Eratosthenes and Aristarchus' Observations

Eratosthenes and Aristarchus’ Observations

Eratosthenes’ Observations

  • Eratosthenes was a Greek mathematician, geographer, and astronomer, most famous for accurately determining the Earth’s circumference.
  • He estimated the Earth’s circumference using observations of shadows made in two different places at the same time.
  • Eratosthenes knew that in the town of Syene (now Aswan, Egypt), at noon on the summer solstice, the sun was directly overhead and objects cast no shadow. At the same time in Alexandria, he measured the shadow cast by a stick placed vertically into the ground.
  • By calculating the angle of the shadow in Alexandria, and knowing the distance between the two cities, he was able to estimate the Earth’s circumference.
  • His estimates were astonishingly accurate - he calculated the Earth’s circumference to be about 40,000 kilometres, which is very close to the modern value of 40,075 kilometres.

Aristarchus’ Observations

  • Aristarchus of Samos was a Greek astronomer and mathematician who proposed a heliocentric model of the solar system, placing the sun, not the Earth, at the centre of the known universe.
  • He also made important contributions to determine the relative sizes and distances of the Sun and Moon.
  • By observing lunar eclipses, Aristarchus theorised that the Earth’s shadow was approximately two lunar diameters wide and hence the Sun was both larger and further away than the Moon and Earth.
  • He postulated that the Sun was much larger than the Earth, contradicting the then-popular belief. This theory ultimately supported the idea of a heliocentric solar system.
  • Aristarchus’ observations, however accurate, were rejected in his time in favour of geocentrism (the belief that the Earth is the centre of the universe), but are now considered ground-breaking in the evolution of astronomy.

Remember, the pioneering works of Eratosthenes and Aristarchus laid the groundwork for modern astronomy, and their contributions are key aspects of understanding the Earth-Moon-Sun system.