Ciência Viva

The sundial

The Sun is a star, a very hot ball of gas which gives us the heat and light that are necessary for life.
As you can see, the Sun is a huge ball of fire that lights and heats the Earth.

In summer, the Sun climbs higher in the sky and the days are longer than the nights. During winter, the Sun does not climb so high and there are less hours of sunlight so this is why it is colder.

When the Sun is at its highest in the sky, it is the middle of the day. You can find out when it is midday by looking at shadows. Look at the shadows of trees or buildings when you are going to school in the morning; the shadows are longer in the morning; shorter at midday and longer again in the afternoon.

On your own or with your friends make a mark on the ground in a large area. Make sure that you are allowed to do this. The ideal place would be the school yard. Stand on the mark and ask a friend to mark the end of your shadow in chalk. Repeat this several times during the day.
The length of your shadow will vary throughout the day.
It will be shorter at midday and longer at the beginning and end of the day.

Draw a straight line through the different points that you marked in chalk. Stand on the central mark again and turn towards the closest mark on this line. You have just turned to face the Earth’s North Pole. Check it with your compass.
Now draw the compass rose and the cardinal points on the ground.

The Sun is at its highest in the sky at midday. At sunrise and sunset your shadow is longer. Now you can make a clock which will tell you what time of day it is.

Cut out your sundial from the card and fold it as shown in the diagram.

Cut out a U-shape along the top and sides of the rectangle and fold it along the dotted line.

Cut out the black dot of the clock face so that you can fit a straw through it.

Glue the grey flaps of the card to the bottom of the sundial and push the straw through the hole.

Slope the straw so that the bottom fits around the raised piece of card that you cut out.

Place your sundial in a sunlit spot in the fresh air, on the window or on the balcony.

Using your compass, line up your sundial with the cardinal points marked on the card.

Your sundial is now working. The shadow of the straw shows approximate noon time on the clock face. In spring and summer the shadow of the straw falls on the outer face, in autumn and winter the shadow falls on the inner face.

If you place your sundial on the window you will always know whether it is morning or afternoon or whether it is midday. With a little practice you will also know whether it is spring-summer or autumn-winter.
Note that in summer, sunrise occurs before six o’clock solar time and that sunset is also later. In winter, it is exactly the opposite. This is because in summer the days are longer than the nights and in winter the nights are longer than the days.

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