IGCSE Astronomy Revision Notes Heliocentric (or Copernican) view of the solar system Prior to the enlightenment, the Earth was viewed as the centre of the solar system (Geocentric or Ptolomeic model) with the other planets and the sun moving around the Earth. Given what we see and feel, this view does make sense. We do not feel the motion of the Earth and it looks like the stars are moving above us. We now know that this is not the case and the sun is at the centre of the solar system. There is lots of evidence for the planets orbiting the sun including the phases of Venus and the retrograde motion of mars (and other planets). We also now understand the law of gravity and the relative masses of the sun and the earth. Kepler’s laws Kepler was a scientist who lived in the late 16th century. He made detailed observations of the skies using the then new technology of the telescope. He came up with three laws to describe (but not explain) the motion of the planets: 1/ The planets orbit in an ellipse (a flattened circle) with the sun at one of the foci. 2/ The planets orbit maps out equal areas in equal amounts of time. 3/ The square of the time period of the orbit is directly proportional to the cube of the distance from the sun. 2 3 T αr Gravity Gravity is a force between any two masses. It is the glue that binds the universe together. It is an attractive force only and pulls towards the centre of mass. The force of gravity between two masses is directly proportional to the masses and inversely proportional to the square of the distance between them. 𝑮𝜶𝒎 𝑮𝜶 𝟏 𝒓𝟐 This means that if gravity on earth is 10N/Kg, if a body moves one radius away, it will be or 2.5N/Kg. If one moves out a further radius, it will become be need to understand and be able to apply this. 1 32 = 1 9 1 22 = 1 4 of 10 0f 10 or 1.11N/Kg etc. You do Gravity and circular motion If a body is fired horizontally to a planet at very high speed (4), it will escape the planet’s gravity and continue to move away from the planet. If it is fired at low speed, it will be quickly drawn back down to the planet’s surface (1) . If it is fired at just the right speed, however, it will orbit the planet in a circle(2). Newton imagined that as a canon and you can see this above. The velocity of an orbiting body and its time period are related by the following formula: 𝑉= 2𝛱𝑟 𝑇 This is simply a statement of speed=distance/time where the distance is the circumference of a circle. Astronomical objects Planets These are relatively large objects that orbit stars. You will need to know the order of planets in our solar system. Asteroids These are also objects that orbit stars. They are too small to classify as a planet (or even dwarf planet). There is a large belt of asteroids between Mars and Jupiter where the gravity of Jupiter is too strong for another planet to form. Moons Moons are objects that orbit planets. We have one moon but many planets have several. Satellites A satellite is the general term for an object that orbits a planet. Moons are natural satellites but now there are also many man made satellites, used for communications signals and GPS. Comets Comets orbit the sun in long elliptical orbits. They have long tails made of evaporated ice. These always face away from the sun. Galaxies These are collections of stars, often billions, which orbit a central point. This is often a black hole. Our galaxy is a spiral galaxy called the Milky Way.
