Build a Temporary Sundial
Have you ever wanted to know what time it was, but didn't have a watch? Instead of checking your cell phone or going inside to look at a clock try building a sundial! While these instructions are focused on a simple method for making an accurate sundial on a patch of level ground, there is no reason you cannot make it with more permanent materials and have a discussion piece in your backyard or garden.
Steps
- Clear a circular area of bare ground and place a stick (gnomon) in the center.
- Find which way is north. If you place pebbles throughout a day at the point where the sun casts a shadow from the tip of the gnomon, the stones will describe a hyperbola and North is where the shadow is shortest. A more accurate way will be to find east-west first. Draw a circle centered at your vertical stick, at a radius given by a morning pebble, then wait until afternoon when the shadow just touches the circle. A line drawn between these two points will be due east-west and you can draw a line perpendicular to this to find a true north-south line.
- Draw a new circle as large as you want to make your sundial, with the center where your east-west and north-south lines meet. A good radius is about the same length as your shadow stick.
- Make a mark every 15 degrees on the circle (use a pebble). Start by dividing the arc between east and north in half, then divide each of these into three equal pieces. You should end up with 24 even spaces along the circle.
- Find your approximate latitude, you can look it up online, or one way to find latitude in the northern hemisphere is to determine how far above the horizon the north star (Polaris) lies. Polaris is at the end of the handle of the little dipper. Once you know your latitude, mark the point on the circle that corresponds to that angle (counterclockwise) from east. If your latitude is a multiple of 15 degrees, you can use one of the pebbles you have already used.
- Extend a perpendicular line from the latitude stone to the north-south line
- Draw an ellipse with the minor axis at this point, and the major axis where the circle intersects the east-west line. The point where the ellipse crosses the north-south line will be 12 o'clock. The points where the ellipse crosses the east-west line will be 6 o'clock (AM to the west, PM to the east).
- Extend a line straight south or north from each 15 degree mark on the circle to the ellipse and place a pebble at the intersections. These will be your hours. Note the lines extending east-west out from the inner circle in the diagram and inward north-south from the outer circle, the intersections determine the hour points and instead of drawing an ellipse, you can just find these points.
- Your sundial should look like this (this image was done in a drawing program and the 15 minute marks were added, you can simply divide each hour into 4 with three smaller pebbles):
- Stand up a stick in the center of the circle. The type of sundial you have just made is called an emblematic sundial. The exact position of the stick (gnomon) should change with the season (+/- 23.5 degrees) along the north-south line as the sun moves north and south of the equator, but this is a temporary structure so we will dispense with that for now.
- Watch for a shadow to be cast, whatever number that shadow is cast on, that is your beginning to find out what time it is. You must then correct for your longitude and the equation of time, and daylight savings time (if any).
- Attached is a completed sundial with the construction lines removed and a declination line added. The mono (stick) should lie along the center of this in a position which corresponds to the time of year.
Tips
- Since you are doing this just for fun, you can compare it to a watch to see how accurate your sundial really is! Apart from the accuracy of your construction, there are several factors which affect this.
- If you make the radius of your construction circle about equal to your height, you can be the shadow stick!
- A passable declination line can be made by making a circle 0.4 (really sin(23.45degrees) times the radius of your ellipse major axis and dividing it into 12 equal zones, realizing that the solstices (Jun 21 and Dec 21) are on the north-south line and the equinoxes (Sept 22 and Mar 20) lie on the east-west line. Once you figure out where the beginning and end of the months are, you extend a perpendicular line to the north-south line and you will end up with something like the declination line you see above.
- The orbit of the earth is an ellipse, and the variations in orbital velocity give an extra variance in the time of local noon of +/- 15-16 minutes, depending on the time of year. This is known as the "Equation of time". You can find a chart of corrections by doing a web search.
- You can use the shadow cast by the moon to tell time at night. On the night of the full moon, your moon dial will be correct, but the time will be fast or slow when the moon is not full. Keep records and see if they match theory!
- This presumes you are in the northern hemisphere a reasonable distance from the equator. In England 12 O'Clock is North, in Australia 12 O'Clock is South.
- Your sundial will read local solar time, you need to find out how far you are from the standard longitude of your time zone (the standard longitudes are every 15 degrees, multiplied by the number of hours your clock time is different from Greenwich mean time). In the USA, Eastern time has the standard longitude of 75 degrees west (GMT - 0500) and Pacific time is 120 degrees west (GMT - 0800). The sun travels 15 degrees in an hour, so if you are 7.5 degrees west of your time zone standard longitude, your sundial will be a half hour slow. If you are east of your timezone your sundial will run fast. If you don't know your longitude, your sundial can help you find it as long as you know the correct time. Don't forget that any daylight savings time adjustments must also be accounted for.
Things You'll Need
- a flat spot of clear ground
- stick (for casting a shadow)
- pebbles
- String (for drawing circles)
