As you know that earth two type of motions.namely rotation and revolution ROTATIO is the movement of the earth around
the sun in a fixed path or orbit is called Revolution.
The axis of the earth which is an imaginary line makes an angle of 66/ with its orbital plane the plane formed by the orbit
is known as the orbital plane .the earth receives light from the sun due to spherical shape of the earth only half of it gets
light from the at a time(figure 3.2) .
The portion facing the sun experience night the circle that divides the day from night on the globe is called the circle of
illumination this circle does not coincide with the axis as you see in the figure 3.2 the earth takes about 24 hours to complete one rotation around its axis the period of rotation is know as the earthday this is the daily motion of earth.
What would happen if earth did not rotate? the portion of the earth facing the sun would always experience day thus
bringing continuous warmth to the region the other half would remain in darkness and be freezing cold all the time
life would not have been possible in such extreme conditions.
REVOLUTION
The second motion of the earth around sun in its orbit is called revolution it takes 365 days (one year)to revolve around
the sun we consider a year as consisting of 365 days only and ignore six hours for the sake convenience
Six hours saved every year are added to make one day (24 hours) over a span of four years. this surplus day is added to the month of February . thus every fourth year February is of 29 day instead of 28 days.
such a year eith 366 days is called a leap year find out when will the next leap year be?
From the figure 3.3 it is clear that the earth is going around the sun in an elliptical orbit.
notice that throughout its orbit the earth is inclined in the same direction.
A year is usually divided into summer winter spring and autumn seasons seasons change due to the change in the position of the earth around the sun.
Look at the figure 3.3 you will see that on 21st june the northern hemisphere is tilted toward the sun the rays the sun fall
directly on the tropic of cancer as a result these areas receive more heat the areas near the poles receive less heat as the rays of the sun are and the north pole is inclined towards the sun and the places beyond the arctic circle experience
continuous day light for about six month.
since a large portion of the northern hemisphere is getting light from the sun its is summer in the regions north of the
equator the longest day and the shortest night at there places occur on 21st june at this time in the southern hemisphere all these conditions are reversed it is winter season there the nights are longer than the days. this position of the earth is called the summer solstic.
ON 22st december the tropic of capricorn receive direct rays of the sun as the south pole tilts towards it as the sun as the
sun's rays fall vertically at the tropic of capricorn (23 s) a larger portion of the southern hemisphere gets light therefore,
it is summer in the southern hemisphere with longer days and shorter night the reverse happens in the northern hemisphere this position of the earth is called the winter solstice do you know that christmas is celebrated in australia in
the summer season ?
on 21st march and september 23st direct rays of the sun fall on the equator at this position neither of the poles is tilted
towards the sun so the whole earth experience equal days and equal nights. this is called an equinox.
ON 23st september it is autumn season in the northern hemisphere and spring season in the southern hemisphere the opposite is the case on 21st march when it is spring in the northern hemisphere and autumn in the southern hemisphere thus you find that there are days and nights and changes in the season because of the rotation and revolution of the earth respectively...
