Teachpedia

A sundial is a device that uses the position of the sun to reflect the time. An upright stick, called a gnomon, is positioned to cast a shadow onto a pre-marked sundial face. As the sun moves across the sky, the shadow also moves. The concept can be easily demonstrated in your backyard with a very basic sundial built with a stick and a handful of small stones. There are many easy projects that kids can do to learn the concept, as well. For something a little more advanced, you can build a permanent sundial in your garden or backyard. After some measuring and a little carpentry, it will reflect the time precisely.

[Edit]Steps

[Edit]Using Sticks and Stones

Gather your tools.^[1] This extremely basic sundial is a great way to explain the concept with very little planning. All you need to create it are a few simple items found in your backyard. These tools are a straight stick (about two feet long), a handful of pebbles and a wristwatch or cell phone to tell the time.
Find a sunny area to plant the stick.^[2] Look for a place that gets full sun exposure all day long. Push one end of the stick into the grass or earth. If you live in the Northern Hemisphere, slightly slant the stick toward the north. If you live in the Southern Hemisphere, slight it slightly toward the south.^[3]
- If you don’t have access to a grassy area with soft earth, you can improvise.
- Fill a small bucket with sand or gravel and plant the stick directly in the center of it.
Start at 7:00 a.m.^[4] If you want to complete the sundial in a single day, begin in the morning after the sun has fully risen. Survey the stick at 7:00 a.m. As the sun shines down on it, the stick will cast a shadow. Use one of your pebbles to mark the place where the shadow falls on the ground.
Return to the stick every hour.^[5] Set an alarm or keep an eye on your watch so that you can update the dial at the top of every hour. Return at 8:00 a.m. and use another pebble to mark where the stick’s shadow falls on the ground. Do the same thing at 9:00 a.m., 10:00 a.m. and so on.
- If you want the greatest amount of accuracy, use chalk to mark each pebble with the exact time you placed it on the ground.
- The shadow will move in a clockwise direction.^[6]
Continue this process until dusk. Return every hour and mark it with a pebble on the ground. Do this until there is no more sunlight left in the day. Your sundial will be complete at day’s end. As long as the sun is shining, you can use this simple device to tell what time of the day it is.

[Edit]Using a Paper Plate and Straw

Gather your tools. This easy sundial is a great project for kids on a summer day. The tools required are very simple – you probably already have everything you need at home. The items required are crayons/markers, a paper plate, a sharpened pencil, pushpins, a ruler and a straight plastic straw.
- Start preparing the plate at around 11:30 a.m on a sunny, cloudless day.
Write the number 12 on the very edge of the plate. Use a crayon or marker for this. Take the sharpened pencil and push it through the center of the paper plate. Remove the pencil so that you’re left with a hole in the center.
Use a ruler to draw a straight line. Draw it from the 12 to the hole you made in the center of the plate. This number represents 12 o’clock noon.
Use a compass to determine the closest celestial pole. Your straw, or gnomon, should point towards the closest celestial pole, which is parallel with the Earth's axis. That's the North Pole for those living in the Northern Hemisphere. If you live in the Southern Hemisphere, it's the South Pole.^[7]
Bring the plate outside shortly before noon. Place it on the ground in an area that will get full sun exposure all day. Stick the straw through the hole in the center of the plate.
Push the straw slightly so. Do this so that it slants in the direction of the closest celestial pole.
Rotate the plate at exactly noon. Rotate it so that the straw’s shadow is aligned with the line you drew. Since you are only measuring the hours of daylight, the plate will end up looking kind of like a clock, showing only 12 hours.
Secure the plate to the ground. Poke a few pushpins through the plate so that it will stay fixed in the same place on the ground.
Return to the plate one hour later. At 1:00 p.m., go back to the plate and check the position of the straw’s shadow. Write the number 1 at the very edge of the plate, where you see the shadow falling.
Set an alarm and go back outside at the top of every hour. Continue marking the shadow’s position on the edge of the plate. You will observe that the shadow is moving in a clockwise direction.^[8]
Talk to your child about the shadow. Ask them why they think the shadow is moving. Explain what is happening as the shadow moves around the dial.
Repeat this process until dusk. Keep marking the plate every hour until you run out of daylight. At this point, the sundial will be complete.
Check the plate the next day. Have your child return to the plate on the next sunny day and tell you the time based on the shadow’s position. This simple device can be used to tell the time on any sunny day.

[Edit]Building an Advanced Sundial

Cut a 20-inch diameter circle out of ¾-inch plywood.^[9] This circle will be the sundial’s face. Coat both sides of the wooden circle with primer. As the primer dries, think about what you want your sundial to look like. You will need to choose a number style, such as Roman numerals, standard numbers, and so on.
- Pick out the colors you want to use and, if you like, a drawing or illustration to put on the face.
- Sketch out a few different designs until you’ve settled on something final.
Draw your final design on a large piece of circular paper.^[10] You will be using this as a stencil to transfer the design onto the wooden circle, so draw it to scale. Now you need to put the numbers onto the design, which will require some precise measuring. Use a straightedge and a protractor to do this.
- Start with the number 12 at the very top, like a clock’s face.
- Measure where the center of the circle is, then use the straightedge to draw a precise line from the 12 to the center.
Use the protractor to measure exactly 15 degrees to the right.^[11] Mark the number 1 there. Use the straightedge to draw another hour line. Continue marking the numbers exactly 15 degrees apart. Move in a clockwise direction and use the protractor to continue marking off the numbers. Work your way around until you get to the number 12. This will be directly across from the first 12. These represent noon and midnight.
- Then start over with 1 again until you end up back at the original 12 at the very top. The numbers are now accurately marked onto the paper.
- The full 24 hours is represented for the most precise accuracy. When seasons change, so does the position of Earth. In summer, days are longer. In winter, they are shorter.
- There are days in the summer when there are more than 12 hours of daylight.^[12]
Paint your design onto the wooden circle. Use your paper as a stencil so that the numbers and hour lines match what you’ve measured out precisely. Use paint markers to put the numbers onto the wood, since they will involve fine detail work. Paint markers are preferable to permanent markers, since they’re more resilient to the elements.
Obtain the gnomon.^[13] The gnomon is the part of the sundial that will cast the shadow. It is a length of threaded pipe, and you’ll need it to be approximately two or three inches long. Its diameter should be a half inch. Make sure the diameter of the gnomon is slightly wider than the pipe itself. Improvise a conical tip.
- The length of the pipe and the gnomon tip should be no longer than three inches total.
- Paint the gnomon in whatever color you’d like. This will keep it from rusting.
Prepare the sundial post for mounting. The post is what the sundial’s face, the wooden circle, will be mounted on. You will need a 4x4x8 pressure-formed wooden post that has been outdoor-treated. It needs to be perfectly straight and have no large cracks in it. To mount it correctly, the top of the post must be cut with a precise angle.^[14]
- To get this angle, subtract your current latitude from 90 degrees.
- For example, if you are located at 40 degrees N. latitude, you would draw a 50 degree angle on the 4x4.
Cut the angle into the post. Draw a line at right angles using a carpenter’s square. Draw this line six inches from the top of the post. The line is the bottom side of the angle. Use a protractor to measure it, then cut the angle with a table saw.
- Then measure the center of the sundial’s face and drill a hole there.^[15]
- Test the attachment of the post to the sundial face with a 5/16-inch lag screw, just to make sure everything fits together properly.
Dig a hole for the post. Find a sunny spot for your sundial and dig a hole for the post. Be sure you aren’t disturbing any buried cables or lines underground. Put the post in the hole. Test to make sure it’s no taller than five feet off the ground when standing erect. Use a compass to make sure the angle you cut into the post is facing north. Use a carpenter’s level to make sure the post is standing exactly vertical.^[16]
- Permanently put the post in place by pouring and setting it in cement.
- Allow a few days to go by before mounting the sundial face, so that the cement has dried completely.
Attach the sundial face to the post. Use a 5/16-inch by 2-inch lag screw to attach the face. Tighten the screw enough so that it holds the face in place, but you are still able to turn the face easily. Place the flange directly over the sundial’s face.^[17]
- You should be able to see the lag screw in the flange’s center hole.
- Use your right hand to screw the gnomon pipe into the flange, which you should hold in your left hand.
Rotate the sundial face so that the 6 a.m. and 6 p.m. lines are horizontal. Then align the gnomon so that those same lines look like they are going straight through the center. Make sure the line at 12 noon also looks like it is going directly through the gnomon.^[18]
Set the time and attach the gnomon. You must set the time during Daylight Saving Time to read accurately. Hold the flange steady with your left hand. Use your right hand to turn the sundial face. Check the current time. Keep turning the face until the shadow of the gnomon shows the same time on the sundial. Use a pencil to mark where the four flange screws are and then take the flange away.^[19]
- Now tighten the lag screw. Do not move the sundial face as you do this.
- Drill holes for the four screws and then screw the flange onto the sundial.
- Lastly, screw the gnomon in.