Why are days longer and night shorter in Southern Hemisphere?

Apr 4, 2018

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Did you know that when it’s summer in North America it is winter in some other continents, like South America and Australia? Picture Earth as a round ball with a line drawn around the middle. That line is the equator (ee-QUAY-ter), and it divides our planet into two halves: the Northern Hemisphere (HEM-iss-feer) and the Southern Hemisphere. When it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere. In summer the days are longer, while in winter they are shorter.

December 21 is the Winter Solstice, and that means it’s the shortest day of the year on our part of the planet. But why do days and nights get longer and shorter?

From our perspective, it looks like the sun moves in the sky all the time. But we’re the ones moving: Earth orbits, or revolves, around the sun at 67,000 miles per hour. It also spins around on an imaginary line called an axis at 1,000 miles per hour (slower at places closer to the poles). Imagine a basketball player twirling the Earth on her finger while also running in a circle around a spot on the floor. That spot is the sun, and our planet is the doubly twirling basketball! Each twirl on the player’s finger makes up one day, while each circle she completes on the floor is a year. The sun doesn’t move, but we experience different levels of light — a burst of sunshine at noon, the pitch-black of night, and everything between — because we’re spinning.

If half the world were facing the sun and the other half were facing out into darkness at any given time, you’d expect days and nights to be equal. But our orbit is a little more complicated than that.

The tricky bit is that Earth’s axis — the imaginary line it spins on like a basketball player’s finger — is tilted instead of standing straight. Instead of the top and bottom of the planet each being half in darkness and half in light, one end is always skewed more into the sun’s rays than the other. The sunnier side gradually flips in the course of the Earth’s orbit around the sun, slowly shifting from one part of the planet to the other.

Right now, the top half of the Earth (the Northern Hemisphere) is tilting almost as far away from the sun as possible. The opposite is true for the Southern Hemisphere, where days have been getting longer — and will start getting shorter just as we steal our precious daylight back. This angle change also creates the seasons by shifting how directly the sun’s light hits us, which is why summer in the Southern Hemisphere falls during our winter.

Not all parts of the world experience the solstice quite like we do. Near the equator — an imaginary belt going around the planet’s middle — days and nights always stay close to 12 hours each, because the way the top or bottom of the planet is tilting doesn’t much change where the middle sits. But up at the North Pole, it’s been totally dark since October — and for a few weeks before then, the area was in perpetual twilight. It won’t really feel like daytime there until March, but then the sun will seem to stay up all summer long! Be glad you live in a place where the sun always comes out — even if it’ll be out for a little less time tomorrow.

Clarification: An earlier headline on this story asked why days get shorter in winter. Scientifically, the season of winter begins December 21, at which point the days begin getting longer. The headline has been changed.

Updated June 25, 2019

By Amy Dusto

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).

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.

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.