IOP Institute of Physics

# Useful facts

Earth and the Solar System

Scaling factors: if the distance between the Earth and the Sun is scaled to one inch (2.5 cm) then one light year is one mile (1.6 km) [this is a remarkably good scale, correct to better than 1%!]. Therefore:

• If the distance between the Sun and the Earth is one inch, the nearest star is over 4 miles away
• If the distance between the Sun and the Earth is one inch, the distance to the centre of our Galaxy is 25000 miles (once round the Earth)
• If the distance between the Sun and the Earth is one inch, the nearest large galaxy is two million miles away (8 times the distance to the Moon)

Also useful:
"If your teacher drives at 70 mph down the Solar System motorway, it will take him/her:

• Nearly 5 months to reach the Moon
• 150 years to reach the Sun
• About 4500 years to reach Neptune
• 40 million years to reach the nearest star

Speeds:

• The Earth's orbital speed around the Sun: 30 km/s (108,000 km/h, ~70,000 mph)
• The Sun's orbital speed around the Galaxy: ~200 km/s (720,000 km/h, 450,000 mph)
• The speed of the ground beneath your feet, as a result of the Earth's rotation: 1000 km/h (600 mph) at the latitude of Sheffield (53 degrees); it goes up to 1670 km/h (1000 mph) at the equator
• The speed of light: 300,000 km/s (1.08 billion km/h, 670 million mph)
• The speed a rocket needs to attain to escape the Earth's gravity: about 8 km/s (5 miles/second, 29000 km/h, 18000 mph)
• The speed a rocket needs to attain to escape the Sun's gravity, starting from Earth orbit: about 45 km/s (i.e. about 15 km/s in addition to Earth's orbital speed)
• The speed a rocket needs to attain to reach the Sun: about 30 km/s, because it needs to cancel out Earth's orbital speed - so it is about twice as hard, in terms of speed needed, to reach the Sun as it is to reach the outer planets (about 4 times as hard in energy terms, because energy is proportional to speed squared)

Ages:

• Of the Solar System: 4.6 billion years (1 billion = 1000 million)
• Of the oldest stars in the Galaxy: about 12 or 13 billion years
• Of the Universe: about 14 billion years
• Of multicellular life on Earth: about 700 million years
• Of tool-using hominids: about 3 million years
• Of modern humans: about 35000 years
• Of writing: about 5000 years

Names:

• The object in the Kuiper belt that is larger than Pluto: Eris (the Greek goddess of strife, because of all the fuss it caused!)
• The Galilean moons of Jupiter, in order from Jupiter out: Io, Europa, Ganymede, Callisto
• The moons which are larger than Pluto, in decreasing order of size:
Ganymede [J], Titan [S], [the planet Mercury], Callisto [J], Io [J], the Moon [E], Europa [J], Triton [N], [the dwarf planet Eris], [the dwarf planet Pluto] (the initials after each moon indicate its primary planet)
• The largest asteroids, in decreasing order of size:
1 Ceres, 2 Pallas, 4 Vesta, 10 Hygiea (the number in front of an asteroid's name is order of discovery; 10 Hygiea is very dark, and so fainter than several smaller asteroids, hence its slightly later discovery (1849)).

Numbers of moons:

• Mercury 0
• Venus 0
• Earth 1
• Mars 2
• Jupiter (at least) 63
• Saturn (at least) 56, not counting ring particles
• Uranus (at least) 27
• Neptune (at least) 13
• Pluto 3
• Eris 1

Several asteroids have moons.

All four giant planets also have ring systems, though Saturn's is much the largest and the only one visible with a small telescope.

Most of the moons of the giant planets, as well as Mars' two, are very small and are probably captured asteroids or fragments of larger bodies which broke up.