Earth and the Solar System
Day and night, seasons, the Moon, the planets, orbital speed and the formation of the Solar System.
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 6.1.1Explain day, night and the apparent motion of the Sun in terms of the Earth's rotation.
- 6.1.2Describe the Solar System, the order of the planets, and the role of gravity in orbits.
- 6.1.3Recall and use v = 2πr / T for orbital speed.Supplement
7 minute read
The Earth and the Solar System
Day and night
The Earth rotates on its axis once approximately every 24 hours. The side facing the Sun experiences day; the side facing away experiences night. The Sun's apparent daily motion across the sky is caused by this rotation, not by the Sun actually moving around the Earth.
The Solar System
The Solar System consists of the Sun, the planets and their moons, and smaller bodies such as asteroids and comets, all held in orbit by the Sun's gravity. In order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
Orbits and gravity
A planet stays in its roughly circular orbit because the Sun's gravitational attraction acts as a centripetal force, continuously pulling the planet towards the Sun and changing its direction without (in a perfectly circular orbit) changing its speed.
Orbital speed is given by v = 2πr / T, where r is the orbital radius (the circumference of the orbit is 2πr) and T is the time for one complete orbit.
Planets further from the Sun have both a longer path to travel and a weaker gravitational pull, so they move more slowly — both effects make their orbital period much longer than Earth's.
Think of it like this
Orbiting under gravity is like swinging a ball on a string in a circle: the string constantly pulls the ball towards your hand (like gravity pulling a planet towards the Sun), changing its direction without ever touching it, and that inward pull is exactly what keeps the ball moving in a curve instead of flying off in a straight line.
Worked examples
Method, step by step
A planet orbits at a radius of 2.0 × 10¹¹ m with an orbital period of 3.0 × 10⁷ s. Calculate its orbital speed.
- 1v = 2πr / T
- 2v = (2 × π × 2.0 × 10¹¹) / (3.0 × 10⁷)
v ≈ 4.2 × 10⁴ m/s
Common misconceptions
- Believing the Sun orbits the Earth, or that day and night are caused by the Sun moving. It is the Earth's rotation on its axis that causes the day-night cycle.
- Thinking a planet in orbit has no force acting on it because its speed is constant. A resultant force is still needed to continuously change its direction — that is gravity, acting as a centripetal force.
- Assuming all planets orbit at the same speed. Orbital speed depends on distance from the Sun; planets further out move more slowly.
In the exam
- When asked why outer planets take longer to orbit, give both reasons: a longer path (larger circumference) AND a weaker gravitational field giving a lower orbital speed.
- Remember 2πr is the circumference of the orbit — deriving it this way avoids needing to memorise the formula separately from what it means.