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Size and scale

Introduction

Image of six of the nine planets taken by Voyager 1 on 14 February 1990 as it left the solar system
The solar system is made up of the Sun, the planets, moons, asteroids, comets and dust in orbit around it, along with a great deal of empty space.


Image of six of the nine planets taken by the Voyager 1 spacecraft on 14 February 1990 as it left the solar system.
Image courtesy NASA/JPL-Caltech

Both the sizes of the planets and their distances from the Sun vary enormously. At its closest, Mercury is just 0.31 AU (or 46.5 million km) from the Sun whereas Pluto can be 7400 million km away. Similarly, Jupiter has a diameter of 142,800 km and tiny Pluto is a mere 2445 km across. For a given planet, the closest point to the Sun is perihelion and the most distant is aphelion. All of the planets are tiny compared with the distances between them.

The tables give basic information on the size of planets and their orbits.

Planetary orbits
Name Sidereal
period
Perihelion
(AU)
Aphelion
(AU)
Inclination
(degrees)
Mercury 87.97d 0.31 0.47 7
Venus 224.7d 0.72 0.73 3.4
Earth 365.26d 0.98 1.02 0
Mars 686.98d 1.38 1.67 1.8
Jupiter 11.86y 4.95 5.45 1.3
Saturn 29.46y 9.01 10.07 2.5
Uranus 84.01y 18.28 20.09 0.8
Neptune 164.79y 29.8 30.32 1.8
Pluto 247.7y 29.6 49.3 17.2

The sidereal period is the time taken for the planet to return to the same place in its orbit, relative to the star background and inclination is the tilt of a planet's orbit with respect to the ecliptic plane.

Physical characteristics of the planets
Name Equitorial
diameter
Oblateness Mass
(Earth=1)
Rotation
period
Tilt
(degrees)
Mercury 4878 0 0.06 58.65d 7
Venus 12,100 0 0.82 243d 177
Earth 12,756 0.0034 1 23.934h 23
Mars 6794 0.005 0.11 24.623h 25
Jupiter 142,800 0.065 317.89 9.842h 3
Saturn 120,000 0.108 95.17 10.233h 27
Uranus 52,400 0.03 14.56 16-28h 98
Neptune 48,400 0.02 17.24 18-20h 30
Pluto 2445 0 0.002 6.39d 118

Oblateness indicates how much a planet's shape departs from a sphere. In this case, the rotation periods listed are sidereal i.e. measured with respect to the star background. The axial tilt describes how far a planet's rotation axis leans away from the perpendicular to the ecliptic.

Unlike stars, the planets in our solar system shine by reflecting visible light from the Sun.


Questions to think about

1. What is the essential difference between a planet and a star?


2.

What is the astronomical unit?


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