Know that planets, minor planets and comets have elliptical orbits, and recall that the Sun is not at the centre of the elliptical orbit, except when the orbit is approximately circular

6.1.2 The Solar System 🌞

In the Solar System, every planet, minor planet (like asteroids) and comet travels around the Sun in a path that is not a perfect circle but an ellipse. Think of an ellipse as a circle that has been gently stretched in one direction – like a rubber band pulled tight at one end. The Sun sits not in the middle of this oval, but at one of its two special points called foci. Only when the ellipse is almost a circle (eccentricity close to zero) does the Sun appear near the centre.

Elliptical Orbits of Planets, Minor Planets and Comets

• Planets – All eight planets follow slightly elliptical paths. For example, Earth’s orbit has an eccentricity of \$e = 0.0167\$, meaning it’s almost circular but still a bit oval.


• Minor planets (asteroids) – Most asteroids in the belt between Mars and Jupiter also trace ellipses, sometimes crossing each other’s paths.


• Comets – Comets have very elongated ellipses, with eccentricities often close to 1. This is why they can swing very close to the Sun (perihelion) and then travel far out into space (aphelion).

Where is the Sun in an Ellipse?

In an ellipse, the Sun is located at one of the two foci. The distance from the centre of the ellipse to a focus is given by

\$c = a\,e,\$

where \$a\$ is the semi‑major axis and \$e\$ is the eccentricity. When \$e = 0\$ (a perfect circle), \$c = 0\$ and the Sun sits at the centre. For most planets, \$e\$ is small, so the Sun is close to the centre but not exactly there.

Analogy: Imagine a racetrack that is oval-shaped. If you place a light at one end of the oval, that light is the Sun. The cars (planets) race around the track, always staying closer to the light when they are near the end of the oval (perihelion) and farther away when they are on the opposite side (aphelion).

ParameterSymbolDescription
Semi‑major axis\$a\$Half the longest diameter of the ellipse.
Semi‑minor axis\$b\$Half the shortest diameter of the ellipse.
Eccentricity\$e\$Measure of how stretched the ellipse is (\$0 \le e < 1\$).
Distance to focus\$c\$\$c = a\,e\$ – distance from centre to the Sun.

Exam Tip: When asked to describe the Sun’s position in an orbit, remember: the Sun is at one focus of the ellipse; it is only at the centre if the orbit is a perfect circle (eccentricity \$e = 0\$). Use the formula \$c = a\,e\$ if you need to calculate the distance from the centre to the Sun.

Quick Check:

  1. What is the eccentricity of a circular orbit? Answer: \$e = 0\$.

  2. Where is the Sun in an elliptical orbit? Answer: at one focus.

  3. Which planets have the most elliptical orbits? Answer: comets (very high \$e\$).