Northern Hemisphere dwellers, or most of the Earth's population, have probably all noticed longer days and shorter nights in the summer and the opposite in winter. This phenomenon occurs because the Earth's axis is not straight up and down at a 90 degree angle, but it is instead tilted a bit.
Therefore, as the planet orbits the sun every 365 days, sometimes the Northern hemisphere is closer to the sun (summer) while sometimes it is farther away (winter).
Summer: Longer Days and Shorter Nights
To explain why days are longer in summer and shorter in winter, first consider the two ways the Earth is rotating all the time.
It spins around its axis, or the imaginary line running through the North and South poles, every 24 hours so that part of the planet is always facing the sun (experiencing daytime) while the opposite side of the planet is not (experiencing nighttime). Meanwhile, the Earth is also orbiting the sun, completing its circle every 365 days.
If the Earth's axis was straight up and down at 90 degrees, the length of time spent facing the sun would always equal the length of time facing away. But it isn't.
Instead, the Earth is tilted slightly at 23.5 degrees to be exact. Additionally, this tilt is always pointed in the same direction in space, toward Polaris (the North Star), even as the planet travels in a circle around the sun. This means that throughout its yearly orbit, sometimes the Northern hemisphere is closer to the sun (summer) while sometimes it is farther away (winter).
Depending where you are on the planet, the difference in the length of day from season to season can be larger or smaller.
Latitude is a measurement that locates a point on a planet in relation to its distance from the equator. Higher latitudes are closer to the poles, while 0 degrees in latitude is the equator itself.
Because the Earth is a sphere, the higher latitudes near the poles are already curving away from the Sun and therefore receiving less sunlight every 24 hours. This is why the poles stay colder than the rest of the planet.
Therefore, with an additional 23.5 degree tilt away from the Sun, a pole receives even less light, and it will only experience daytime in the short window when its lowest part is in line with the Sun's rays. In fact, in the middle of winter, the sun never fully rises above the horizon, and it is essentially 24 hours of night; in the summer, the reverse is true.
Equinoxes and Solstices
The combination of the Earth's tilt and its rotation about the Sun mean that on one day a year, the North Pole ends up tilting as far as possible toward the Sun while the South Pole is tilted as far away as possible. This results in the longest day of the year, also known as the summer solstice, for all locations in the Northern Hemisphere, and the shortest day in the Southern Hemisphere, called the winter solstice.
Halfway between the solstices are the equinoxes. This marks the point in Earth's orbit where the planet's tilt switches its orientation either toward or away from the Sun. At one hemisphere's spring equinox, the tilt changes from away to toward the Sun, lengthening the subsequent days until the fall equinox, when the opposite occurs.
The solstices and equinoxes have variable dates due to small accounting differences in the Earth's orbit (a year is slightly more than 365 days) and calendar systems.
However, the first day of a season as usually defined on a calendar falls near the same dates as these astronomical events. In the Northern Hemisphere, the winter solstice occurs around December 22; summer solstice, June 22; spring equinox, March 21; and fall equinox, September 23.
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