Introduction: (Initial Observation)
From sunrise to sunset, shadows of buildings, trees and other objects move slowly, but continuously. In the northern hemisphere, shadows cast west in the morning, north at noon time and east in the late afternoon. Knowing the direction of shadows is very helpful for telling the time and the directions. For example, at noon time, you can look at the shadows of people, light posts, traffic signs or trees to tell which way is north.
When you stand towards the north, your right hand will be east and your left hand will be west. South is your back. The same way, if you know which way is north, you can look at the shadow and tell the estimated time.
In this project you will make a sun clock (sundial).
This is a multi-level project that can be performed as a display science project, experimental science project or a classroom activity. Students as low as 1st grade and as high as high school may attempt making or testing sundials at different levels. Lower grades will only go as far as the first experiment; however they may choose to continue this project in future years with more advanced sundials suggested in experiments 2 and 3.
Think, make observations or study the position of the sun in the sky. Also gather information on the direction and the size of shadows and how they change with the different hours of the day. Find out how you can use shadows to tell the time or make a sun clock in your backyard, school yard or a local park.
The following are samples of information that you may find.
Prior to the late nineteenth century, time keeping was a purely local phenomenon. Each town would set their clocks to noon (solar noon) when the sun reached its zenith each day. A time set based on the position of the sun on the sky is known as the Solar Time or Local Apparent Time. A sundial or sun clock shows the solar time.
Note: Solar Noon at your location is when the Sun reaches its zenith or it crosses your line of longitude. This is when the shadows are shortest. In contrast, when the Sun crosses the Standard Time Meridian for your time zone, it is 12 noon Standard Time (regardless of the Sun’s position relative to your meridian).
Because of the earths continuous rotation, the relative position of the sun in the sky varies in different places on the earth; So, the solar time in your town is different from the solar time of another town a few miles to your east or a few miles to your west. Variations of the time from town to town makes it impossible to make a usable schedule for trains, radios and other communication systems. Problems caused by time variations forced railroads to establish a new time known as standard time.
Standard Time or wristwatch time is the worldwide time-keeping standard based upon Mean Solar Time for selected lines of longitude (located in the middle of each time zone) known as Standard Time Meridians. Around the world, there are 24 Standard Time Meridians, beginning with the Prime Meridian (0° longitude, Greenwich, Britain). In the United States, the Standard Time Meridians are 75°W, 90°W, 105°W, and 120°W for the Eastern, Central, Mountain, and Pacific Time Zones, respectively.
Each time zone has one standard time meridian. The local time on each standard time meridian is considered the standard time for all areas within that time zone.
Daylight Saving Time
We’ve learned to save energy and enjoy sunny summer evenings by switching our clocks an hour forward in the summer.
When we change our clocks
Daylight Saving Time begins for most of the United States at 2 a.m. on the first Sunday of April. Time reverts to standard time at 2 a.m. on the last Sunday of October. In the U.S., each time zone switches at a different time.
In the European Union, Summer Time begins and ends at 1 am Universal Time (Greenwich Mean Time). It starts the last Sunday in March, and ends the last Sunday in October. In the EU, all time zones change at the same moment.
Rationale & original idea
The main purpose of Daylight Saving Time (called “Summer Time” in many places around the world) is to make better use of daylight. We change our clocks during the summer months to move an hour of daylight from the morning to the evening.
Idea of Daylight Saving Time
The idea of daylight saving was first conceived by Benjamin Franklin (portrait at right) during his sojourn as an American delegate in Paris in 1784, in an essay, “An Economical Project.” Read more about Franklin’s essay.
What else do I need to know?
During this project you may also need to know what are latitude and longitudes. Following links will explain what are they.
What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.
The purpose of this project is to construct a sundial for showing time in sunny days. When our sundial is ready, use it to determine if the length of shadow changes from day to day. Can you use the shadow length to determine the season or the month of the year? Can you say in what season the shadows are longer?
Can you use the shadow of a vertical pole to tell the time?
When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.
Since we want to know how the shadow lengths change from date to date:
- Independent variable is the date of the year.
- Dependent variable is the shadow length.
- Constants are the sundial and its position.
Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.
This is a sample hypothesis:
I think the length of shadows increase in the winter. My hypothesis is based on my personal observation of shadows in my backyard.
Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”
Can you use the shadow of a vertical pole to tell the time?
Introduction: In order to see if the shadow of a vertical pole can be used to tell the time, we have to make a sundial with a vertical pole and observe the position of shadows on different days.
- Get a wooden board or foam board about 12″ x 12″. This will be the dial face
- Get a wood dowel or pencil about 6″ long. In a sundial this is called a gnomon (pronounced no-mon) or Style
- Locate the center of the board. To do that you can draw lines to connect the opposite corners of your board. The center of the board is where these lines cross.
- Mount the wood dowel or pencil vertically on the center of the board. Use glue to secure it in place.
5. Mark four sides of the board as North, South, East and West.
6. Mount a compass on the board and align it such that the north of compass is at the same side as the north of your board. Use glue to secure the compass in place.
7. Align the entire board to the north.
8. On the top of every hour, from morning to the evening, visit your plain sundial and mark the location of shadows. You can do it by drawing a dot or by drawing lines on both sides of shadow line.
9. Number the marks that you make with standard time in your area.
10. Paint your sundial or do your final decorations as you like.
11. Following shows a simple sundial before and after painting.
12. Start your daily observations. For the next 7 days, observe the shadow of the vertical pole at 10 a.m., 12:00 p.m. and 3:00 p.m., your standard time. Does your sundial show the accurate time in the next 7 days? For best results, you may not move or change the location of your sundial during your experiment.
13. Instead of making the above sundial, you could also use the shadow of any existing vertical pole and mark the position of shadows in 7 different days at certain hours.
14. Report any change in the position of shadows that may make a vertical pole sundial unusable.
Standard horizontal sundial
Introduction: One might imagine that we could measure time simply by setting up a vertical pole on level ground. After a few days, it would become clear that this idea does not work. The shadow of a vertical object does not fall in the same direction nor extend to the same distance at the same time on successive days. This is because the Sun passes across the sky each day on a different path, which rises and falls with the changing seasons.
This difficulty is overcome by observing the shadow of a pole or straight edge, parallel to the Earth’s polar axis. This pole is known as style or gnomon. The angle of style with the dial face must be equal to the latitude in your area.
Note: The latitude is 0º at the equator. It will increase as you move to the north. The latitude will be 90º at the north pole.
Of course, our sundial will indicate Sun Time, which may differ from your local standard time. For one thing, the apparent circular motion of the Sun varies in speed with the time of year, so that a day may last for slightly more, or slightly less, than the annual average of 24 hours. At certain times of year, this causes Sundial Time and Clock Time to differ by as much as 16 minutes.
Sundials may be constructed for horizontal installation on the ground or vertical installation on a south facing wall. In either case, if you change the orientation or the location of a sundial, it may not show the accurate time anymore. Since you may need to move your sundial from outside to inside or from home to school, you will also install a compass on your sundial. The compass will help you in placing the sundial at the right orientation again if you need to move it from one place to another.
The sundial that you construct in this experiment is very similar to what you made in experiment #1 (above); however, instead of a vertical pole as gnomon, you will use a pole that is bent towards the true north and makes an angle with the dial face. This angle is equal to the latitude at your location. Such a pole will be parallel to the earth rotation axis.
- Find out what is to the latitude of your place
You need this because the angle of your gnomon with the dial face must be equal to the latitude in your area. You can use a map or a globe to find the latitude of your town. You may also use the U.S. Naval Observatory website to find your latitude. In the northern parts of the united states the latitude is about 45º while in the southern parts the latitude is about 30º.
(Also see Latitude, Longitude Lookup for the latitude of cities in United States, Canada or other countries.)
For this example, I looked up the latitude of New York City. It is 40° 47′ N (40 degrees and 47 minutes). Since every 60 minutes is one degree, we can estimate the latitude of New York City to be 41°. The longitude for New York City is 73° 58′ W.
- Find the direction of true north (Not magnetic north
True north can be located by using a magnetic compass and making an appropriate correction. Magnetic north is substantially off from true north — the exact amount varies by location. You can use the magnetic declination calculator and enter the latitude and longitude of your area to determine your magnetic declination. You can then use the declination angle and the magnetic north shown by your compass to determine the true north.
For example magnetic declination of New York City is 13ºW, So the true north is 13º on the east of what the magnetic compass shows.
Another Method of finding the true north: The most accurate way to find a true north south orientation is by using the sun itself to find the direction of a shadow cast by a vertical object when the sun is at its zenith. This is easier than it sounds, and can be done by measuring the length of the shadow cast by the upright before and after noon. Set up a vertical pole (or a use a rope with a weight) to cast a shadow on the ground. If you use a rope you will need to make the reference point somewhere near the top cast a visible shadow — like a stick knotted into the rope) The base of the shadow will be the first point for your south-north axis and the reference point or top of the pole will trace the second point. At some time in the morning, mark the spot on the ground where the reference point casts its shadow. Measure the length from the base to the end of the shadow, and using a string of that length, trace out a semi-circle on the ground with the base of the shadow as its center point.
As the sun rises higher in the sky, the shadow will first shorten as noon approaches, and afterwards lengthen.
At some time in the afternoon it will reach the semi-circle you traced in the morning. Note the spot when it touches the arc the second time. The midway point between the morning and afternoon points will be directly north of the base point of vertical object.
3. Choose your materials for the style and dial face on the basis of appearance, durability and ease of working.
For the purpose of this project guide, I assume that you will be using a wooden board or foam board about of about 12″ x 12″ for your dial face.
4. Get a wood dowel or pencil about 6″ long to be used as the gnomon (pronounced no-mon) or Style. Alternatively you may cut a triangle from cardboard, foam board or balsa wood to be used as gnomon. For New York City, your triangle gnomon must have a 41º angle.
5. Locate the center of the board. To do that you can draw lines to connect the opposite corners of your board. The center of the board is where these lines cross.
6. Cut the style from cardboard, balsa wood or any other material that you have chosen. Note that the angle of the style must be equal to the latitude in your area. The style in the right has a 41º angle that is the same as the latitude in central park of New York City.
If using a wood dowel, you will still need a smaller triangle that helps the dowel to stay at the proper angle. Mount the style on the dial face and use glue to secure it in place.
7. Place your newly constructed sundial in a sunny spot in a way that the base of the style is in line with the North/South axis (Gnomon rod is facing the true north). To compensate the magnetic declination, your compass needle must be showing the declination angle equal to the magnetic declination of your area.
8. On the top of every hour, from morning to the evening, visit your plain sundial and mark the location of shadows. You can do it by drawing dots or by drawing lines where the style casts its shadow. If you are observing daylight saving in your area, then the real time is one hour less than what your clock shows. So it is really 12 noon when you’re watching TV shows at 1 p.m.
Standard horizontal sundial (Better method)
Introduction: In the last steps of the previous experiment you used your clock and standard time to mark the dial face and hour lines. There are two other methods that you can do this with and mark the dial face for accurate sun time. These two methods are:
2. Drawing hour line angles
Draw a semi-circle on the lower half of a piece of stiff paper, marking out angles of 15 degrees with a protractor. Fold back the upper half of the paper.
Fit the paper semi-circle so that it rests against the end of the style plate opposite to the measured angle and fits closely around the sloping rod.
Attach one end of a length of cord around the rod and, by stretching it in line with each 15 degree line, mark the point to the left and right across the dial face. Temporary extensions to left and right of the dial face will be needed, where points can be added in line with the wider angles.
Remove the paper protractor and with a straight ruler, draw in the hour lines of the sundial, joining the marked points to the point of emergence of the sloping style.
Another Standard horizontal sundial
In this sundial, a round wooden or cardboard plate is used as the dial face. A right triangle is used as the Style plate. Style is the component that casts the shadow.
Sundial is made by mounting the Style plate on the center of the dial face.
Position and direction of a sundial is very important. This sundial should then be mounted horizontally in a way that the style base is oriented on a north/south direction. In the northern hemisphere the style angle must be towards the north. In the southern hemisphere the style angle must be faced to the south.
To make this sundial more precise, the style must have the proper angle for your location.
You must also calculate and draw hour lines.
Following are the details.
Locate the center of the dial face and draw two perpendicular lines crossing at the center.
Mark the ends of these lines with N, S, E, W for North, South, East and West, respectively.
The base of the style may be about the radius of the dial face; however, it won’t matter if it is smaller or larger.
Temporarily place the style on the middle of NS line and mark the point where style hits the dial face. (Point X)
Draw a line perpendicular to the NS line at this point. This line will also be the hour line for 6 a.m. and 6 p.m.
Use the hour line angle calculator to determine the angles of hour lines for your specific latitude (all originated from the point X).
Use a protractor, center it at point X and mark the angles that you calculate for the hour lines.
Note that the angles calculated are for hours 1 to 6. You may use the same angles for 6 to 12 in an opposite direction.
Draw the hour lines. To do that draw lines from the point X to the marks that you made in previous step. Continue the lines to the edges of the dial face.
When all the hour lines are ready, you may optionally do some decorative painting or write down the numbers for different hours.
Use some glue to mount the style plate in the middle of NS line starting from point X.
You may optionally mount a compass on your sundial that can help you to adjust the orientation of your sundial.
This sundial will only be accurate if it is properly aligned to the true north.
At night true north is the direction of the north star or Polaris.
The shadow of vertical poles at noon time cast directly to the north of the pole. At this time a vertical pole has the shortest shadow.
Materials and Equipment:
This is a sample list of materials for this experiment:
- Wooden board 12″ x 12″ (30cm x 30 cm)
- Wood dowel, 6″ long (15 cm). You can use bamboo skewers for this.
- Compass, protractor
- Paint, paint brush, marker, pencil, ruler, glue (for decorating)
Results of Experiment (Observation):
Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run, you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. For example, when raw data gets processed mathematically, it becomes results.
No calculation is required for this project while it is done by 3rd grade to 8th grade students.
High school level or college level students may also try this project. In this case, the hour line angles can be calculated using the formula:
H=Atan(Sin L * Tan(15h)) where H is the hour line angle, L is the latitude and h is hour.
Summary of Results:
Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.
It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.
Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.
In your conclusion you may report that shadows of vertical columns may be used to tell the time; however, for best results, the column (known as style) must have an angle that vary depending on the latitude of the location where the sundial is being used.
Related Questions & Answers:
What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.
If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.
If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.
List of References
- Learn about angles
- magnetic declination calculator
- A sun clock template for latitude 39º N
- Another sundial template for latitude 39º N
- Horizontal Sundial Nomenclature
- The North American Sundial Society
By: Mitsumasa Anno
Philomel Books, 1987
Sundials; the Art and Science of gnomonics
By: Frank W. Cousins
Pica Press, 1969