How to build 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 RetseckAdvertisement

Key concepts Physics Earth’s rotation Time

Sundial

Introduction
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!

Background
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.

Materials

• Sidewalk chalk
• Tape measure or yardstick
• Pen or pencil
• Large concrete space with no shadows
• Clock
• Paper plate
• Plastic straw
• Ruler
• Markers or crayons
• Paperweight or a few small stones
• Adult helper
• Sunny weather

Preparation

• 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.

Procedure

• 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

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April 16, 2020 April 16, 2020/ Guest User

Centenary sundial, Alfriston Clergy House, East Sussex, England (Image Credit: Murgatroyd49 / wikimedia commons)

A sundial tells time using sunlight. Just like the hour-hand moves around a clock face and points to the current hour, a sundial shows the current hour with a shadow. The shadow is created by the sun shining onto a pointer called a gnomon.

If the sundial stays in place, the gnomon’s shadow changes position as the sun moves through the sky over the course of the day. If the sundial is aligned correctly, the shadow will point to the current hour!

• Paper plate

• Crayons, markers, or colored pencils

• Ruler/straight edge

• Geographic compass

• A sharp pencil

• Plastic drinking straw

• Rock or paperweight

• Open outdoor area that receives direct sunlight throughout the day

• Flashlight (required for Take It Further activity only)

1. Write the number 12 along one edge of your plate; this will anchor the rest of your clock

2. Use the sharp pencil to poke a hole in the center of the plate, and put the straw into this hole so that it is sticking straight upward

3. Use a ruler to draw a straight line from the hole to the number 12

4. JUST BEFORE NOON, take your plate outside with the compass. Use the compass to find magnetic north—this is the direction the arrow is pointing. Place your compass on the ground with the arrow pointing north.

5. Place your plate on the ground next to the compass.

6. RIGHT AT NOON, position your plate so that the gnomon’s shadow is right over the line pointing to 12:00, without moving the gnomon. You’ll need to hold the gnomon in place with one hand and rotate the plate from side-to-side with your other hand, until the shadow aligns with the line.

You have now calibrated your sundial. The sun moves from east to west in the sky. At noon, the sun is in the middle of the horizon, to the south—so it casts a shadow to the north.

7. Use a rock or a paperweight to hold your sundial in place on the ground. You may take the compass inside.

8. Come out every hour throughout the afternoon, and use the gnomon’s shadow to mark each hour on the plate.

9. You’ll have to return in the morning to mark the remaining hours (for example, you’ll find your marks on the left side of your dial from 8, 9, 10, and 11 A.M.) Your sundial is then ready to tell the time whenever the sun is out! Just keep in mind that the angle at which the sun hits its surface changes throughout the year…What do you think would happen if you tried to use the same sundial in the same location six months from now?

Note: If you’d rather complete this activity in one day, you can start early in the morning, at any even hour. Don’t write any numbers on your face before going outside. Position your plate on the ground, and make a mark where the gnomon’s shadow currently casts. Label this mark with the current hour. Then, at noon, you can double-check your work by finding magnetic north with the compass and making sure that the gnomon’s shadow points in that direction.

When you’ve recorded every hour, take a look at your sundial: how is this different from a clock face?

Clocks tell time by dividing a circle (360 degrees) into 12 equal segments, each evenly spaced 30 degrees apart. (360 degrees / 12 hours = 30 degrees per hour.) A whole circle represents 12 hours, and the hands go all the way around in the course of a 24-hour day. Can you think of why your “clock” face only looks like half a circle?

Try taking your sundial inside, and YOU can be the sun!

• Place your sundial on the floor (or onto a tabletop or other flat surface)

• Now dim the lights, and use your flashlight to simulate the sun’s movement through the sky. As you move the light from “east” to “west” (right to left) over your sundial, watch the gnomon shadow move in a clockwise direction. Does this help you to understand what happened when your sundial was outside?

There is a large outdoor sundial just outside the McAuliffe-Shepard Discovery Center—have you seen it? Look down! This ground-mounted clock is located just outside our building, in a public area that can be enjoyed any time, free of charge.

To use the clock, stand with your feet on the brick showing the current month. YOU are the gnomon: the shadow of your head will fall over the current time of day! (We took this photograph at 12:45 PM.) If your shadow doesn’t reach the hour-bricks, raise one arm and the shadow of your hand will mark the hour.

This sundial is slightly different than the one you made at home. The builders already oriented the clock to magnetic north. However, you’ll notice that you have to adjust the gnomon’s (your body’s) position to account for the time of year. This is because the sun’s position in the sky changes with the seasons. Because McAuliffe-Shepard’s clock is located outside year-round, we have options for the entire year: the gnomon shadow will correspond with the correct time of day only when its base is at the correct month.

April 16, 2020/ Guest User/