How the Earth-centred model gave way to the Sun-centred one and the evidence that did it, what telescopes add, why some telescopes must be in space, and what each part of the electromagnetic spectrum shows us.
What you will learn
- Describe the geocentric and heliocentric models and the evidence that changed them (Edexcel)
- Explain why some telescopes are put in space, and weigh up the cost (Edexcel)
- Match objects in space to the wavelengths that show them (Edexcel)
Two models of the solar system
This lesson is for Edexcel (1PH0).
AQA and OCR do not ask about it, so skip it if you sit one of those.
- Geocentric (geo = Earth): the Earth is at the centre, and the Sun, Moon, planets and stars orbit it. Ptolemy set it out in detail around AD 150. It matched everyday experience: we do not feel the Earth move, and the Sun seems to cross the sky.
- Heliocentric (helio = Sun): the Sun is at the centre, and the Earth is one of the planets orbiting it. Copernicus published it in 1543.
Planets sometimes seem to drift backwards across the night sky for a few weeks. To explain this, the geocentric model needed extra small circles (epicycles) on top of the orbits, which made it complicated. In the heliocentric model it happens naturally, as the Earth overtakes a slower outer planet.
The rest of this lesson and its 10 practice questions are locked.
In this lesson
- Two models of the solar system
- The evidence that changed the model
- Telescopes, on the ground and in space
- Seeing in other wavelengths
- Try it: Which wavelength?
10 practice questions, marked as you go; about 20 minutes in all.
Key terms
Geocentric, Heliocentric, Epicycles, Galileo, Kepler, Ellipses, Newton, Collects far more light, Magnifies, Absorbs
In this unit: Space physics
The solar system and the Milky Way, the life cycle of a star, orbits and orbital speed, red-shift and the expanding universe, and what is still unknown.
- The solar system and the Milky Way
- The life cycle of a star
- Orbits and orbital speed
- Red-shift and the expanding universe
- Dark matter, dark energy and reading space data
- Models of the solar system and observing the universe