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GCSE Astronomy Topic 1 Key Facts The Earth Eratosthenes first measured the circumference of the Earth accurately. He measured the altitude of the Sun North of a place where the Sun was at the zenith at exactly the same time. Knowing the distance and the angle one can work out the circumference. The Earth is a slightly flattened sphere with average diameter 13,000km The Earth takes 23 hours 56 minutes to rotate. This is a sidereal day. The Earth is the only known planet with liquid water and a range of temperatures that could support life.        Poles – the points that the axis of rotation of the Earth passes through Equator – a circle around the middle of the Earth at equal distances from each pole. Zenith – the point directly above you in the sky Horizon – an imaginary line where the land meets the sky Latitude – the angle of a location North or South of the equator Meridian – an imaginary circle that passes through both poles. The prime meridian goes through Greenwich Longitude – the angle of a location East or West of the prime meridian

The Atmosphere The sky is blue because blue light from the Sun is scattered more than bigger wavelengths so appears to come from all directions Most of the atmosphere is Nitrogen. The 21% oxygen and water vapour it contains is important for human survival. Visible light, microwaves and some radio waves can pass through the atmosphere Infra red, ultra violet and x rays are mostly absorbed so observatories tend to be high up or in space. The light from stars is slightly distorted as it refracts through different layers in the Earth’s atmosphere, especially if pollution is present. Light pollution may be caused by light from human activity. If the background sky is less dark the stars stand out less and so are harder to see. Telescopes A refracting telescope uses lenses to produce a magnified image A reflecting telescope includes a large mirror to produce a magnified image It is much easier and cheaper to produce a...
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