Natural satellite or moon is a celestial object
that orbits a planet or minor planet under
the influence of their gravity. On the 7th
of January 1610, Italian astronomer and mathematician
Galileo Galilei discovered the first space
object orbiting Jupiter. It was the first
celestial body found to be orbiting another
planet instead of the earth. In few days three
more similar objects were discovered orbiting
Jupiter. These objects including Earth’s
moon were called planets at that time but
later termed satellite by the German astronomer
Johannes Kepler. The term satellite is derived
from the Latin word “satelles” which means
companion. These satellites are labeled IO,
Europa, Ganymede, and Callisto after the characters
in Greek mythology. The names were suggested
by German astronomer Simon Marius who also
claimed to have found these four moons before
Galileo. Collectively these four moons are
called Galilean moons in honor of the discoverer.
After this groundbreaking discovery, the ancient
theory of the geocentric model became obsolete
in which Earth was placed at the center of
the universe. This gave way to the disputed
Copernican model which was developed and published
in 1543 by the Polish astronomer Nicolaus
Copernicus. In this model, Sun was rightly
placed at the center of the Solar System instead
of the Earth.
After the launch of the first man-made satellite
Sputnik1, the satellite was classified into
two different types: natural and artificial
or man-made. Soon artificial satellites were
simply called satellites and natural satellites
were commonly called moons.
Till now a total of 205 natural satellites
have been discovered orbiting planets excluding
mercury and venus. The majority of these moons
are under the gravitational influence of the
gas giants Jupiter and Saturn which have at
least 79 and 82 moons at their disposal. Outermost
ice giants planets Neptune and Uranus hold
14 and 27 moons respectively. Rocky planets
have only 3 moons, two small moons of Mars
and one large Moon of the Earth.
Pluto, now categorized as a dwarf planet has
got 5 moons up its sleeves. Other Dwarf and
minor planets such as Haumea, Eris, Makemake,
Orcus also have their own moon. Moons are
also found orbiting asteroids that are small
and called moonlets.
Like our Earth-Moon system, most moons have
prograde orbits and are tidally locked meaning
one side of the moon is always facing towards
its parent body. Moons that have prograde
orbit revolve in the same direction to the
axial rotation of their parent and when it
revolves in the opposite direction the orbit
is called a retrograde orbit.
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