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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 large mirror than a large lens so big telescopes tend to be reflectors

The Moon The Moon’s diameter is 3,500km and it is 380,000km from Earth Its rotational and orbital periods are both 27.3 days. Because of this we only see one side from Earth The Moon has virtually no atmosphere as its gravity is too low to keep one

Manned and unmanned vehicles have orbited the Moon and mapped its far side. The far side has more craters and fewer maria. This may be because its crust is thicker so molten rock does not flow to the surface as easily.

Moon Features Named features you should recognise are the Sea of Tranquillity, Ocean of Storms, Sea of Crises, Craters Tycho, Copernicus and Kepler, Apennine mountain range Lunar seas are called mare. Molten rock due to giant impacts cools to form smooth darker regions Craters are the result of impacts. The edges of large craters form highlands Rilles are grooves formed by collapsed lava tunnels Wrinkle ridges were formed as the crust cooled

Apollo The Apollo missions were to explore the Moon They left behind A.L.S.E.P.s (Apollo Lunar Surface Experiments Packages) Amongst other things these measure seismic activity, gravity strength and devices to measure the distance to Earth precisely The Moon may have been formed by a collision between the Earth and an object the size of Mars (Giant Impact Hypothesis) Some of the rocks brought back from the Moon contain certain isotopes which suggest they have the same origin as some rocks on Earth

The Sun The Sun’s diameter is 1.4 million km and its average distance to Earth is 150 million km or 1 AU (Astronomical Unit) The outer layers of the Sun are    The Photosphere – the visible surface, 5,800K The Chromosphere – gases in this absorb certain wavelengths so we get dark lines in the solar spectrum The Corona – The Crown. Faint and not dense but very hot, about 2 million K

Sunspots These are patches on the surface of the Sun which appear dark. They are slightly cooler than the...
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