* The Galilean moons or Galilean satellites are the four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto.

* They were first seen by Galileo Galilei in December 1609 or January 1610 and recognized by him as satellites of Jupiter in March 1610.

* Ganymede, a satellite of Jupiter, is the largest and most massive of the Solar System's moons. The ninth-largest object of the Solar System, it is the largest without a substantial atmosphere.

* Our Solar system consists of eight planets.

* Mercury, Venus, Earth and Mars are called as the inner planets.

* They lie between the sun and the belt of asteroids the other four planets are called the outer planets.

* The first four are called Terrestrial planets, meaning earth-like as they are made up of rock and metals, and have relatively high densities.

* The rest four are called Jovian or Gas Giant planets.

* Jovian means jupiter-like.

* The Perigee is the point in the orbit of the moon or a satellite at which it is nearest to the earth.

* An apsis is the farthest or nearest point in the orbit of a planetary body about its primary body.

* An apogee is the point in the orbit of the moon or a satellite at which it is furthest from the earth.

* An eclipse is an astronomical state in which a celestial body such as a planet or satellite comes between a source of light such as the Sun and another celestial body such as the Earth, thereby blocking the light for some time.

* A galaxy is a space system which is huge collection of stars, dust, interstellar gas, stellar remnants and dark matter and all held together by gravity.

* IC 1101 is the largest galaxy that has ever been found in the observable universe. It is a supergiant elliptical galaxy located almost a billion light-years away from earth.

* Our galaxy is known as the Milky way galaxy because it appears as a milky/white band of light in the sky.

* The Milky Way is a spiral galaxy.

● The moon is an example of a Non-luminous object.

● Objects which do not emit light on their own are called non-luminous objects.

● Moon does not emit its own light and glows by reflecting the light of the sun.

● Saturn has the most number of moons.

● Titan is the largest moon of Saturn and It is the second-largest moon in our solar system.

● Saturn is the second-largest in the Solar System and It is the sixth planet from the Sun.

Saturn has 82 moons (53 confirmed, 29 provisional).

● Jupiter is the largest in the Solar System and It is the fifth planet from the Sun.

Jupiter has 53 named moons and another 26 awaiting official names.

● Neptune is the eighth and farthest known Solar planet from the Sun and It is the third-most-massive planet.

Neptune has 14 moons.

● Uranus is the seventh planet from the Sun and It has the third-largest planetary radius.

Uranus has 27 moons.

● Jupiter is known as a gas giant. It is the fifth planet from the Sun and the largest in the Solar System. It is primarily composed of hydrogen, with a quarter of its mass being helium.

● A gas giant is a large planet that has a solid core but a very thick atmosphere. There are four gas giants in our outer Solar System: Jupiter, Saturn, Uranus, and Neptune. These are also called the Jovian planets.

• Pluto is no longer considered as a planet within Earth's Solar system.

• It was discovered in 1930 by US astronomer Clyde Tombaugh.

• On August 24, 2006 the International Astronomical Union (IAU) downgraded the status of Pluto to that of “dwarf planet.”