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The planets of the Solar System orbit the Sun along elliptical paths. The Sun is at one focus of the elliptical orbits. An ellipse is a very easy shape to draw if you have two drawing pins, a piece of string and a pencil. First, fasten the ends of the string to the ‘pin-parts’ of the two drawing pins. Now press the drawing pins into your drawing surface a distance d apart, where d is less than the length of the string. Take a sharp pencil, and with the tip, extend the string until it is taut. Now draw the curve which the pencil follows when it is moved in such a way as to keep the string taut. The curve is an ellipse; the pins are at the foci of the ellipse. An ellipse can be defined as that curve for which the sum of the distances from the two foci to any point on the curve, is constant, i.e. in the diagram, r1 + r = 2constant. The shape of the ellipse can be altered in one of two ways. The distance between the two foci can be changed without changing the length of the string, or the length of the string can be changed without changing the positions of the foci. The distance AO (in the diagram) is known as the semi-major axis, and the distance OB as the semi-minor axis. If the two foci are made coincident (i.e. d = 0) the ellipse reduces to a circle.
Kepler’s first law says that the planets orbit the Sun along elliptical paths, with the Sun at one focus of the elliptical orbits. The other focus has no significance in the case of planetary motion.
This is basically a set of instructions which tell us how to perform a particular task. The task itself is designed to help us understand scientific law. The style is direct and straightforward as it wants to avoid any ambiguity. Some examples of specialised vocabulary are: ellipse, foci, constant. The author uses inverted commas to identify a madeup expression (‘pin-parts’), which is used to describe something otherwise indescribable.