Daytime on earth can roughly be defined as the time of the day when a given point on earth experiences natural illumination from the sun. Other planets and satellites also experience daytime of some sort. The period of daytime starts when the sun appears above the horizon and ends when the sun disappears below the horizon.
At any given time, approximately half of the earth is illuminated by sunlight. Whereas the ideal area subject to direct illumination is exactly half of the earth, indirect illumination may occur due to atmospheric factors and other effects making the total area illuminated by the sun at any point to be slightly more than half the planet.
Daytime Variations with Latitudes and Seasons
The length of daytime on earth varies from one point to another depending on the latitudes and seasons. Daytime latitude variations come about because of variations in the axis of the Earth’s rotation to the plane of its path around the sun. Seasonal variation in the length of day at night occurs because the rotation axis of the earth is fixed compared to the stars axis and moves with respect to the sun when the earth revolves around the sun.
The earth’s rotation axis is tilted about 23.5° to the line perpendicular to its orbital plane, the ecliptic. Depending on the observer’s latitude, daytime length will vary with seasons. Regions of the planet tilted towards the sun experience longer hours of daylight and summer. Areas tilted away from the sun experience winter and shorter hours of daylight.
Duration of Daytime at the Equator
The duration of daytime at the equator remains almost twelve hours throughout all seasons but varies at all the other latitudes. In the other latitudes, for instance, daytime last shorter than twelve hours during winter and longer than twelve hours during summer. At the equator, the sun rises at almost a right angle to the horizon. The sun rises a little bit south-east and it sets a little bit in the south-west starting from the September equinox to the March equinox and rises a little to the northeast and it sets a little to the North West starting from the March equinox to the September equinox. In the period of the March equinox to the September equinox, the Suns' pathway is wholly in the northern hemisphere of the sky while it’s in the southern hemisphere during the period of the September to March equinox.
During the equinox, that is around 20-21 March and 23-24 September, when the equator passes through the center of the suns disk, the sun passes directly overhead at noon. This fact explains the hot equatorial temperatures on the planet. Besides, sunrise and sunset occur very fast at the equator requiring only two minutes to occur. This is because at the equinox of the pathway of the sun across the sky is almost vertical with the horizon.
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