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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.
The Earth is tilted 23.5 degrees on its axis. During the course of its path or revolution around the Sun, it is either tilted toward or away from the Sun. During the summer months, in the Northern hemisphere, although Earth is further away from the sun than in winter, it is actually tilted more directly toward the sun. Because of this, the sun is above the horizon longer, with more rays directly hitting Earth's surface, resulting in longer daylight hours and warmer temperatures. However, in winter months, although the Earth is closer to the sun, it is tilted away from the sun, in the Northern hemisphere. This results in the sun rising lower in the sky and above the horizon for a shorter period of time, producing less hours of daylight and cooler temperatures.
The Earth moves around the sun in its tilted position . It rotates around the sun every 365.25 days and spins on its axis once in every 24 hours. The Earth's axis is tilted in 23.5 degrees. As the Earth reaches a particular point during summer solstice in its orbit about the sun, the northern hemisphere tilts towards the sun while the southern hemisphere tilts away from it. The imaginary line that divides the Earth into the northern and southern hemispheres is the equator. The season when we(Indians) are tilted toward the Sun is our summer. During our summer, the daylight lasts more than 12 hours because the sun is at it's northernmost position from the equator. During winter, just the reverse,daylight lasts less than 12 hours for the northern hemisphere because the sun is at its southernmost position from the equator. The farther north from the equator,shorter the daylight hours.
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