Today, the Northern half of the Earth will be treated to the longest day of the year: the Summer Solstice. For astronomers (except the solar or radio variety), this also marks the shortest night of the year.
So why does the days change length at all? It all has to do with the Earth’s 23 degree tilt.
If the Earth were spinning on its axis with no tilt at all, everyone would be treated to days of identical length every day of the year, with latitudes nearer the equator having longer days than those nearer the poles. However, with the tilt, the angle of the Earth relative to the Sun changes as or planet moves about its orbit. On the Winter Solstice (shortest day of the year) the Northern Hemisphere is tilted up and away from the Sun. On the Summer Solstice this month, the Northern Hemisphere will be tilted down toward the Sun. On the equinoxes, the tilt is half way between the solstices. To see this effect, go out and observe the path the Sun takes through the sky for the course of day of winter).
As seen on the Summer Solstice, the Sun rises in the Northeast, arcs into the Southern sky, peaking at a height of about 72 degrees at local noon (about 1:30pm), and then starts heading down to its set in the Northwest. On the solstice, the day will be over 15 hours long. For about a month after the solstice, you will notice that the rise/set points of the Sun hardly change, hence “solstice” from the Latin words “sol” and “sitre,” literally, “Sun stands still.”
By early August though, the days start to shorten and the Sun is noticeably moving South, towards due West at sunset. The shortening of the days will accelerate until the Sun reaches the day of the Autumnal Equinox, where it will rise/set exactly due East/West. The Sun will climb about 50 degrees high and the day and the night will be exactly 12 hours long. The Sun will never leave the Southern celestial hemisphere until the next Vernal Equinox.
The shortening of the days will continue until the Sun finally reaches its most Southerly rise/set on the Winter Solstice, the shortest day of the year. On this day, the Sun will rise/set low in the Southeast, get only about 25 degrees high at local noon (at about 12:30 thanks to a return to Standard Time). The final result: a day that is only 9 hours long.
So there it is, the mechanics of why we have the seasons.
Just for fun:
Why not try a year long photo shoot of sunrises or sets to track solar movement?
Simply go out at sunrise/set, take the camera, and snap a picture just before the solar disk starts to disappear into the horizon. Cloudy tonight? Don't worry. As the Sun is standing still, it will be rising/setting at virtually the same spot for about a week or so. Repeat the dawn/dusk shoot from the same location for the Autumnal Equinox and Winter Solstice to gain a full picture of solar motion.
For someone really dedicated, go out around the 20th of each month for a year.
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