Why is it Dark at Night?

Have you ever wondered why you look up and see a dark sky at night?

Night or nighttime (sp. night-time or night time) is the period of time between sunset and sunrise when the Sun is below the horizon.
Complete darkness or astronomical night is the period of time between astronomical dusk and astronomical dawn when the sun is between 18 and 90 degrees below the horizon and the sky is not illuminated by the sun. At latitudes between 48.5 and 66.5 degrees north or south of the equator, complete darkness does not occur near the summer solstice because even though the sun sets, it is never more than 18 degrees below the horizon at its lower culmination. The opposite of night is day (or "daytime", to distinguish it from "day" as used for a 24-hour period). The start and end points of time for a night vary, based on factors such as seasonlatitudelongitude and timezoneTwilight is the time of night when the sky is illuminated by the sun but the sun is below the horizon.

At any given time, one side of the planet Earth is bathed in light from the Sun (the daytime) and the other side of the Earth is in the shadow caused by the Earth blocking the light of the sun. This shadow is called the umbra. Natural illumination is still provided by a combination of moonlightplanetary light, starlight, diffuse zodiacal lightgegenschein, and airglow. In some circumstances, bioluminescenceaurorae, and lightning can provide some illumination. The glow provided by artificial illumination is sometimes referred to as light pollution because it can interfere with observational astronomy and ecosystems.

Duration and geography
Nights are shorter than days on average due to two factors. Firstly, the sun is not a point but has an apparent size of about 32 arc minutes. Secondly, the atmosphere refracts sunlight so that some of it reaches the ground when the sun is below the horizon by about 34 arc minutes. The combination of these two factors means that light reaches the ground when the center of the sun is below the horizon by about 50 arc minutes. Without these effects, day and night would be the same length at the autumnal (autumn/fall) and vernal (spring) equinoxes, the moments when the sun passes over the equator. In reality, around the equinoxes, the day is almost 14 minutes longer than the night at the equator, and even more towards the poles. The summer and winter solstices mark the shortest and the longest night, respectively. The closer a location is to either the North Pole or the South Pole, the larger the range of variation in the night's length. Although equinoxes occur with a day and night close to equal length, before and after an equinox the ratio of night to day changes more rapidly in high latitude locations than in low latitude locations. In the Northern HemisphereDenmark has shorter nights in June than India. In the Southern HemisphereAntarctica has longer nights in June than Chile. The Northern and Southern Hemispheres of the world experience the same patterns of night length at the same latitudes, but the cycles are 6 months apart so that one hemisphere experiences long nights (winter) while the other is experiencing short nights (summer).

Between the pole and the polar circle, the variation in daylight hours is so extreme that for a portion of the summer, there is no longer an intervening night between consecutive days and in the winter there is a period that there is no intervening day between consecutive nights.


O NIGHT! Thou friend of Thought, of Song, of winged
Inspiration! So gentle is thy tread,
Thy hand so soft, thy look so deep, the sea
Is not so deep as thy mysterious gaze.
Revealest thou what worlds have thought in distant,
Unfathomable dream? Thou knowest wonders,
And tellest them in whispers to the dreamer.
Thou art alive with silence, gentle Night,
The silence of the Past and of the Future,{59}
Of things untold, but not forgotten, dreams
Unreal, yet full of burning truth, and clad
In image, that they startle not our heart,
Nor wake its nerveless beating till it sounds.
In silence, wondrous Night, thou teachest what
The noisy Day would never understand:
Thou makest us descend into the mine
Yet unexplorèd of our soul, that hoards
The many destinies of thousand years
And other thousand years it wandered through.
Search in the darkness of that mine, behold!
The gold that shineth forth into thine eyes,
The treasures of those other lives that death
Transformed and left them unremembered. In
The stillness that surrounds thy search thy soul
Will show thee all its strength and weakness, all
Those errors that condemned it to another{60}
And yet another life, to die again,
And rise again and wander, yet a stranger,
Into the changing world, but laden with
The knowledge of the past it seems to learn
And calls it history, perchance its own
Forgotten past, the very person that
It seemed to be. And now it wonders why
That person acted so and erred and wrought
Such destinies. And all the time it is
Itself that learns itself. Neglect not dreams
Nor call them worthless. Great the truths they bring,
Revealed in sights and legendary lore.
When understood they are a blessing. Learn
To understand the vision's soul, the thought
Which it conveys, the future it reveals,
The past it fetches out of yonder mine
Thy brain was far too tired or far too weak
To search. When plunged in sleep that brain that now
Is thine will listen and may learn such things
Thy soul will tell, as never book or school
Or present life will teach. Oh, blessed Night!
Spread o'er my soul thy wings and carry me
Into those worlds, my brain can never reach!
Fathom not memories, but let me feel
At one with all those lights that lie upon
Thy bosom, breathing, shining there in silence.
Thanks to Wikipedia: Night
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