Last update: 29 July 2012

A Few 'Facts' to Whet the Appetite

... hopefully.

Pre-amble - why 'Facts' is in quotes: Science doesn't really 'know' anything. Observations lead to theories which lead to predictions which lead to more observations which lead to refinements or radical overhauls of the theories which lead to theories being broadly accepted but still subject to being proved wrong or only approximations or only valid in certain circumstances (there won't be any longer sentences!). Despite this it's an incredibly impressive process - we apply the theories to build phones, medical scanners, nuclear power, satellites, bombs, .... Point of this is that some of the things below we are pretty confident are close to being correct (the close being why I keep saying 'roughly' or 'approximately'),and others we are less confident on the accuracy or sometimes the broader truth. Plus I haven't tried to explain the basis of any of these yet, so you should retain a healthy scepticism. Anyway ...

• The Earth revolves around (or orbits) the Sun anti-clockwise¹ approximately once every 365.25 days.
  • The orbit around the Sun follows an elliptical path in a two dimensional plane called the ecliptic plane. More or less.
  • The more or less elliptical two dimensional orbit is quite close to a circular orbit with the Sun at the centre 150 million km away (93 million miles). That's about 400 times the distance from Earth to Moon.
• The Earth rotates anti-clockwise¹ approximately once every 23 hours 56 minutes.
• This explains why there is day and night and why the sun rises in the east and sets in the west. [Well, roughly - it depends where you are and the time of year. In the UK sunrise moves approximately between north east and south east.]
• The rotation of the Earth also explains other effects: missiles and aeroplanes veer to the right and a freely swinging pendulum rotates clockwise (in the northern hemisphere - the opposite in the south); the weather is significantly affected via the impact on winds and currents; a vertically hanging object doesn't point exactly at the centre of the Earth.
• The axis of rotation is a line through the two poles. This is not perpendicular to the motion around the Sun (the ecliptic plane); the equator (the line round the Earth midway between the poles and perpendicular to the axis of rotation) is at 23.5° to the ecliptic. This axial tilt explains why there are seasons and the variable length of day and night through the year.
• The radius of the Earth is approximately 6,400 km. It's a bit bigger at the equator than the poles - about 20 km. The circumference of the Earth is about 40,000 km (25,000 miles).
• The Earth's radius is about 3.7 times larger than the Moon's; the Sun's radius is about 109 times larger than the Earth's.
• The mass of the Earth is approximately 6 x 10²⁴kg (6,000 billion billion tonnes or 6 sextillion tonnes; or 6.6 sextillion tons, if you prefer imperial).
• The Earth is about 81 times more massive than the Moon; the Sun is about 333,000 times more massive than the Earth.
• The Moon orbits the Earth anti-clockwise¹ approximately once every 27.3 days. In the same time it takes to orbit the Earth, the Moon also rotates once (anti-clockwise¹ again). This explains why we only ever see one side of the Moon.
• The Moon is approximately 385,000 km from the Earth on average, varying by up to 20,000 km each way as it revolves in an (approximately) elliptical orbit. So getting to the Moon is like driving round the Earth 10 times (except you'd use a rocket). The orbital path of the moon is complicated.
• How fast is the Earth traveling?
• Due to its daily rotation: about 1,600 kmph (1,000 mph) at the equator reducing to 0 at the poles.
• Due to the orbit around the Sun: about 100,000 kmph (67,000 mph).
• Due to the movement of the Sun within our Milky Way galaxy: about 72,000 kmph (45,000 mph).
• Due to the rotation of the whole Milky Way galaxy: about 790,000 kmph (480,000 mph) - the galaxy rotates about once every 225 million years.
• Due to the movement of the whole Milky Way galaxy through space: relative to the cosmic background radiation (details on what this means later) - about 2.2 million kmph (1.4 million mph).
• Overall: damn fast, and my hair's not even out of place.
• How many stars are there?
• How many stars in our Milky Way galaxy: about 300 billion, give or take.
• How many galaxies in the observable universe: about 500 billion, on current estimates. The observable universe is the subset of the whole universe that it is possible to see (i.e. that light could have reached the earth from since the big bang approx 13.7 billion years ago - more on this later) - it extends about 45 billion light years from us in all directions.
• How many galaxies in the whole universe: Not a clue, nor how big it is (though there are some theories that take a guess). This is the scariest thing on this page: the total size of the universe could be a ludicrous amount of times bigger than the part of the universe we can ever see.
• Are there other universes? Who knows? A couple of popular theories are:
• The multiverse: there are a large (or infinite) number of 'pocket' universes similar to but separated from our own, created in big bang type scenarios. This is sometimes used to explain why our universe is the way it is - essentially, there are so many universes that one is bound to be like ours.
• Parallel universes: for every event that is realised in our universe (e.g. I find the secret of the universe and become famous) there are parallel universes where the event turned out in all other possible ways. This has arisen to explain how quantum mechanics works (more later). Adds up to a lot of other universes and is obviously wrong.

1. The anti-clockwise direction of rotation assumes the observer is looking down on the Earth from above the North Pole.