Earth completes one rotation every 24hrs about its axis and the direction of rotation is from west to east. This rotation causes Day and Night that we observe. Had the Earth been stationary, only one part of the earth would have received sunlight while the other part would have been dark, never experiencing any light, and hence it would have been night all the time in the darker part. It's because of the rotation that the darker part sees the daylight and vice versa. It's important to remember that the sun is stationary it's the earth that rotates.
Earth is tilted on its axis. The tilt is around 23.5 degrees from the vertical or 66.5 degrees from the horizontal plane. Due to the inclination of its axis, there are different seasons like summers and winters and the tilt also causes varying lengths of Day and night. Both concepts are interrelated to each other as longer days cause summers and vice versa.
Why is that?
When the earth is tilted towards the sun, the greater part (area wise) of the northern hemisphere receives sunlight and the lesser part is in darkness because of which heat received is more than heat lost, also the days are longer than nights for the same reason. Since heat received is more than heat loss the net effect is of heat gain and hence summers are caused. If the axis would have been perpendicular all the days and nights would have been of the same length and hence there would have been no seasons at all.
To visualize try to draw a circle around the axis of the earth when the earth is tilted towards the sun you will notice a greater area of the circle in the daylight.
So you have to remember that inclination of the axis of the earth is responsible for causing different seasons and the varying length of day and night.
The Earth completes one full revolution around the sun in 365.25 days. 1 year is 365 days so to account for 0.25 days we add 1 day every 4 years (4*0.25=1 day) which is called Leap year. The orbit of the earth around the sun is elliptical in shape. The point at which the earth is nearest to the sun is called perihelion and the point where the earth is farthest from the sun is called aphelion. It's a misconception that at perihelion the earth is the warmest, It's NOT, in fact, it's around 4th January that the earth is nearest to the sun or at perihelion. The earth is at aphelion in around early July. This means that when the earth is nearest to the sun it is winter in the northern hemisphere and summers in the southern hemisphere. Let's look into this more closely. Play the video below to visualize the concept :
Let's start with summers. The Sun is overhead on the tropic of cancer on 21st June (Summer solstice). It's summers in the northern hemisphere and winters in the southern hemisphere. As time progresses the earth revolves to its new position where the sun is overhead on the equator. At this point in time, all parts of the earth have equal days and equal nights and this is called Equinox. It's 21st September or the date around this day that the sun is overhead on the equator and equinox is observed. Revolving further the earth now moves to a new position where the sun is now overhead on the Tropic of Capricorn and its 22nd December or date around this day that this phenomenon happens. The earth is now tilted away from the sun and the northern hemisphere experiences winter at this point in time. After this earth again revolves and around 21st March, the sun is overhead on the equator once again and equinox is experienced. Revolving further the earth enters summer solstice at around May-June when summers arrive and the sun is again overhead on the topic of cancer thus completing the cycles.
Points to remember :
Sun is never overhead on any latitude beyond 23.5 N (Tropic of Cancer) & 23.5 S (tropic of Capricorn) thus tropics mark the limits of the overhead sun.
Within the tropics, the mid-day sun varies little from its vertical position at noon daily, the 4 seasons are almost indistinguishable.
At the Arctic circle(66.5 N) there is complete darkness on 22 December (Winter Solstice).
At the North pole (90 N) there are 6 months of darkness in winter. In summers it's reversed ie. there is daylight for 6 months. Hence the north pole is also called the Land of the midnight sun.
See the above image now can you see why it's daylight for 24hours in the north pole when summers are there in the northern hemisphere?
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