The path or trajectory followed by a body through
space is known as its orbit. The path is generally a curve described
by the body as it progresses. Bodies like planets, comets and
asteroids among others have an orbit. The word ‘trajectory’ used to
describe an orbit, comes from Latin words - traicere, which
means to cause to cross, trajectorius, which means passing
and jacere, which means to throw. A body in space
goes through all these motions.
There are many factors
that determine the exact path a body takes. The gravitational force
of one celestial body working upon another celestial body that
crosses its path determines the path along which the body with the
weaker gravitational force moves. For example, the earth and the
moon. The moon orbits around the earth. The moon completes one
revolution in an elliptical orbit about Earth in 27 days 7 hours 43
minutes and 11.5 seconds. Similarly all the planets in our solar
system orbit around the sun.
Orbits exist in each
and every atom too. Electrical forces cause electrons in an atom
to orbit around the nucleus.
Take a conical shape and cut through it. The shape you see –
a circle or an ellipse – depending on how the section was cut,
is the shape of an orbit. The central body around which the
other body is orbiting is at the focus point of the curve taken
by the orbit. The point on the orbit at which the central body
is most distant is called the apogee and the point at which the
central body is closest is called the perigee.
Do man made
satellites also have orbits?
Man-made satellites, once they are launched into space, orbit
round the earth. For example,
the Infrared Space Observatory (ISO), launched by the
European Space Agency (ESA) in 1995, completed one orbit around the
earth every 24 hours and its apogee was at an altitude of about
70,500 kilometers. Many factors like irregularities in mass within a
body, planetary attractions and atmospheric drag may affect the
orbit of a celestial body. The main characters that define an orbit
are its shape, altitude and in the case of man-made satellites
orbiting the earth, the angle made by the orbit with the equator.
Man-made satellites are launched in space with a particular mission
to be fulfilled. So the satellite’s controllers choose the best
combination of shape, altitude and angle that will serve the
By looking at the name affixed to an orbit one can tell whether
the central body is the earth, the sun, any other star or an unknown
body. The suffix ‘gee’ is used to refer to a body’s orbit around the
earth. Orbits around the sun are described using words with the
suffix ‘helion’ – perihelion and aphelion. Orbits about stars are
described using the suffix ‘astron’ and the suffix ‘apsis’ is used
when the central body is not specified.
The basic physical laws governing orbits were discovered by
Isaac Newton and Johannes Kepler in the seventeenth century. More
insight into the topic was gained in the twentieth century through
Albert Einstein’s general theory of relativity. Johannes Kepler
deduced three basic laws that described the movement of planets
around the sun. Isaac Newton’s Laws of Motion explained the physical
causes of Kepler’s observations and Einstein’s work clarified these
concepts in a more comprehensive manner.