Changing ideas about Solar System

Physics · Evidence or opinion?

Why do we believe the Earth is a sphere?

Sort each reason for believing the Earth is a sphere. Is it evidence or opinion?

5 of 5 still to sort.

Predict, then check

Use the Sun-centred (heliocentric) model.

Different constellations are seen at night at different times of the year. Which explanation fits the heliocentric model?

What you need to know

• Ancient civilizations thought the Earth was stationary while the Sun, Moon and stars moved across the sky. That is the geocentric (Earth-centred) model.
• Have a goYour classmate Priya is absolutely certain that 'geocentric' means 'Sun-centred'. She has already told three people. What do you say to her?

  Geocentric means Earth-centred: the Earth at the centre, with everything orbiting it.

  The ancient view put a still Earth in the middle with the Sun, Moon and stars moving across its sky, so the name has to point at the Earth, not the Sun.

• Planets were seen changing direction against the patterns of stars, which is why 'planet' means 'wanderer'. Picture the simplest Earth-centred model: each planet on one perfect circle round the Earth. A planet on one circle just keeps going the same way, so it can't do that. Later geocentric models (Ptolemy's is the famous one) fixed it by adding circles on circles, called epicycles. They could copy the motion, but the model got a lot more complicated.
• In the 16th century Copernicus proposed the heliocentric model: the Earth and the other planets orbit the Sun. Helio is Greek for Sun.
• Have a goFill the gaps: in the heliocentric model the ___ is at the centre and the Earth goes round it. In the geocentric model the ___ is at the centre.

  Sun; Earth.

  Helio is Greek for Sun, so heliocentric means Sun-centred. Geocentric is the Earth-centred model the ancients used.

• The world was not convinced. His model was just as complex, and it went against the widely held opinions of powerful religious organisations such as the Catholic Church.
• Tycho Brahe collected huge amounts of extremely accurate measurements. Kepler used them to build a much simpler heliocentric model that fitted almost perfectly with what is seen.
• Telescopes were invented in 1608. Galileo saw craters on the Moon and specks of light going round Jupiter, which showed that not everything orbits the Earth.
• In the late 1600s Newton worked out the laws of gravity. They tied Kepler's model, Galileo's observations and Brahe's data together and explained the orbits of all the planets.
• The Earth orbits the Sun once a year, so different constellations are seen at night at different times of the year. The same one returns after a year.
• Have a goYou spot a particular constellation in the night sky tonight. How long until you see that same constellation again at this time of year, and why?

  After one year, because the Earth orbits the Sun once a year.

  The Earth's trip round the Sun takes a year, so the same constellation is seen again after one orbit.

• Evidence is an observation you can test. Opinion is not. Ships disappearing from the bottom up is evidence that the Earth is a sphere; 'a sphere is the most perfect shape' is opinion.
• The Sun is the centre of the solar system only. It is one of many thousands of millions of stars in the Milky Way.

The big picture

For a very long time the Sun, Moon and stars seemed to wheel round a still Earth, so the Earth-centred (geocentric) model made sense. Then evidence piled up: planets that changed direction, specks of light going round Jupiter, Brahe's measurements, Kepler's simpler model and Newton's gravity all pointed to the Sun-centred (heliocentric) model. It was slow going: Copernicus did not convince the world in the 1500s, and his model was not widely adopted until well into the 1600s and 1700s. Today we know the Sun is the centre of the solar system only, and the Earth is a sphere because of evidence, not opinion.

⚠ Watch out

Thinking people simply 'found out' the Earth goes round the Sun, or that the Sun is the centre of everything. Evidence built up slowly, and the Sun is the centre of the solar system only.

Memory hook

GEO = Earth in the middle. HELIO = Sun in the middle. And the balance tipped because evidence kept piling onto one side, not because anyone shouted louder.

Check yourself

Copernicus proposed the heliocentric model in the 16th century, but it was not widely adopted until well into the 1600s and 1700s. In your own words, what happened in between to tip the balance?