ESS
Grade-9
Easy
Question
Select the model that allows a scientist to predict the exact positions of the planets in the night sky over many years.
- The planets' orbits are straight in a geocentric model.
- The planets' orbits are ellipses in a heliocentric model.
- The planets' orbits are zigzag in a heliocentric model.
- The planets' orbits are round in a geocentric model.
Hint:
In order to explain the motion of the planets, Ptolemy combined eccentricity with an epicyclic model.
The correct answer is: The planets' orbits are ellipses in a heliocentric model.
- In the Ptolemaic system each planet revolves uniformly along a circular path (epicycle), the centre of which revolves around Earth along a larger circular path (deferent).
- The heliocentric model of the planets’ motions, as modified by Johannes Kepler, so as to use elliptical orbits for the planets. It is still quite effective in predicting the motions of the planets over many years.
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