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The seasons and the history of astronomy

Why we have seasons, solstices and equinoxes, and the history of astronomy up to the SKA. Read one part at a time and tap “I know this”. Then test yourself in the quiz.

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The seasons

Between June and December the days in South Africa get longer, because our hemisphere tilts more and more towards the Sun. In winter the midday sun stands lower in the northern sky than in summer. Near the equator the Sun is high all year, so there are no strong seasons.

Date (about)In South Africa
21 DecemberLongest day (summer solstice)
21 JuneShortest day (winter solstice)
March and SeptemberEquinox: day and night equally long
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The history of astronomy

WhenWhoIdea
Long agoAristotle, PtolemyGeocentric: the Earth stands still in the middle
1543CopernicusHeliocentric: the Sun in the middle
1609KeplerOrbits are ellipses (ovals)
1610GalileoTelescope: moons orbiting Jupiter
1687NewtonGravity holds the planets in their orbits
  • People believed the geocentric model for a long time, because from the ground the Sun seems to move around a still Earth.
  • Galileo (in 1610) saw moons moving around Jupiter: not everything circles the Earth. Kepler: orbits are ellipses (ovals).
  • Sputnik 1 (1957): the first artificial satellite. Gagarin (1961): first person in space. 1969: people on the Moon.
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Stars and South Africa

  • The Southern Cross helps people in the southern hemisphere find south; sailors used the stars to navigate.
  • SALT at Sutherland is a huge telescope; MeerKAT is a radio telescope of many dishes, part of the SKA (Square Kilometre Array).
  • Big telescopes stand far from cities: city lights hide faint stars. The Hubble telescope orbits the Earth.
Summer hereWinter here
The tilted Earth around the Sun. On the left (December) the south leans towards the Sun: summer here. On the right (June): winter here.
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Direct and slanting rays, and astronomy long ago

  • When the Southern Hemisphere tilts towards the Sun, sunlight falls directly on us: the energy spreads over a smaller area, it is warmer, and it is summer.
  • When we tilt away, the rays fall at a slant: the same energy spreads over a bigger area, and it is winter.
  • In summer the days are longer than in winter, also because of the Earth’s tilt.
  • Long ago people used the patterns of the Sun, Moon and stars to measure time: a day, a month (from full moon to full moon) and a year. They knew when to plant, how to find direction, and they passed the knowledge on in stories.
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Remember this and test yourself

Remember this: your summary
  • The axis is tilted 23.5°; without the tilt there would be almost no seasons.
  • Summer is warmer: the rays are more direct and the days are longer.
  • Longest day in South Africa: about 21 December; shortest: about 21 June; at an equinox day and night are equally long.
  • Geocentric (Ptolemy): Earth in the middle. Heliocentric (Copernicus, 1543): Sun in the middle.
  • Indigenous communities used the stars to know when to plant; isiLimela is the Seven Sisters (Pleiades).

Write these points in your book. Read them again the night before the test.

Memory trick

The seasons come from the tilt, not the distance. Geo = Earth in the middle; Helio = Sun (Helios) in the middle.

Flash cards · Tap to turn over

Important words

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