Petrosains, The Discovery Centre | Discover STEM

How is the length of a day calculated?

Did you know, last year, on July 19th, scientists observed that it was the shortest day ever recorded? It was 1.4602 milliseconds shorter than the standard.

For most of the history of mankind, time has been marked by the 24-hour day/night cycle, which is governed by the speed at which Earth spins on its axis. Because of that, the length of a day has become the standard by which time is marked—each day lasts approximately 86,400 seconds.

How did people tell time without using a clock?

Nowadays, International timekeepers use painfully sensitive atomic clocks to define the Coordinated Universal Time (UTC) that dictates what time each location on Earth sets their clocks. But, before the invention of the modern clock, how do you think people can tell the time?

One of the ways is to track the position of the sun in the sky by using a SUNDIAL.

How does a SUNDIAL help people to TELL TIME?

The surface of a sundial has markings for each hour of daylight. As the Sun moves across the sky, another part of the sundial casts a shadow on these markings. The position of the shadow shows what time it is.

Thus, it is recommended for you to do this activity starting at 12 noon and test it out regularly until 3 pm, so you have many hours of sunlight for testing.

Let’s try to make one!



Paper plate




First, we need to PREPARE THE SUNDIAL!


Use the pencil or pen to poke a hole through the center of the paper plate. Poke the straw through the hole and secure it, or you can also use the pencil or pen as the gnomon (the pin of a sundial).


Then write number 12 on the outer edge of the plate.

Then, we SET THE DIAL…


Place the sundial in direct sunlight at 12 noon, so that the shadow projected by the gnomon is pointing towards number 12. Slant the gnomon slightly towards the number and leave your sundial for about an hour (as shown at step 1).


Before you observe the shadow again at 1 pm, mark the board with your guess of where the shadow is going to be.

Next, we need to do TIME CHECK…


Come back at 1 pm and check to see how accurate your guess was. Mark the actual place for 1 pm and have your guess again for 2 pm and the rest of the timing.

*Do not remove the paper plate once you start marking

How does a sundial really work?

The sun’s light will make the long stick cast a shadow. Shadows are formed when an opaque object (in this case, the pin of the sundial) is placed in the path of rays of light. As the sundial pin is opaque, it does not let the light pass through it. The light rays that go past the edges of the material make an outline for the shadow.

The shadow will change its angle depending how the sun’s light is hitting the stick because our earth is constantly rotating and revolving around the sun.

Try out this activity to discover how sundial works by yourself!

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