The Physics of the Universe - Difficult Topics Made Understandable
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How far is it to space, the Moon, the Sun, the stars, etc?

Earth’s atmosphere is divided up into several layers: the troposphere from about 6 - 20 kilometres up; the stratosphere from 20 - 50 kilometres; the mesosphere from 50 - 85 kilometres; the thermosphere from 85 - 690 kilometres; and the exosphere out to about 10,000 kilometres. “Space” is often considered to start at about 100 kilometres up, known as the Kármán line, where the Earth's atmosphere becomes too thin for aeronautical purposes. The International Space Station orbits the Earth about 350 kilometres up (in the thermosphere).

The Moon is about 360,000 kilometres away from the Earth (and it is receding from us at a rate of about 4 centimetres a year as its orbit gradually speeds up). The nearest planet to the Earth is either Venus, which varies between 42 million kilometres and 258 million kilometres away (its orbit is highly irregular), or Mars which varies between 56 million kilometres and 100 million kilometres away. The Sun is about 150 million kilometres away from the Earth (sometimes referred to as 1 astronomical unit, or 1 AU).

The next nearest star to us (other than the Sun) is Proxima Centauri, in the Alpha Centauri star system (still part of our Milky Way galaxy), which is about 40 trillion (40,000,000,000,000) miles away or, using the more convenient unit based on the distance light travels in a year (which is about 9.46 trillion kilometres), 4.24 light years. Sirius A and B, in the Sirius star system, are about 81 trillion (81,000,000,000,000) kilometres or 8.58 light years away.

The centre of the Milky Way galaxy is about 26,000 light years, or roughly 246 quadrillion (246,000,000,000,000,000) kilometres. Our next closest galaxy is the Andromeda Galaxy, which is about 2.5 million light years away, or roughly 24,000,000,000,000,000,000 kilometres. The best estimate of the size of the observable universe (given that it has been expanding for 13.7 billion years), is about 93 billion light years (880,000,000,000,000,000,000,000 kilometres) across.

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