How do Sundials Work | Border Sundials Handmade Sundials
Since pre-history people have regulated their lives by the apparent motions of the Sun and the shadows cast it casts on the ground
Of course we all know it’s really the rotation of the Earth on its axis which causes the shadows to move, but for convenience we say the sun moves.
For every degree that the Earth rotates, the sun appears to move one degree across the sky. It takes the sun 4 minutes to cover 1 degree, and an hour to cover 15 degrees, therefore 360 degrees or one full rotation in 24 hours.
If you know this and where South is, you have all you need to tell the time. Let me explain: The Sun goes round the Earth in 24 hours. Therefore if it is due South at midday, and due North (hidden from us by the Earth) at midnight, then at 6am it must be due East, and at 6pm it will be at due West. If you look to the South, East is over your left shoulder and West on your right shoulder, so it is easy enough to estimate the angle of the sun between these points and guess the time. Possibly not good enough to keep an appointment, but a good trick, and you get better with practice.
A sundial is a device that tells the time of day by measuring this apparent movement of the Sun accurately. The simplest sundials consists of a flat plate (the dial) and a shadow caster (the gnomon) which casts a shadow onto the dial. As the sun appears to move across the sky, the shadow aligns with different hour-lines which are marked on the dial to indicate the time of day.
The gnomon usually casts a broad shadow so that it is easy to see, however this can lead to uncertainty over which side of the shadow to read. To avoid this one side is usually curved and the style will be a straight line which lines up with the hour lines.
On more complicated sundials the gnomon may also be a rod (like an armillary sphere), a wire or anything else (such as your own body).
The style of the gnomon must be parallel to the axis of the Earth for the sundial to be accurate throughout the year. Therefore the style’s angle from horizontal is equal to the sundial’s geographical latitude.
You can see from this picture that a sundial on the equator would have a gnomon angle of 0 degrees, while one on the poles would have an angle of 90 degrees.
Using the sun’s light to cast a shadow onto the hour lines, sundials tell ‘Local Apparent Time’ rather than ‘Mean Time’ as conventional clocks do. To make modern life work, the world is divided into time zones and our clocks are set to national or regional time (GMT for the UK).
For each degree of Longitude to the West of Greenwich (or your local meridian) the sundial will appear to be four minutes slow and for every degree to the East it will appear four minutes fast. This is the time it takes the sun to travel one degree, and is known as Longitudinal Variation.
Sundials do not understand daylight saving time, so in the summer you will need to add an hour onto the time shown to take account of British Summer Time.
By adding or subtracting the Longitude Variation and the Equation of Time you are able to read GMT at any position in the country.
The Equation of Time
Longitudinal variation and Summer Time are fixed and easy to understand. Much more complicated and deserving of it’s own page is The Equation of Time.
It’s about Time… to Make a Sundial!
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Clock time: Measure the minutes as the sun shifts overhead. All you need is some sunshine, simple household items—and some time! Credit: George Retseck
Have you ever watched a movie set in an earlier era, and when a character asks what time it is the other characters, who don’t have watches or cell phones, all look at the sky? They are not looking at a giant digital clock above, they are using the position of the sun in the sky to tell time, as people have done for generations.
The oldest known instrument for telling time, the sundial, allows us to track the position of the sun more accurately. Up until the early 19th century sundials were the main instrument people used to tell time. If they are correctly placed, sundials can be used to accurately tell time down to the minute!
In this activity you will be making your own sundial and using your body to track the movement of the sun across the sky!
For millennia people have used sundials to tell the time of day based on the apparent position of the sun in the sky. There are many types of sundials, but in general each consists of a gnomon, a thin rod that casts a shadow onto a dial, and a flat plate or platform. The apparent movement of the sun across the sky is the result of Earth’s rotation on its axis. As our planet spins, the sun appears to move across the sky—but really we’re the ones who are moving!
As the sun’s position changes in our sky, the shadow it casts will align with lines marking each hour indicating the time of day. The accuracy of a sundial is affected by a number of factors, including the fact that the angle of Earth’s rotation isn’t perfectly perpendicular, and Earth isn’t perfectly round. As a result, corrections have to be made to sundials to account for these changes.
For this activity we’ll be making a simple sundial (using a clock to help us!) as well as tracking the position of the sun by observing our shadows.
- Sidewalk chalk
- Tape measure or yardstick
- Pen or pencil
- Large concrete space with no shadows
- Paper plate
- Plastic straw
- Markers or crayons
- Paperweight or a few small stones
- Adult helper
- Sunny weather
- This activity works best if you start early in the day, so you have a few hours of daylight to do your testing. We recommend starting at 9 A.M. and testing off and on until noon—or starting at noon and testing off and on until 3 P.M.
- Start by choosing a place where you will always stand during this activity. Make sure it is in the middle of the open concrete space, with no shadows nearby. Mark this space by using your chalk to outline your shoes.
- Stand in your chosen spot and have your helper use the sidewalk chalk to trace the outline of your shadow on the concrete.
- Use your chalk to write the time at the top of your shadow.
- Repeat these steps every 30 minutes, each time marking the time of day at the top of your shadow.
- While you are waiting to trace your shadow use a pencil or pen to carefully poke a hole through the center of your paper plate.
- Check the time, round up to the nearest hour, and write this number at the very edge of your plate with a crayon or marker. For example, if the clock says 9:45 A.M., write “10” on the plate. Use your ruler to draw a straight line from the number you wrote to the hole in the center of the plate.
- Wait until the clock reads the hour that you wrote before proceeding to the next step (for example, if you started at 9:45 A.M., you would wait 15 minutes until the clock reads 10:00 A.M.).
- Take your plate and plastic straw outside. Put the plate on the ground and poke the straw through the hole you made. Slant the straw slightly toward the line you drew.
- Carefully rotate the plate so the shadow of the straw lines up with the line you drew. Do you think the shadow will stay in the same place all day? Why or why not?
- Place the paperweight or stones on the very edges of the plate to hold it in place.
- Every hour check your sundial and the position of the shadow on your plate. If you started at 10 A.M., note the position of the shadow at 11 A. M. and write “11” on the edge of the plate where this shadow falls. Each time you check the sundial, write the hour on the edge of the plate where the shadow falls. Why do think the shadow is moving? What does your sundial remind you of?
- After several hours of tracing your own shadow observe the positions of each tracing. Did your shadow move during the day? What else changed about your shadow with each tracing? What do you think caused these changes?
Observations and results
In this activity you observed the movement and changes in shadows over the course of the day. In the case of your own shadow the pattern you noticed depends on the time of day you started. If you started this activity in the morning, you should have observed your shadow started out long and by the middle of the day it looked much shorter. If you started in the middle of the day, you noticed the opposite—your shadow started off shorter and grew longer over the course of the afternoon. Regardless of when you started, however, you should have noticed the position of your shadow changed over time. As it got later your shadow moved in a clockwise direction from the first outline you drew—as long as you were completing this activity in the Northern Hemisphere!
For your sundial you should have noticed something similar. At each hour the shadow of the straw was in a different position, each time moving clockwise from the start position. After a few hours you should have noticed the sundial looks like the face of a clock with the numbers evenly spaced out around the plate.
The reason for your shadow’s change in shape and position has to do with Earth’s rotation on its axis. As Earth spins, the sun appears to move across the sky. The sun is highest in the sky at noon or midday, and at this point it casts its shortest shadow because it is most directly above us in the sky. In the morning and later in the afternoon the sun is more off-center and therefore casts a longer shadow.
The position of the shadow also changes as the sun appears to move across our sky. You can see something similar if you shine a flashlight on your hand, and then move the flashlight. As you move the light, the position of your hand’s shadow will change with the movement of the flashlight. The position of the sun in our sky is dictated by the speed of Earth’s rotation—it turns on its axis at a speed of 460 meters per second, or approximately 1,000 miles per hour! When Earth rotates 15 degrees on its axis, it’s just as though the sun has moved 15 degrees across our sky. As a result of this movement (and depending on where you live), the shadow cast by the sun moves approximately 15 degrees each hour so that over the course of 24 hours it travels a full 360 degrees around your sundial.
More to explore
A Matter of Time, from Science Buddies
Make Moon Cycles—with an Orange!, from Scientific American
Changing Constellations, from Science Buddies
Science Activities for All Ages!, from Science Buddies
ABOUT THE AUTHOR(S)
How to determine the time by the sun.
How to make a sundial with your own hands. Determining the time from the shadow of the sun
How to determine the time from the sun?
Determining the time by the sun in a certain situation can help you out, for example, you will know the exact time if you forget your watch at home and do not miss the bus or train. The method of determining time by the sun is useful not only for travelers and summer residents, but also for all other people who do not have watches. There are different ways to determine the time by the sun, in fact, we will tell you about them today.
HOW TO KNOW THE TIME FROM THE SUN IN THE NORTHERN HEMISPHERE OF THE EARTH
So, to determine the time from the sun, you need to make (make) the simplest sundial. To do this, you need to determine the exact direction of the cardinal points, a thin stick and the sun. The easiest way to tell time from the sun is to make a sundial out of a compass and a match.
HOW TO TELL THE SUN TIME WITH A MATCH AND A COMPASS : Place the compass on a level surface, then accurately determine the direction of the cardinal direction NORTH, set the compass dial so that the compass needle points north and the compass number 180 degrees azimuth. Place a match exactly on the center of the compass. Everything, the sundial is ready. Now, in order to determine the time from the sun and the given sundial, you need to look at where the shadow of the match falls. It turns out that if the shadow indicates 180 degrees on the dial, this is equal to 12 o’clock in the afternoon, if 270 degrees, then this equals 18 hours, and 90 degrees 6 am. It turns out that one hour of time is equal to 15 degrees on the compass. With this definition of time by the sun, it is necessary that the sun shines directly on the compass and match.
HOW TO MAKE A PROFESSIONAL SUNDIAL : In this case, a little more effort is required. We will not tell you how to carve a sundial out of wood, since no one will spend time on it on a hike, but we will tell you how to make a sundial on the sand or on the ground and determine the time with the help of the sun. So, for example, you are fishing on the river bank, and you periodically need to know the time, but you don’t want to constantly get the compass and carry out the above manipulations. To do this, you can make a professional sundial on sand or earth, for which you need to draw on the ground a semblance of a compass with a degree scale (the figure should point strictly to the north with the number 180) and put a long stick in the center, from which a shadow will fall by degrees, and show time. And even in place of degrees you can write the numbers of time. The sun will move across the horizon, the shadow will move, and you will always determine and know the time.
All other methods will not give you an accurate determination of time by the sun, for example, you cannot determine the exact time by sunrise and sunset, since not everyone can know what time sunrise and sunset occur, because it changes depending on the month of the year .
HOW TO KNOW THE TIME FROM THE SUN IN THE SOUTHERN HEMISPHERE OF THE EARTH
In the southern hemisphere of the earth, the time from the sun and the compass is recognized in the same way as in the northern hemisphere, except that the compass needle must point not to the north, but to the south .
HOW TO KNOW THE TIME FROM THE SUN ON THE EQUATOR
At the equator the sun passes exactly above the horizon, so you do not need to know the direction of the north, and to determine the time from the sun, you need to make the same clock as in the first case, but just do not place them horizontally but vertically.
SUN DIAL ERROR
You must remember that you will determine the physical time using the methods described above, but it may differ from the actual time in your region. So, for example, Moscow actual time is 12:00, at the same minute, according to the law of Russia, the actual time in Kazan is 12:00, but the physical time determined by the sun in Kazan is 13:00, since the distance between Moscow and Kazan equals approximately one geographic time zone, and if you take into account the transition to summer and winter time, then the difference can be 2 hours. Therefore, make a sundial, compare it with the actual time and make adjustments. Thus, for the future, you will know how to make an amendment when determining the time using a sundial.
How does the sundial tell time?. Everything about everything. Volume 2
How does a sundial tell time?. Everything about everything. Volume 2
Everything about everything. Volume 2
How does a sundial tell time?
The Sun was the first clock for man. Long ago, man determined what time it was by looking at the sun as it moved across the sky. It was easy to distinguish between sunrise and sunset, but it was much more difficult to know the time when the sun rose above the horizon. It was during these daylight hours that it was difficult to tell the time, guided by the sun. The man noticed that the length of the shadow changes throughout the day. It became clear that the time could be more accurately determined by looking at the shadow rather than at the sun. This discovery was only one step away from the invention of the sundial, which is actually a shadow clock. Instead of trying to look at the sun and associate the time of day with it, it is better to look at the shadow, which reflects the position of the sun in the sky. The first sundial was simply a pole stuck into the ground.
Stones placed around the pillar showed the position of the shadow as it moved during the day. So a person could measure the current time. Later they began to use huge stone columns. Cleopatra’s Needle, now on the Thames in London, is part of such a sundial. But smaller sundials were also used. One small Egyptian clock, which is 3500 years old, is in the shape of the letter L. It lies flat on the long side of this letter, and the marks show 6 periods of time. About 300 BC. e. the ancient Babylonian astronomer invented a new type of sundial, shaped like a bowl, a ball. The shadow cast by the pointer moved and marked 12 hours a day. This type of sundial was very accurate and has been used for centuries.
A sundial can now be seen in the gardens, but it was built more for fun than real use. However, crude sundials can sometimes be seen on walls and window shutters. They are designed so that the crutch or edge of the window casing casts a shadow. In an accurate sundial, the pointer should be placed at an angle equal to the latitude of the place where the clock is used. Just a vertical stick shows the correct time only at a certain latitude, at a certain time of the year. If the dial is flat, the hour markers should be unevenly spaced on it.
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