurate sundial. Because of the tilt of the Earth, hour characters remain same all over the year,.Following this discovery, people were able to built much better sundial.

Throughout the history numerous types of sundials were developed to provide time more and more accurately. Egyptians invented time stick in the form of T. It consisted of a vertical post and the crossbar from five hours were written characters on a stick. The stick was faced north in the morning. Shadows of the cross fall on the stick and move till noon. 

In the evening, the stick turned to the west. Egyptians built smaller sundial after some trial and errors. The opening of smaller versions of portable sundial tells the time and became important for the Egyptian. this smaller version of obelisk turned as sundial.

Peasants of the middle ages in Northern Europe using the sun set on the bottom of his wooden clogs. To find out the time, the peasants  stands on facing the sun. The hour was reported from the shoe heel shadow cast on the dial.

Another device, medieval Europe has used is the wheel by hand. Gnomon of the sundial was just a stick; carried out on the corner with the thumb. Gnomon was held in his left hand in the morning, and hand horizontal to the ground and pointed to the west. In the afternoon of the gnomon in the right hand and arm pointed to the east. In search of greater precision in the movement of the Earth's position during the year, flat plate solar hours have changed ever after then.

In the Renaissance; sundial change rapidly and in a variety of designs have been created. In addition to the hour and minute hands tell the time brands have other properties that are sometimes recorded. Some of the selected sundial during Renaissance show the season, date, time of sunrise and sunset, zodiac signs, and directions.

People still make the sundial even today`s digital age. Sundial could count on him to keep the correct time, and is often used to set the time on the clock that stopped! Sundial still the most popular way to set the time before clocks to make it easier for people who say that it is time to think of our heritage.

Today, most people have in their garden sundial for decorations, not as a means of time. With the advent of digital clocks, you should take no more portable sundial. But the sundial should not be forgotten to modern man. The sun always raises in the morning and sets in the evening, resulting in a sundial to tell one of the most reliable method of telling time at present.

Idea behind Making a Sundial

You may heard about the term "Noon". I bet you might heard of this term. Noon is a way to say it is exactly 12 hrs. The sun at its highest point at noon or evening.  Therefore, when the sun is directly above or high in the sky above you, then the day is at noon. You can tell when it is noon if you don’t have any shade on sunlight.

Sundial is just one way is to visualize it, the dome of the sky or heaven is divided into sections. As the sun rises in the east and move to the west in different blocks of the sky within each hours. These  movement of sun in sky gives us about 21 ordinary day time hours in summer and 12 hours of average daytime time all over the year. So if you see the sun in the sky in the morning and when it close to the West, you can see it is afternoon.

The eastern side of the dome of the sky shows imaginary time of 6AM to 12 AM. The western part of sets the rest 6 hours from 12 PM to 6 PM. Share the idea of heaven, we can split the sky in 12 sections representing the 12 hours. Although it is difficult to imagine on March 12, representing the divisions on each side and then think of splitting into two parts.

You can also use the stick in the ground and measure the time from the shadows. This is called the shadow of the stick. Even as the gnomon. Shadow of the gnomon indicates the time. Mark the shadow on the ground working every hour. This will help you know when this time of year.

It is very easy to make a sundial of your own. Just draw a circle on white cardboard or paper with a compass. Make sure you mark the center of the circle.  Bring a vertical gnomon in the middle of the circle. Alternatively, a piece of cardboard, a pencil, pipe cleaner, floor, etc. can be used for the gnomon. Then find a sunny spot to place the sundial. Look at all hours and make a shadow on the edge of the circle at every hour.  Mark it with roman numbers that sync with the shadow; and you get your own sundial.

3 Adjustment of Sundial for Precise Time

Sundial is an ancient way of telling time. Some people even say that solar time is more accurate than the clock, sundial is a important alternative to modern clock that gives time reading based on sunlight.

However, a sundial cannot give time accurately if it is not adjusted properly. The planetary society recommends three types of adjustment need to read the time of a sundial more correctly. 

First of all, configuring the location of the sundial is very important that depends upon your position on earth that means you time zone. If you live in the western hemisphere, you should add 4 minutes for every degree of latitude. Alternatively, if you live on Eastern hemisphere, you should deduct 4 minutes for each degree. 


It is very easy to know how much degree ahead or behind you are living from central time zone or GMT. Just calculate the difference between your local time and London time; then multiply the result with 15.  This method allows you to play can convert solar time on your exact location at this time on your watch.

It is also important to set the summer time. In the summer you need to add one hour more in the reading of the sundial, to find the correct time.

The final configuration, you need to do is the most difficult and complex. With this readjestment, the clock was set to perform absolute uniformity, every hour a day without little distortion of a milisecond. However, this does not mean it is a easy task to do.

This meager adjustment that credits the sundial as most accurate clock in the world. In solar time, it is exactly 24 hours the earth rotates on its axis. But this time vary slightly except 4 days in a year. This time variation is very meager; can be 30 seconds more or less. It happens because, earth is bit tilted about 23.4 degree on its axis and the circular path the earth orbits around the sun is not standard circle rather than elliptical.

So, if you calculate this 30 seconds variation during a month, you will get about total 15 minutes per month. You may need a PHD in Mathematics to understand this, however if you are interested enough, you  can visit wikipedia  to have a gimps of the complexity working here. Fortunately, sophisticated commercial sundial this equation of time correction is done in a automated way.

If you have these three parameters, you can read the exact time on a sundial. Most people do not want to go through all this work, it is out of line subject, mostly people prefer to view a rough time. Regardless of how much detail you claim your time by sundial; it is an interesting addition to your yard or garden. They come in different styles and from different materials depending on what your aesthetic taste demands.