For the activity ‘The large sundial’, you will need a playing field that is in sunlight most of the day.
What time is it?
Ask if any of the students are wearing a watch. Why is it handy to have a watch? Explain that 600 years ago nobody had a watch. Ask how the people back then knew what time it was. Before the mechanical clock was invented, it was much more difficult to know the time of the day. People found out that they could use the Sun to tell the time. They did this using a sundial.
Tip. Long ago people also used other devices to tell the time, such as an hourglass.
Have any students ever seen a sundial? Do they know how it works? Explain that a sundial has a stick or pointer that makes a shadow. This is called the gnomon. It is important that in the Northern Hemisphere, the gnomon always points north, or you will not be able to read the sundial. Explain that the Earth turns on its axis. This means that the position of the Sun with regard to the Earth is always changing. If necessary demonstrate this using a torch and an orange or globe. Explain that the shadow of an object also changes as the Earth rotates. The sundial uses this fact. By looking at the position of the shadow of the gnomon on the sundial, you can tell what time it is.
The students make two sundials.
Good to know. When the Sun is due south and the shadow is pointing to the north, it is noon. That means it is exactly 12 o’clock in solar time. Solar time is not always exactly the same as the time shown on your watch. That is because the time we use today is not based on the Sun’s actual position in the sky.
Make a sundial
Hand out scissors, glue, and the activity sheet.
The students complete Task 1 on the worksheet.
Important: To calculate the angle for the gnomon, you need to know the latitude of your town. You can look this up in an atlas or on the internet. For example the latitude of London is 51 degrees N, so the angle needed for a sundial in London is 51 degrees. The instructions are on the worksheet. When the sundial is ready, the students should put it somewhere with the arrow facing south.
The students read the time shown by the sundial. Can they see what time it is? The students complete Task 1 on the worksheet. Discuss the tasks. Come to the conclusion that today we always know exactly what time it is because there are so many clocks around us. Long ago, when there weren't any watches and clocks, it was much more difficult to tell the time. And of course they could not use a sundial at night!
Good to know. This sundial is based on GMT+1. This may need adjusting depending on your local time. For example, in the UK, this would match British Summer Time, but for wintertime every hour number would need to be one hour earlier. So 12 would be 11, 1 would be 12, etc. If you are making it in Central European Summer Time (GMT+2), you will need to change the numbers. Every hour number will need to be one hour later. So will be 1, 1 will be 2, etc.
The large sundial at least
Make a large sundial with the students. Take the students outside to a location where the Sun shines most of the day. Mark the direction of north, using a compass if necessary.
Stand the large protractor upright on its long side in the grass. Use it to measure the correct angle to the ground, as described above. Stick the stick firmly in the ground at the chosen angle, facing north. See the picture for how this should be done.
Every hour the students place a large stone on the ground where the shadow of the stick falls. One of the students uses the marker pen to write the number of the hour on the stone. You can use the smaller stones to mark the quarter and half hours.
Image: Horizontal Sundial
If you don't want to take the students outside every hour, you can just place two stone markers, one in the morning (for example at 9.00) and one in the afternoon (for example at 14.00). Of course your sundial will be less accurate.
To finish the sundial, the rest of the day after school-time needs to be divided using the stones. In the example shown below, five hours have passed and so the time in between needs to be divided into five. Encourage the students to write the numbers of the hours on the stones and place them in the correct position. The next day, take the students outside to see if they can read what time it is. They compare the time they read on the sundial with the time shown by a watch.
How accurate is their sundial? Discuss that long ago, it was more difficult and less precise to tell the time than nowadays.