Scientists historically held controversial views regarding a
heliocentric (sun-centered) and
geocentric (earth-centered) view of the solar system.
These two concepts correspond to descriptions of the same physics using different coordinate systems.
Today, everybody uses coordinate systems in everyday life. A GPS (Global Positioning System)
helps finding a particular location anywhere on Earth
while using terrestrial coordinates. Using spherical geometry, this instrument can
even calculate distances and thus display the estimated travel time.
However, the terrestrial coordinate system is only applicable on the Earth.
The celestial coordinate system can be used to determine the position of
celestial bodies while travelling on the oceans where other landmarks are not present.
The galactic coordinate system is useful for locating objects beyond the solar system.
A 5000 light-year long jet being produced
by the active nucleus of an active galaxy.
(image from Hubble Space Telescope)
image courtesy from: http://en.wikipedia.org/wiki/File:M87_jet.jpg