How to tell time with help from the Earth turning on its axis
Discovery Place Science
Before clocks and watches existed, people used sundials to tell time. The earliest known sundials were used by Egyptians in 3500 B.C. and were used up until the early 19th century. If placed correctly, they can tell time accurately to the minute.
A sundial works by placing a gnomon, which is the stick-like object in the center of a sundial, upright to cast a shadow onto the ground. Throughout the day the sun will move across the sky due to Earth's rotation on its axis, changing the position of the shadow and indicating the time of day.
The Earth turns on its axis at an incredible speed of about 1,000 miles per hour. As a result, the shadow cast by a sundial moves about 15 degrees every hour. Over 24 hours it will travel 360 degrees around the sundial.
Across time zones, sundials must be corrected. Each time zone has its own reference longitude so with every longitudinal degree away from its reference, the sundial will be off by an additional four minutes.
Today, we will make our own sundials and look a little deeper into how they work.
As you create your sundial think about what direction the shadow is moving. Is it moving clockwise or counterclockwise? Are the marks evenly spaced on your dial plate? Does the shadow change in size or length?
- Paper plate for base
- Writing utensils to make marks, like Crayons, markers, pencil or pen
- Pencil, straw, or stick to serve as your gnomon
- Tape or clay
- Compass (Most phones have one)
- Rocks or something heavy to weigh your plate down
- Using your stick, poke a hole through the center of your plate and secure it using tape or clay. Make sure you do this so that your stick doesn’t move while it is outside.
- Place the plate on the ground outside. When choosing a spot, look for an area that is sunny and unobscured by shadows for hours to come. Place something heavy like rocks to weigh it down so it doesn’t fly away.
- Using a compass, find North and gently lean your stick in that direction.
- Starting in the morning mark the hour where the shadow is pointing using whatever writing tool you choose! For example, if you begin at 9:00 a.m. you will write 9 where the shadow falls, at 10:00 a.m., you will record a 10 where the shadow falls.
- Set a timer to remind yourself to check back every hour and record the spot of the shadow on that hour.
- Continue this process until the shadow has made it all the way around the plate and your sundial is complete!
- Test out your sundial the next day for accuracy!