sundial – Science Without a Laboratory

Materials needed: face plate (made in Sundial part I), protractor, glue, BBQ skewer, cardboard scrap, cardboard rectangle large enough to hold sundial/BBQ skewer arrangement

Now that you have made the face of the sundial, you need to set it. That requires two pieces of knowledge about where you live. In this brief lesson, we will discuss your latitude.

You can find your latitude by googling “latitude your town“, or you can check a map. The face plate of your sundial must be angled to that same angle. There are a number of interesting ways to make this happen depending on your sundial design. For instance, I have made jiffy sundials from folded postcards of the place I’m visiting. Here is one easy way to use your latitude angle:

Make a hole in the very center of your sundial face with a thumbtack.
Insert a BBQ skewer, having a tight friction fit. The skewer will be your gnomon. The gnomon casts the shadow. “Gnomon” is Greek meaning one who knows.
Prop the other end of the BBQ skewer against a triangle which you cut from cardboard. The triangle will be a right triangle (one angle is 90 degrees) with your latitude angle positioned as shown in the drawing below. Make sure you have the triangle positioned correctly. Apply a strip of glue to the triangle and glue it to the skewer. Optional: Leave a small extending point below the triangle. You can use this small point to stick the skewer into a slab of cardboard as a base for your sundial and move it around more easily.
For example, my latitude in San Francisco is 38 degrees. See the drawing to look at the correct angle position.
The friction fit of the skewer in the face plate will hold the face plate at the correct angle. The friction fit will also hold the skewer at a 90 degree angle to the face place – which is necessary for correct shadow casting. You can double-check that 90 degree angle by putting a square piece of cardboard under the gnomon.

There is one more step: finding true north. That will be addressed in the next lesson. Apologies for the simple sketch; I will replace it later with a photo.

The shadows sweeping across a sundial’s face tells far more than the time. It marks our latitude and longitude, tells us our position in relation to Polaris the North Star, whether the sun has passed through an equinox, and whether the sun is near its zenith in mid-summer or has dropped to its lowest arc in the heart of winter.

Most of us can delight in the knowledge that we have lassoed the sun for a clock when we make a sundial. Even children in kindergarten laugh to see how a shadow marks the hour for recess or a snack. But the study of sundials can pull you deeper and deeper into joyous realms of knowledge: astronomy, geometry, seasons and equinoxes, magnetishm and the north pole, art and poetry, latitude and longitude, time telling around the world, carpentry.

Shadow and sun clocks have been in use over many parts of the world thousands of years. Ancient examples in Babylonia and Egypt were mere sticks in the ground. These sticks evolved into great obelisks, sun temples, sundials, navigational tools, implements used in solstice rituals, and timepieces for the common citizen. Sundials graced buildings, gardens, wrists, necks, fingers and even gravestones. Until the 1700’s, sundials were relied upon for accurate time, and helped navigators find their way across endless oceans.

You can make a sundial that tells accurate time. It will give you correct solar time, without the fudge factors built into modern watches or cellphones to make our lives more standardized. The directions included in this series of sundial lessons will teach you how to correlate true sun time with modern devices. But it is always the sundial that is correct; your cellphone or computer that is off!

The subject matter is simply too great for one lesson, so I have divided it into several. But you’ll want to get started now, because this equatorial sundial does something special on the equinoxes and one is just around the corner on March 21.

Materials

Protractor, 4″ (approx) square card stock or cardboard, pushpin, crayons or colored pencils (optional but nice), pencil, pen

Make the Sundial faces. I have put all the photos below, to make reading faster.

Use a pencil to lightly draw two diagonal lines across the card stock to locate the middle. Mark that point neatly but darkly because it will be your beginning point. The diagonal lines will not be used further.
Use a push pin to make a hole through the center point.
Drop a noon line from the center point to the “bottom” of your square. This line must be exactly perpendicular to the bottom edge, or your sundial will not work. Remember, a perpendicular line will have a 90 degree angle. Use a protractor for this step.
Use a protractor to measure 15 degree angles on either side of the noon line. These are the hours lines (photo will help). Mark 9 angles/hour lines on each side of noon, but do not label them yet. Do the same thing on the other side. The noon line must drop to the same edge on both sides. Read these notes first:
1. Note 1: Why 15 degrees? If the sun travels 360 degrees during a 24-hour day, you divide 360 by 24 to find that the sun travels 15 degrees each hour.
2. Note 2: Draw as many hour lines as daylight would permit on each side. For instance, you’ll probably need 6 hour lines on each side of the noon line during the winter, (6 a.m. to 6 p.m.) but you will need more during the summer.
3. Note 3: There are two sides to this protractor because one side is read during summer and the other side read during winter. More on this later; just draw the hour angles now.
4. I have shown how to use two types of protactor to draw these lines. Since many students have difficulty using the standard protractor, my husband and I patented another type of protractor and that one is deep red. You can obtain one of these protractors by contacting me, but the standard protractor can be used.
Label one face “Summer” and the other “Winter”. The photos will provide other labels you need to add. The labels may seem to be reversed but do as shown.
Now label your hours as shown below on each face. Again, the hours may appear to be reversed but they won’t be once you orient your sundial in a later step of the lesson.