CoachA planet is a big, round world that travels around a star and makes no light of its own — we see planets, as we see the Moon, by the sunlight they send back. The Earth is a planet.
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Example 26.5 (Rocky planets and gas giants)
The four inner planets — Mercury, Venus, Earth, Mars — are balls of rock, small and solid: worlds you could stand on. The four outer ones — Jupiter, Saturn, Uranus, Neptune — are the giants: enormous balls mostly of thick gas, with no ground to stand on at all. Jupiter, the biggest, could swallow more than a thousand Earths; Saturn wears its famous shining rings — countless little pieces of ice circling it like a crowd of tiny moons.
Example 26.6 (Moons are not planets)
Our Moon circles the Earth, not the Sun directly — so it is not a planet but a moon, a companion of a planet. Other planets have moons of their own: Mars has two small ones, Jupiter and Saturn dozens each. A moon belongs to its planet; the planet belongs to the Sun.
Example 26.8 (The wanderers in our sky)
Venus is often the first “star” of the evening, blazing in the west after sunset — so bright because it is our nearest neighbor, wrapped in white clouds that send sunlight back generously. Mars glows faintly orange; Jupiter and Saturn shine steady and bright. Watch one of them for a few weeks against the fixed stars: it drifts — the very wandering that gave planets their ancient name.