The celestial bodies in space like the planets and satellites move in paths that obey the laws of motion. These paths are usually elliptical or parabolic. The planets move in elliptical orbits around the sun whereas satellites move in elliptical orbits around the planets. The sun and the solar system itself move in an elliptical orbit around the centre of our galaxy – The Milky Way!. Branch of astronomy that deals with the study of the motion of objects in space is known as celestial mechanics.
These heavenly bodies move in such a way due to the attractive force of gravity. The elliptical orbit is kind of an elongated circle. Instead of having a single centre like a circle, ellipses have two centres called foci (singular=focus).
For planets in space, the centre of the sun is always at one of the foci as shown in the figure above. The larger the distance between the two foci, the more elongated the ellipse is. The amount of the elongation of the orbit is given by the eccentricity of the orbit. Most planets have a very small eccentricity. The Earth’s orbit has such a low eccentricity that both the foci lie within the sun itself! So you can say that the earth’s orbit is almost circular.
Kepler’s Laws of Planetary Motion in Space
Motion in elliptical orbits is governed by three laws known as Kepler’s Laws of planetary motion. These laws tell us how fast a planet goes at any point in the orbit and the time a planet takes to orbit the sun. The moon also follows an elliptical orbit around the earth following Kepler’s Laws.
Most comets in space have very large eccentric orbits. You can see the blue orbits belonging to comets compared to the orbits of planets in green. These comets can come very close to the sun after many years and can go beyond the orbits of all planets for many years. Some comets only come close to the sun once in their life time. After coming close to the sun, they keep moving away from it forever. Such comets are said to have parabolic orbits.
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