Diurnal and annual motion of the earth

The earth move round on its axis as well as in its orbit around the sun.The motion of the earth on its axis is called diurnal motion and a complete round on its axis is called a rotation.The earth takes about 24 hours (23 hr 59 min and 4.09 s) to complete one rotation.The motion of the earth in its orbits is called annual motion and a complete round around the sun is called a revolution.The revolution of the earth has a direct concern with the change in seasons and varying length of day and night.The rotation of the earth causes day and night on the earth.The earth takes about 365 days (365 days, 6 hr, 9 min and 9.54 s) to complete one revolution.
The axis of the earth is inclined to the plane of ecliptic at an angle of 66.5 degree.It causes different seasons and varying lengths of day and night.If the axis were perpendicular to this plane,all parts of the earth would have equal day and nights at all times of the year.

Day and night: an effect of the rotation of the earth

Day and night on the earth occur due to its rotation.when the earth rotates on it axis,only one portion of its surface faces the rays of the sun and experiences day.The other portion which is always from the sun's ray will be in darkness which experiences night.
To understand better about the cause of day and night, place a globe or ball as  the earth in a dark room and use a flash of light or a table lamp representing the sun. Do you see all the parts of the globe facing light?
When the earth rotates eastwards,every part of the earth's surface will be exposed to the sun at some time or the other.The part of the earth's surface that emerges from darkness into the sun's rays experiences sunrise.Later, when it is gradually obscured from the sun's rays,it experiences sunset.

The earth in the universe

The universe

The universe has unlimited size and unknown shapes.All the existing things including different galaxies, constellations, planets, satellites, comets, meteors, meteorites, asteroids etc.constitute it.Our earth is a planet, which is a member of the solar system.The solar system is situated in a galaxy called milky way.The milky way is a medium size galaxy,which consists of about 150 billions of stars. Astonomers believes that there are about 10 power 12 such galaxies in the univers. Now you can imagine that how hug the universe is.Because of its so huge and unlimited size,the distance between two heavenly bodies is inconvenient to measure in metres and kilometres. Hence, astronomical unit(AU), Light year(LY) and parsec are used to measure the distance in the universe.

1 AU= 1.5*10 power 11 m
1 LY= 9.46*10 power 15 m
1 Parsec = 3.26 Ly

Solar system

Only the sun is a known star, which has a group like solar family and only one planet the earth of it has suitable conditions for organisms to survive on it. The solar system consists of the sun and all the objects around it are influenced by its gravity.These objects include planets, dwarf planets and satellites. Countless other smaller objects such as meteroids (meteors and meteorites), and an electrically charged gas the sun. Drifting particles called interplanetary dust and an electrically charged gas known as plasma exist in the space of the solar system. Although solar system is huge, it is only a small part of a galaxy. The eight planets of the solar system are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
The international convention of astronomers held on 2006 in Prague of Czech Republic has removed the Pluto from the list of planets.A new definition of planet is also given by the convention. According to it:
1.A planet should have its own round orbit (elliptical) around the sun.
2.Its shap must be round.
3.Its orbit must not cross the orbit of other planets.
The Pluto fails to fulfill the last condition, due to which it is kept in the category of Dwarf planets.Eris and Ceres are also kept under this category.
In the solar system,the asteroids belt lies between the orbits of Mars and Jupiters.