We can easily observe that length of day (LOD) changes according to the time of year, but how and why does this happen.
To understand this, we have to consider the structure and movement of the Earth in relation to the Sun. All the planets in the Solar System orbit around the Sun in an easterly direction, it takes the Earth 365.25 days to complete its rotation around the Sun. The Earth also rotates in an easterly direction on its own axis, in 24 hours it rotates a full circle (360 degrees) so by division it can be calculated that every 4 minutes the Earth turns 1 degree.
The Earth’s axis is also tilted at an angle of 23.5 degrees, and it is the change in orientation of the tilt that causes the seasons. When the top half of the Earth tilts towards the sun it is Northern Hemisphere summer/Southern Hemisphere winter, when the lower half of the Earth tilts towards the sun it is Northern Hemisphere winter/Southern Hemisphere summer; autumn and spring are transitional seasons. The reason that it is hot in summer and cold in winter is not because the tilt makes the Earth’s surface closer or farther from the Sun, but because it changes the angle of the Sun’s rays and therefore spreads or concentrates the heat.
The measurement of the length of a day (LOD) refers to a temporal length of a day - 24 hours- during which there is daylight. Sunrise and sunset is not calculated as the time when light appears or disappears, because even when the sun is below the horizon, its light can be seen, before sunrise this is dawn and after sunset it is twilight. The times for sunrise and sunset are calculated from the leading and trailing edges of the sun.
LOD for a given day, at a given location, is determined by the time of year and the latitude of the location, because these determine position in relation to the sun. On the equator there is very little change in position so LOD is an almost constant 12 hours. At the poles there is only one sunrise and sunset per year and these occur about the time of solstice. On any day, at a given latitude, length of day in one hemisphere equals length of night in the other.
The solstices occur around the 21 June and 21 December, this marks the time of maximum tilt of the equator and consequently they are the dates of the longest and shortest days. The vernal and autumnal equinox occur on 21 March and 23 September respectively, this mark zero tilt of the equator, when the sun is directly above the equator and everywhere on Earth has a 12 hour day.
Now that we can measure LOD more accurately we are also discovering that the 24-hour day – 1 rotation- is only an average. Every day actually varies minutely in length, this is caused by many factors including weather, ocean currents, tides, earthquakes, changes in land surfaces, large scale pumping of groundwater, construction of reservoirs and activity in the Earth’s core.
Days are getting longer, about 4.5 billion years ago a day lasted for only 6 hours, but considering the rate of increase is an imperceptible 1.7 milliseconds per century, we do not need to worry too much about the extra sunlight fading the curtains.
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