How science ideas change

Scientists use models to explain what they observe. A good model explains the evidence. When new evidence is found that the model cannot explain, scientists improve the model or replace it with a better one.

For a very long time, many people believed in an Earth-centred model. In this model, Earth stood still at the centre of everything and the Sun, Moon, planets and stars moved around it. About 1,900 years ago, the astronomer Ptolemy described a detailed version of this model that was used for over a thousand years.

The Earth-centred model made sense with the evidence people had. Every day the Sun seems to rise, cross the sky and set. The stars seem to wheel across the sky at night. And we cannot feel Earth moving under our feet.

But some observations were hard to explain. Planets such as Mars sometimes seem to slow down, move backwards across the stars for a while, and then move forwards again. Ptolemy had to add extra small circles to the planets' paths to make his model match.

In 1543 Nicolaus Copernicus published a Sun-centred model. He suggested that the Sun is at the centre and that Earth is a planet that spins once a day and travels around the Sun. This explained the backwards loops of Mars simply: Earth, on its faster inside path, overtakes Mars. A few ancient Greek thinkers had suggested a similar idea, but it had not been widely accepted.

Around 1610 Galileo Galilei used the newly invented telescope to look at the sky. He saw four moons orbiting Jupiter, which showed that not everything travels around Earth. He also saw Venus go through a full set of phases, like the Moon, which fits Venus orbiting the Sun. Johannes Kepler showed that planets travel in stretched-circle (oval) paths, and later Isaac Newton explained how gravity keeps the planets in orbit.

The new idea was not accepted overnight. Many people found it hard to give up such an old, familiar idea, and Galileo got into serious trouble with powerful leaders for supporting it. Over time, as the evidence kept building, the Sun-centred model became accepted. This shows that science changes as new evidence is gathered.

Careful sky-watching is much older than telescopes. First Nations peoples, including Aboriginal and Torres Strait Islander peoples, have watched the night sky for thousands of years. The appearance of particular stars or star patterns at dawn or dusk told them that the time of year was changing, which was a signal to gather certain foods, move camp or hold ceremonies. Many other cultures, such as the ancient Egyptians and Polynesian navigators, also used the stars to keep track of time and to find their way.