## What Causes Day and Night?

For most of here on planet Earth, sunrise, sunset, and the cycle of day and night (aka. the diurnal cycle) are just simple facts of life. As a result of seasonal changes that happen with every passing year, the length of day and night can vary – and be either longer or shorter – by just a few hours. But in some regions of the world (i.e. the poles) the Sun does not set during certain times of the year. And there are also seasonal periods where a single night can last many days.

Naturally, this gives rise to certain questions. Namely, what causes the cycle of day and night, and why don’t all places on the planet experience the same patterns? As with many other seasonal experiences, the answer has to do with two facts: One, the Earth rotates on its axis as it orbits the Sun. And two, the fact that Earth’s axis is tilted.

## Earth’s Rotation:

Earth’s rotation occurs from west to east, which is why the Sun always appears to be rising on the eastern horizon and setting on the western. If you could view the Earth from above, looking down at the northern polar region, the planet would appear to be rotating counter-clockwise. However, viewed from the southern polar region, it appears to be rotating clockwise.

The Earth rotates once in about 24 hours with respect to the Sun and once every 23 hours 56 minutes and 4 seconds with respect to the stars.  What’s more, its central axis is aligned with two stars. The northern axis points outward to Polaris, hence why it is called “the North Star”, while its southern axis points to Sigma Octantis.

## Axial Tilt:

As already noted, due to the Earth’s axial tilt (or obliquity), day and night are not evenly divided. If the Earth’s axis were perpendicular to its orbital plane around the Sun, all places on Earth would experience equal amounts of day and night (i.e. 12 hours of day and night, respectively) every day during the year and there would be no seasonal variability.

Instead, at any given time of the year, one hemisphere is pointed slightly more towards the Sun, leaving the other pointed away. During this time, one hemisphere will be experiencing warmer temperatures and longer days while the other will experience colder temperatures and longer nights.

## Seasonal Changes:

Of course, since the Earth is rotating around the Sun and not just on its axis, this process is reversed during the course of a year. Every six months, the Earth undergoes a half orbit and changes positions to the other side of the Sun, allowing the other hemisphere to experience longer days and warmer temperatures.

Consequently, in extreme places like the North and South pole, daylight or nighttime can last for days. Those times of the year when the northern and southern hemispheres experience their longest days and nights are called solstices, which occur twice a year for the northern and southern hemispheres.

The Summer Solstice takes place between June 20th and 22nd in the northern hemisphere and between December 20th and 23rd each year in the southern hemisphere. The Winter Solstice occurs at the same time but in reverse – between Dec. 20th and 23rd for the northern hemisphere and June 20th and 22nd for the southern hemisphere.

According to NOAA, around the Winter Solstice at the North Pole there will be no sunlight or even twilight beginning in early October, and the darkness lasts until the beginning of dawn in early March. Conversely, around the Summer Solstice, the North Pole stays in full sunlight all day long throughout the entire summer (unless there are clouds). After the Summer Solstice, the sun starts to sink towards the horizon.

Another common feature in the cycle of day and night is the visibility of the Moon, the stars, and other celestial bodies. Technically, we don’t always see the Moon at night. On certain days, when the Moon is well-positioned between the Earth and the Sun, it is visible during the daytime. However, the stars and other planets of our Solar System are only visible at night after the Sun has fully set.

The reason for this is because the light of these objects is too faint to be seen during daylight hours. The Sun, being the closest star to us and the most radiant object visible from Earth, naturally obscures them when it is overhead. However, with the Earth tilted away from the Sun, we are able to see the Moon radiating the Sun’s light more clearly, and the stars light is detectable.

On an especially clear night, and assuming light pollution is not a major factor, the glowing band of the Milky Way and other clouds of dust and gas may also be visible in the night sky. These objects are more distant than the stars in our vicinity of the Galaxy, and therefore have less luminosity and are more difficult to see.

Another interesting thing about the cycle of day and night is that it is getting slower with time. This is due to the tidal effects the Moon has on Earth’s rotation, which is making days longer (but only marginally). According to atomic clocks around the world, the modern day is about 1.7 milliseconds longer than it was a century ago – a change which may require the addition of more leap seconds in the future.

We have many interesting articles on Earth’s Rotation here at Universe Today. To learn more about solstices here in Universe Today, be sure to check out our articles on the Shortest Day of the Year and the Summer Solstice.

More information can be found at NASA, Seasons of the Year, The Sun at Solstice

Check out this podcast at Astronomy Cast: The Life of the Sun

## The Arctic Circle

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The Arctic Circle is a region on Earth that is marked as one of the five major circles of latitude on maps of our planet. It is located between the Arctic to its north and the Northern Temperate Zone to its south. Its counterpart in the Southern Hemisphere is known as the Antarctic Circle. In addition to the Antarctic Circle, the other three major latitude lines are the Tropic of Cancer, the Tropic of Capricorn, and the Equator.

The Arctic Circle is the farthest southern region that experiences polar day and polar night. A polar day is where it is 24 hours of continuous daylight, and a polar night is 24 continuous hours of darkness. North of the Arctic Circle more than one polar day and night occur per year. However, the Arctic Circle only experiences a polar day and night once a year on the June and December solstices respectively. Usually, those days fall on the 21st of each month. Polar day is also known as midnight sun and polar-summer while polar night is also known as darkness at noon or midwinter darkness. The Antarctic Circle also experiences a polar day and a polar night once a year.

Due to the Earth’s shifting axial tilt – a fluctuation of 2° during 40,000 years – which is especially a result of the Moon’s orbit, the Arctic Circle is moving. It is drifting north about 15 kilometers per year. The Earth’s axial tilt is the same thing that causes the different seasons on Earth.

The Arctic Circle passes through seven countries that have a considerable portion of land within the Arctic Circle. The countries are the United States of America, Greenland, Canada, Russia, Norway, Sweden, and Finland. Iceland has a tiny region – less than one square km – inside the Arctic Circle. Some areas within the Arctic Circle include Lapland Province in Finland; Yukon, the Northwest Territories, and part of Nunavut in Canada; Davis Strait in the Atlantic Ocean; the Island of Grimsey, which is part of Iceland; and the Greenland and Norwegian Seas in the Atlantic Ocean. The Arctic Circle is also a popular tourist site. People travel from different parts of the world to experience the region. Many places in the Arctic Circle also offer tours; Alaska has quite a few tours, although you can also find them in other countries. Some of these involve hiking around the Arctic Circle region during the summer.

Universe Today has articles on why Earth has seasons  and 10 interesting facts about Earth in case you want to learn more about our planet.

If you are looking for more information, check out what is the Arctic and the Arctic.

Astronomy Cast has an episode on Earth.