The different lengths of day and night

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In addition to its movement around the sun, the earth rotates counterclockwise around itself; that much is clear. We owe the alternation between day and night to this circumstance, as only a portion of the earth’s surface receives the sun’s rays, and the objects in the sky rise in the east and set in the west. But how do the pleasantly long days of summer and the nasty short ones in winter result when the rotational speed of the earth remains constant throughout the year? And why do the points of rising and setting of the sun shift through the seasons, so that the old saying „In the east the sun rises, to the south it takes its course, in the west it will set, in the north it is never to be seen“ is actually valid on only two days in the year?

There are two crucial questions that necessitate a thorough examination of this subject to ensure clarity when planning a shot featuring a desired motif. After all, the time and direction from which the light falls decisively determine the effect of the image.

The interaction between the apparent movement of the sun, which in reality remains motionless in space, and the actual movement of our earth explains both facts, just like the emergence of the seasons. In the course of the celestial rotation, the sun moves on an orbit, which would correspond to a circle of constant geographical latitude on a globe. The oblique position of the earth’s axis causes the positions of the two hemispheres relative to our light source to fluctuate during the earth’s orbit around the sun, sometimes tilting more towards it and at other times more away from it. But the position of the sun does not change, and therefore it describes the up and down dancing line of the ecliptic.

The ecliptic slopes 23.5° against the celestial equator, positioning one half above (north) and the other half below (south) of its plane. The points marking that half, where the ecliptic intersects the celestial equator, lie in an exact west-east direction and are called the vernal and autumnal points. Both the vernal and autumnal points hold significance for our discussion, as during the vernal equinox on March 21, the sun is vertically above the equator, resulting in equal day and night lengths of 12 hours each, with the sun rising in the west and setting in the east. Following this equinox, the sun moves along the ecliptic north of the celestial equator, causing it to rise and set in the exact west-east direction. This is the case until the summer solstice on June 21, when the sun has reached the northernmost point of the ecliptic, the summer point, and the day is longest and the night shortest. In our terrestrial coordinate system, the sun now stands vertically above the northern tropic. After passing this position, the sun follows the ecliptic back to the south. The days continue to shorten until the sun reaches the autumn point, which is the opposite of the spring point, and once again rises vertically over our equator. This is the case on September 21, the date of the autumnal equinox. From now on, the days are shorter than the nights because the sun moves on the part of the ecliptic south of the celestial equator. The winter solstice on December 21, the winter equinox, marks the extreme of the shortest day and longest night. Following this, the ecliptic shifts northward once more, causing the days to lengthen until they reach the vernal equinox, marking the resumption of the seasonal cycle.

Therefore, everything hinges on the fact that during summer, a greater portion of the apparent solar orbit is visible above the horizon compared to winter. The most extreme example of this is polar summer and polar winter, when the sun does not rise or set over the polar circles. Refer to figure 22 in section „Special case low sun position – The combined scattering of light brings the strongest colors“ for the apparent solar path for 66.5° north latitude, the Arctic Circle. During the winter solstice, the sun’s apparent path never crosses the horizon plane, preventing it from rising, while during the summer solstice, it passes completely above it, preventing it from setting.

Practically, you can understand all this quite simply. Take a sheet of paper and draw a straight line in the middle. Now take a glass, preferably one with a large opening, and place it on the line with half of this diameter. This corresponds to the position of the sun in spring and autumn, when it is exactly on the celestial equator and one half of its daily circle is above the horizon and the other below. Day and night are now of equal length. Now move the glass from this middle position 1/3 upwards. Transferred to the real conditions in space, the earth now inclines its northern hemisphere to the sun, like at the beginning of summer. Therefore, the sun is now positioned further north in the sky, with more than half of its daily course culminating in a higher point above the horizon, resulting in a longer day than night. Back over the midpoint and an additional third of the opening below the line would simulate the position at the beginning of winter when night lasts longer than day and the sun’s apparent orbit peaks lower. So the sun does not really describe a steeper orbit in summer and a flatter one in winter because the angle of the orbital plane to the horizontal plane always remains the same for a given location and only the part of its circle visible from our respective position changes. Therefore, the migration of the sunrise and sunset points is merely a symptom of these processes.

Still not convinced by the bare theory? Take a moment and place your globe a few meters away from the sofa. Now you can comfortably observe how the light glow from the ceiling floodlight in the corner changes on its surface when you place its tilted axis towards you, away from you or parallel to you. See how the north pole gets some light and the south pole remains in darkness? How this ratio is reversed and how the light is evenly distributed over the sphere in the 3rd position? – It is exactly the same on the large scale of reality.

Next Uniform Scattering of Light to Nearly White – The Mie Scattering and Haze

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