Watching the sky: early ideas about the cosmos
Every culture watched the sky. The Sun, Moon, stars and five bright "wandering stars" (planets: Mercury, Venus, Mars, Jupiter, Saturn) gave a calendar for farming and festivals. Babylonian, Egyptian, Chinese, Indian and Greek observers wrote down careful records.
The Earth-centred model
Look up: the Sun rises, crosses the sky and sets. The ground feels still. So it was natural to think Earth does not move and everything goes round it. This is the geocentric (Earth-centred) model. The Greek thinker Aristotle supported it. Around 150 CE Ptolemy turned it into a working method to predict where the planets would be.
Why it became complicated
Planets do not just go round smoothly. Sometimes a planet slows, goes backwards for weeks (retrograde motion) and then goes forward again. Ptolemy added small circles on big circles (epicycles) to copy this. It gave good predictions, but it needed many circles.
Not everyone agreed that Earth is still. About 250 BCE Aristarchus suggested the Sun is in the centre. In India, Aryabhata (about 500 CE) taught that the Earth turns on its axis and that this makes the stars seem to move. Eratosthenes (about 240 BCE) even measured the size of the Earth from the shadows of sticks.
A new view: the Sun in the middle
The ideas below were built one on another. Each person used the work of the people before.
Copernicus (1543): Sun-centred model
The Polish astronomer Nicolaus Copernicus wrote that Earth is one planet among others, all circling the Sun. This is the heliocentric (Sun-centred) model. It explains retrograde motion in a simple way: a faster planet on an inner track passes a slower one, so the slower one seems to move backwards for a while.
Tycho and Kepler: better data, better shapes
Tycho Brahe measured planet positions very carefully, without a telescope. His student Johannes Kepler studied the Mars data and found that orbits are not perfect circles but ellipses with the Sun at one focus. Kepler's three laws (1609 and 1619): (1) orbits are ellipses; (2) a planet sweeps equal areas in equal times, so it is fastest near the Sun; (3) T² is proportional to a³.
Galileo (1610): the telescope
Galileo built a telescope and saw new things: mountains and craters on the Moon, four moons going round Jupiter, many more stars in the Milky Way, and the phases of Venus. He also showed the value of testing ideas by looking and measuring.
Newton (1687): gravity explains the orbits
Isaac Newton showed that one force, gravity, makes an apple fall and keeps the Moon and planets in their orbits. Kepler's laws then followed from Newton's laws. Later, Herschel found Uranus (1781), and Neptune was found in 1846 at the place that Newton's laws predicted.
Beyond the Solar System
In 1838 Bessel measured the tiny shift of a nearby star (parallax) and found its distance. In the 1920s Hubble showed that some "clouds" are other galaxies and that they move away from us: the universe is expanding. See the lesson on Cosmology.
Evidence for the models we use today
A model is accepted when it explains what we see and gives predictions that come true. Here are the main reasons we now use the Sun-centred model:
- Phases of Venus. In Ptolemy's model Venus stays between Earth and Sun, so it could only look like a thin crescent. Galileo saw nearly round, full Venus too. That fits a Venus that goes round the Sun, sometimes behind it.
- Moons of Jupiter. Four moons circle Jupiter. So Earth is not the only centre of motion.
- Stellar parallax. As Earth moves round the Sun, near stars seem to shift a tiny bit against far stars. This is direct proof that Earth moves. The shift is very small, so it was found only in 1838.
- Simple retrograde motion. One idea (overtaking) explains the loops of all planets, with no extra circles.
- Gravity and predictions. Newton's law predicted comets coming back and the planet Neptune.
- Today. Spacecraft reach planets using these orbit rules. Redshift of galaxies and the cosmic microwave background support the model of an expanding universe.
How science works here
Old models were not "stupid". They fitted the data of their time. A better model replaced them when new tools (the telescope) and new data (Tycho's numbers, Galileo's observations) showed problems. Many people from many countries took part: Babylonian, Greek, Indian, Arab, Chinese and European astronomers all added pieces.
Timeline at a glance
- ~240 BCE: Eratosthenes measures the size of the Earth.
- ~250 BCE: Aristarchus suggests a Sun-centred system.
- ~150 CE: Ptolemy writes the Almagest (Earth-centred, with epicycles).
- ~500 CE: Aryabhata teaches that Earth rotates.
- 1543: Copernicus publishes the Sun-centred model.
- 1609-1619: Kepler's laws.
- 1610: Galileo's telescope discoveries.
- 1687: Newton's gravity.
- 1838: first stellar parallax (Bessel).
- 1929: Hubble shows the universe is expanding.
Key formulas and definitions
- Geocentric = Earth in the centre. Heliocentric = Sun in the centre.
- Kepler's 3rd law: T² = a³ (T in years, a in astronomical units, AU)
- Parallax distance: d (in parsecs) = 1 / p (p in arcseconds); 1 parsec = 3.26 light-years
- Earth turns 360° in 24 h, so the sky seems to turn 15° every hour
- Time between two oppositions of an outer planet: 1/S = 1/T(Earth) − 1/T(planet)
Worked examples
1. Eratosthenes found that at noon a stick makes a shadow angle of 7.2° in one city, and none in a city 800 km north (or south) of it. Estimate the Earth's circumference.
A full circle is 360°. 360 ÷ 7.2 = 50, so 800 km is 1/50 of the circle. Circumference = 50 × 800 = 40 000 km. (The real value is about 40 075 km.)
2. A planet is 4 AU from the Sun. How long is its year?
T² = a³ = 4³ = 64, so T = 8 years.
3. Jupiter takes 11.86 years to go round the Sun. How far is it from the Sun in AU?
a³ = T² = 11.86² ≈ 140.7. a = cube root of 140.7 ≈ 5.2 AU.
4. A star shows a parallax of 0.25 arcsecond. How far is it?
d = 1 / p = 1 / 0.25 = 4 parsecs. In light-years: 4 × 3.26 ≈ 13 light-years.
5. The sky seems to turn through how many degrees in 3 hours? What does this tell us about the Earth?
The sky turns 15° every hour, so 3 × 15° = 45°. This is exactly what we get if the Earth spins once in 24 hours, so the turning of the sky can be explained by Earth's spin.
6. Earth takes 1 year and Mars 1.88 years to go round the Sun. How often does Earth overtake Mars (time between oppositions)?
1/S = 1/1 − 1/1.88 = 1 − 0.532 = 0.468. S = 1 / 0.468 ≈ 2.14 years. So Mars seems to go backwards about every 2 years and 2 months.
Common mistakes
- Thinking people long ago were foolish. Their models matched the data they had; better tools made better models.
- Saying Mars really moves backwards. It only seems so, because Earth overtakes it.
- Thinking Galileo invented the telescope. He improved it and was among the first to use it on the sky.
- Thinking the planets move in perfect circles. They move on ellipses, a bit stretched.