The yearly cycle of variations in length of day - that is
the duration of period between sunrise and sunset - occurs because of a combination of
the the tilt of axis of rotation of the earth.
The earth
spins or rotates around its axis approximately every 24 hours. Different part of the
earth face the sun during this daily rotation, resulting in the phenomenon of day and
night. The earth also orbits around the sun making one circle every one year. The axis
of earth's rotation, which runs between North Pole and South Pole, is tilted at an angle
of about 23.5 degrees in relation to the plane formed by the orbit of earth round the
sun. As a result, the south and north poles are tilted towards or away from the sun to
varying degrees, at different time of the year as the earth circles the sun. In summer,
the North Pole is tilted towards the sun while the South pole is tilted away from the
sun. As a result, areas of the earth in the northern hemisphere are exposed to the
sunlight for longer period each day, resulting in longer daylight time. In winters, as
the tilt of earth's axis in relation to the sun changes, the North Pole tilts away from
the sun resulting in shorter days in the northern hemisphere of the
earth.
It should noted that the cycle of changes in
duration of daylight time in the southern hemisphere is opposite of that in the northern
hemisphere. The southern hemisphere has summer with longer days when the northern
hemisphere has winter. Similarly, southern hemisphere has winter with shorter days when
the northern hemisphere has summer.
It is worthwhile
pointing out that, though the distance of earth changes as it orbits around the sun, it
has no impact on the length of day. Also the effect of this variation of distance from
sun has only minor impact on the changing weather on earth. Thus for the same distance
of the earth from the sun, both summer and winters are experienced in different
hemispheres of the earth at the same time.
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