Latitudes and Longitudes - Ciência Viva

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• Find Cape Verde on the globe. Why do the inhabitants of Cape Verde sometimes see the midday sun in the
north and at other times in the south?
• Find the Equator and the north-south axis on the globe. Light up the globe with a lamp and tilt the axis slightly
towards the light so that the area around the North Pole, outlined by the Arctic Circle, is lit up. Place a pencil
against the globe and move it vertically on the globe.
• As you can see, there is a point where the pencil casts no shadow. Now find the Tropic of Cancer on the globe
and place the pencil against it. Tilt the axis so that there is no shadow when the pencil is on the Tropic of Cancer.
The axis of the globe is now inclined 23º 30’ relative to the vertical. For anyone who is standing on the surface
of the Earth at the point where the pencil meets the globe and who can see the sun in the position of the lamp,
it is midday on June 21st .
• If you move the pencil to the north, its shadow will point north and the Sun will be to the south. If you move
the pencil towards the sun, its shadow will point south because the midday sun is to the north.
North Pole
Winter
Summer
Tropic of Cancer
South Pole
Equator
Tropic of Capricorn
• But if you tilt the axis in the opposite direction so that it is winter in the Northern Hemisphere, the pencil shadow
will still point north in Cape Verde and in Lisbon.
• So those who can see the midday sun to the north or to the south at different times of the year live between
the Tropic of Cancer and the Tropic of Capricorn.
• The answer to our question is simple. As the north-south axis of the Earth is inclined relative to the plane of
the Earth’s orbit, the midday sun on the first day of summer south of the Tropic of Cancer will be in the north.
But in the autumn, between the Equator and the Tropic of Cancer, the midday sun will be in the south.
Eratosthenes was born 2274 years ago in the city of Cyrene in present day Libya. While he was running the famous Library of
Alexandria in Egypt, he noticed that on the June solstice in the Northern Hemisphere, a vertical stick in the city of Siena, 800 km
to the south of Alexandria, cast no shadow at midday. Then based on experiments with stick shadows, he calculated the length
of the Equator and began to include leap years in the Greek calendar.