Astronomy Notes The Lunar Orbiter 1 in 1966 disproved ancient travellers ideas of the shape of the Earth. Our current belief is based on the behaviour and evidence of moons, planets and stars. As new evidence is discovered our belief may change in the future. Astronomy Notes Earth is one of 8 planets to orbit the sun Solar system or from the sun, mercury, Venus, earth, mars, asteroid belt, Jupiter, Saturn, Uranus, Neptune. The orbits are elliptical with the Sun at the centre. Gravitational field strength is the strength of gravity on a planet or moon Astronomy Notes Gravitational Field Strength on Earth is approx. 10N/kg Gravitational Field Strength on moon is 1.6N/kg Planets are non-luminous, do not give off light, they reflect light from the sun Mercury, Venus, Mars, Jupiter and Saturn all visible by the naked eye. Astronomy Notes Uranus and Neptune are so far away they were only discovered after the invention of the telescope. Planets held in orbit by gravitational pull of the Sun. Gravitational pull on Mercury is large compared to that on Neptune, because Neptune is further away. As a consequence Mercury follows a tighter orbit than Neptune. Astronomy Notes Moons are natural satellites that orbit a planet. Their motion depends on gravitation forces. Moons are also non-luminous, we see them as they reflect light from the Sun. Earth’s moon is 400,000km away, has a mass and gravity 1/6th that of Earth. Moon has no atmosphere and has a surface covered with craters caused by impacts of meteorites. Astronomy Notes Takes moon 29.5 days to orbit the Earth. Moon spins on its axis but more slowly than Earth. Completes one full rotation every 29.5 days. Moons rotation=Earth’s rotation Therefore moon always keeps the same part of its surface facing Earth. Some planets have many moons, Jupiter has over 60, some have a small number; mars 2. Astronomy Notes Comets Comets orbit the Sun Approx 1-30km in diameter Made of ice and dust Orbits are elongated Sometimes they are quite close to the sun(strong gravitational pull=fast speed) While other times they are far away (weak gravitational pull=slow speed) Astronomy Notes When it is close to sun the frozen gases evaporate, forming long tails that shine. Most famous comet is Halley’s Comet, visible to earth ever 76 years. Astronomy Notes Asteroids: minor planets or rocks, that orbit the sun. Asteroid belt between Mars and Jupiter Can be few meters to several hundred kilometers. First asteroid was Ceres, diameter 933km Believed that asteroids are broken parts of planets. Astronomy Notes Satellites Satellites are held in orbit around a planet. Two types natural = moons Artificial = manufactured satellites Astronomy Notes Gravitational Forces 1st thought of Universe in history was Ptolemaic System where the Earth was the centre all planets orbited Earth Later: Heliocentric where the Sun is at the centre. Astronomy Notes Newton’s Law of Gravity Gravitational force exists between two bodies They depend on: The masses of the two objects The distance between the masses Inverse Square law exists: see page 53 Fig 6.10 Astronomy Notes Planets travel in orbits around the Sun Sun contains 99% of the mass of Solar System, that's by sun holds the solar system together. Speed of satellites depends on relationship of circumference of the circular orbit and the time it takes. Orbital speed = 2πr/T R is the orbital radius and T is the time period Astronomy Notes See worked example on page 54 Milky way Sun is our nearest star approx 150 mill km away. Surface temp= 15 mill degrees Gravitational forces between stars cause them to form galaxies Galaxies consist of billions of stars Astronomy Notes Our galaxy is a spiral galaxy called Milky Way Universe is simply empty space within which are scattered large numbers of galaxies. Astronomers believe there are billions of galaxies. Distance between galaxies are millions time greater than distances between stars within a galaxy. The distances between the stars in a galaxy are millions of times greater than the distance between planets and the sun.
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