Читать книгу Exploring the Solar System - Peter Bond - Страница 35

1 1 For a time, the ancient Greeks thought there were nine planets. Venus was named both as the Evening Star (Hesperus) and the Morning Star (Phosphorus). Similarly, Mercury was thought to be two different planets – Lucifer and Hermes.

2 2 A Sun‐centered (heliocentric) model of the universe was proposed by the Greek astronomer Aristarchus in the 3rdcentury BCE, but it was not widely accepted.

3 3 A by‐product of this experiment was the discovery that a pendulum swung more slowly at Cayenne than at Paris, showing that gravity is slightly weaker at the equator. Isaac Newton later used this result to show that Earth's diameter is greatest at the equator.

4 4 Kepler's task was made slightly easier by the fact that, of the five known planets, only Mercury followed a more elliptical path than Mars.

5 5 The sidereal rotation period is the time a planet takes to spin once on its axis, with respect to a particular background star.