Omari Gates
Sundials are one of the earliest technologies invented for telling the time. They work by measuring the position of the sun and recording where a shadow falls at each hour of the day. The shadow falls in an anti-clockwise direction in the Southern Hemisphere, which includes Australia. There are several ways to make a sundial, including using a paper plate and straw, or sticks and stones. A nodal sundial uses a pole-like object placed at its centre point to cast a shadow onto a base divided into hours. A gnomon is the part of the sundial that casts the shadow.
| Characteristics | Values |
|---|---|
| Materials | gnomon (stick/pipe), paper plate, straw, stones, protractor, compass, carpenter's level, cement, lag screw, wristwatch/cell phone |
| Gnomon specifications | length: 2-3 inches, diameter: 0.5 inches, conical tip, painted to prevent rusting |
| Post specifications | 4x4x8 pressure-formed wooden post, outdoor-treated, no cracks, angled cut at the top, facing north, vertical, no taller than 5 feet off the ground |
| Process | find a sunny spot, dig a hole, attach post with lag screw, pour cement, attach sundial face, mark hours with stones/on plate, check functionality |
| Notes | shadow movement is anti-clockwise in the Southern Hemisphere, solar hours differ from clock hours due to variations in Earth's orbit and speed |
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What You'll Learn
Choosing a location and materials
When choosing a location for your sundial, it is important to select a spot that will be sunny all day. The sundial should not be placed near any buried cables or lines underground. You will also need to use a compass to ensure that the angle you cut into the post is facing north.
There are various materials you can use to make a sundial, depending on the type of sundial you want to make. For a basic sundial, you can use a straight stick about two feet long, a handful of pebbles, and a wristwatch or cell phone to tell the time. Alternatively, you can use a paper plate and a straw. First, poke a small hole in the centre of the plate and label "12" at the edge of the plate to represent noon. Then, draw a line from the hole to the "12". Place the plate somewhere sunny and stick the straw through the hole, using a compass to slant the straw towards the north. At noon, align the straw's shadow with the line on the plate and secure the plate in place. You can then mark where the shadow falls every hour.
For a more permanent sundial, you will need a 4x4x8 pressure-formed wooden post that has been outdoor-treated. The post should be perfectly straight and have no large cracks. You will also need a 5/16-inch lag screw to attach the sundial face to the post. The gnomon, which is the part of the sundial that casts the shadow, can be made from a length of threaded pipe. It should be approximately two to three inches long, with a diameter of half an inch. You can paint the gnomon to prevent rusting.
Students can also make their own nodal sundial, which uses a pole-like object placed at its centre point to cast a shadow onto a base divided into hours. They will need a protractor for this project.
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Preparing the gnomon
The gnomon is the vertical part of the sundial that casts a shadow to tell the time. To prepare the gnomon, you will need to follow these steps:
Firstly, collect a stick from your backyard. This stick will serve as the gnomon. It is important to ensure that the gnomon is aligned with the slope side pointing towards solar north and at an angle equal to your latitude. This means that the angle between its base and top should be the same as the latitude of your location. This is because when the gnomon points south, it will be parallel to the Earth's axis of rotation.
Next, find a place to position the gnomon. The gnomon should cast a shadow no wider than its thickness at solar noon. You can find solar north by using an analogue watch. Hold the watch parallel to the ground and direct the hour hand towards the sun. Now, bisect the angle between the 12 and the hour, and draw an imaginary line. For example, if it is 4 pm, hold the watch so that the 4 points towards the sun, and the opposite of 2 (which is 8) will point north.
Once you have identified solar north, position the gnomon so that it points in that direction. The gnomon should be placed in a sunny spot and planted firmly into the ground. You can use glue mixed with water and brush it over the gnomon to give it a protective coating.
The gnomon should be positioned so that it casts a shadow onto the faceplate of the sundial. The faceplate is the area where the shadow is cast, showing the time. The gnomon should take up no more than half the length of the faceplate. If using a circular faceplate, the diameter should be between two and three times the length of the gnomon.
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Preparing the sundial post
The next step is to prepare the post for mounting. This involves cutting the top of the post with a precise angle—the angle between its base and top must match the latitude of your location. This is essential because when the sundial points south, it will be parallel to the Earth's axis of rotation. Use a compass to ensure that the angle you cut faces north. This step is critical for the accurate time-telling function of your sundial.
After cutting the post to the correct specifications, it's time to dig a hole at the chosen spot. Ensure that you don't disturb any underground cables or lines during this process. Place the post in the hole and check its height; it should be no taller than five feet from the ground when standing erect. Use a carpenter's level to make sure the post is perfectly vertical. Now, it's time to secure the post in place. Pour cement into the hole and let it set. Allow a few days for the cement to dry completely before proceeding to the next step of mounting the sundial face.
Before mounting the sundial face, it is important to test the attachment of the post. Use a 5/16-inch lag screw to ensure everything fits together properly. This step ensures that your sundial face will be securely attached to the post. Once you've confirmed the proper fit, you can proceed to attach the sundial face using the same type of lag screw. Tighten the screw sufficiently to hold the face in place while still allowing for easy rotation. Now, you're one step closer to telling the time with your very own sundial!
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Attaching the sundial face
Now that you have chosen a sunny spot for your sundial, it's time to attach the sundial face. The sundial's face is the wooden circle that will be mounted on a post. To do this, you will need a 4x4x8 pressure-formed wooden post that has been outdoor-treated. Ensure that the post is perfectly straight and free of large cracks.
The top of the post must be cut at a precise angle—the angle between its base and top must be the same as the latitude of the location in which it is used. This is important because when the sundial points south, it will lie parallel to the Earth's axis of rotation. Use a compass to ensure that the angle you cut into the post is facing north. Check that the post is standing exactly vertical with a carpenter's level.
Now, permanently fix the post in place by pouring and setting it in cement. Allow a few days for the cement to dry completely before mounting the sundial face. You can test the attachment of the post to the sundial face with a 5/16-inch lag screw to ensure everything fits together properly.
Finally, attach the sundial face to the post. Use a 5/16-inch by 2-inch lag screw to secure the face. Tighten the screw enough to hold the face in place, but not so tight that you cannot turn the face easily. And there you have it—your sundial is now ready to tell the time!
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Testing the sundial
Testing your sundial is a process of trial and error. Once you have set up your sundial, you need to check it at different times of the day to see if it is accurate.
Firstly, place your sundial on a flat, horizontal surface. It is crucial that the sundial is in direct sunlight all day, so ensure it is in an open, unshaded area. The sun's position in the sky will determine where the shadow falls on your sundial, so it needs to be able to reach it from dawn to dusk.
Next, you need to establish the correct orientation for your gnomon. If you are in the Southern Hemisphere (which you are if you are in Australia), the gnomon of your sundial must point south. You can use a compass to find true south, or look for the Southern Cross constellation. The gnomon should be parallel to the Earth's axis of rotation, and the angle between its base and top must be the same as the latitude of your location.
Now, check the shadow line. The shadow cast by the gnomon will fall on your sundial, and you need to see which line it is aligning with. This line indicates the time. Check this at various points during the day to see if your sundial is accurate.
You may need to make adjustments to your sundial to account for the longitude of your location. Your sundial could be off by up to an hour compared to clock time, depending on your longitude. To fix this, you need to first figure out how many hours you are ahead or behind GMT (Greenwich Mean Time). For every hour that you are ahead, the centre of your time zone will shift by 15 degrees east; for every hour behind, it will shift by 15 degrees west.
Finally, remember that sundials in the Southern Hemisphere show shadow movement in an anti-clockwise direction. This is the opposite of the Northern Hemisphere, where shadows move in a clockwise direction.
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