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!
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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