Changing Ideas About the Structure of the Solar System
This section is required for Edexcel GCSE students. For the other exam board students, this is still useful as an example that explains how science works as it explains about scientific models and how the models evolve or are changed based on observations and new evidence.
Our understanding of the Solar System hasn’t always been the same. Over thousands of years, ideas have changed dramatically as observations improved and new scientific theories emerged.
- Geocentric Model (Earth-centred): For a very long time, stretching back to ancient Greek philosophers like Aristotle and Ptolemy, the prevailing idea was the geocentric model. In this model, the Earth was believed to be stationary at the centre of the universe, and everything else – the Sun, Moon, planets, and stars – orbited around it. This made sense to people based on their everyday observations: the Sun and Moon appeared to rise and set, and the stars seemed to revolve around the Earth. The apparent ‘backward’ motion (retrograde motion) of planets was explained by complex systems of ‘epicycles’ (small circles along a larger orbit). More about retrograde motion can be found here.
- Heliocentric Model (Sun-centred): In the 16th century, Polish astronomer Nicolaus Copernicus proposed the heliocentric model. He suggested that the Sun, not the Earth, was at the centre of the Solar System, and that the Earth and other planets orbited the Sun. This idea was revolutionary and met with resistance, as it challenged long-held beliefs and religious teachings.
- Galileo Galilei’s observations in the early 17th century provided crucial evidence supporting the heliocentric model. Using his newly improved telescope, he observed:
- The phases of Venus: Like our Moon, Venus shows different phases (full, half, crescent). The geocentric model struggled to explain the full range of phases observed. The heliocentric model, with Venus orbiting the Sun inside Earth’s orbit, naturally explained these phases.
- Moons orbiting Jupiter: Galileo discovered four large moons orbiting Jupiter. This showed that not everything in the heavens orbited the Earth, further undermining the geocentric view.
- The phases of Venus: Like our Moon, Venus shows different phases (full, half, crescent). The geocentric model struggled to explain the full range of phases observed. The heliocentric model, with Venus orbiting the Sun inside Earth’s orbit, naturally explained these phases.
- Later, Johannes Kepler developed laws of planetary motion, showing that planets orbit the Sun in elliptical, not perfectly circular, paths.
- Isaac Newton’s Law of Universal Gravitation in the late 17th century provided the mathematical and physical explanation for why planets orbit the Sun, and why moons orbit planets, firmly establishing the heliocentric model as the correct description of our Solar System’s structure.
- Galileo Galilei’s observations in the early 17th century provided crucial evidence supporting the heliocentric model. Using his newly improved telescope, he observed:
Today, the heliocentric model of our Solar System is universally accepted, based on countless observations, experiments, and theoretical confirmations.