Day and Night

by Conroy

A day/night map of the world

Just a few days ago, September 23rd to be precise, was the September equinox. The date when the sun passes the Equator on its southward journey to the Tropic of Capricorn. Or to put it in a more accurate way, the date (and time) during our planet's revolution of the sun when the Earth's axis of rotation is directly perpendicular to the imaginary line connecting the centers of the Earth and sun. After this date the southern hemisphere is tilted toward the sun (and the northern hemisphere tilted away). From our terrestrial perspective, for the next few months the sun will head ever more southward until the December solstice (December 22 in 2011), when it will reach its southern limit and begin its journey back north, passing the equator for the March equinox (March 20, 2012), and reaching its northern limit at the June solstice (June 22, 2012). Only to head back south again, repeating the cycle, forever.
This familiar cycle, corresponding to the seasonal variation that is such a distinctive feature of life, especially in temperate climates, has always fascinated me. The alternation between summer and winter, light and dark, has directly affected where and how humans live.
Astronomical Facts

A diagram of Earth's axial tilt

Our constantly varying durations of night and day are a result of our planet's axial tilt. If Earth had no axial tilt - if the axis of rotation was perpendicular to the orbital plane - there would be constant equinox with the sun always directly above the equator; all days and nights would be of constant duration. As it is, Earth is titled 23.44 degrees from perpendicular. So, the direct (perpendicular) rays of the sun hit the planet as far south as the Tropic of Capricorn (23.44 degrees south latitude) at the December solstice, and as far north as the Tropic of Cancer (23.44 degrees north latitude) at the June solstice.
Interestingly, the axial tilt of the Earth varies at 41,000 year intervals, ranging between 22.1 degrees and 24.5 degrees. It's currently on the decreasing part of the cycle. Less axial tilt (lower obliquity to use the astronomical term) tends to result in warmer winters and cooler summers with and overall trend of more glaciation, which can lead to an Ice Age. Consider that modern civilization has more or less emerged since the last Ice Age, which ended around 10,000 years ago, around the time the Earth last reached its maximum axial tilt.
(Axial tilt isn't the only way the Earth's orientation to the sun varies. See the end of the post for a brief description of these other variations.)
Day and Night
On Earth the farther you get from the Equator the greater the variation in length of day between the June and December solstices. For instance, in Singapore, just north of the Equator, the difference is just 10 minutes; in Baltimore, a little less than halfway to the North Pole, the difference is 5.5 hours; and in Oslo, two-thirds of the way to the North Pole, the difference is nearly 13 hours! For people who live in the middle latitudes - the majority of humanity - there is a big difference between the long warm days of summer and the long cold nights of winter.

The once annual South Pole sunset

Indeed, the further one travels towards the poles, the more disorienting this difference becomes. At 66.56 degrees north you reach the Arctic Circle (66.56 degrees south is the Antarctic Circle). Above this latitude the sun never sets in mid-summer and never rises in mid-winter. If you go all the way to the North Pole (or South Pole) you lose the distinction of days altogether. At the poles, there is just one day each year. A scientist living at the Amundsen-Scott South Pole Station at the South Pole would see the sun set at the March equinox, the sky slowly darken to blackness by late April, and experience perpetual night until late August when the sky would slowly brighten until sunrise at the September equinox. The sun would then be up - albeit low in the sky - until the March equinox. Just one sunrise and sunset over an entire year, how alien. To those intrepid men and women stationed at the South Pole, the recent equinox must have been a day of celebration.
Fortunately, there are very few cities of any size located north of the Arctic Circle, and none south of the Antarctic Circle, so almost everyone gets daily sunrises and sunsets.
Where Humans Live
The fact is that humans, like most life, love the sun. The closer to the equator the more direct the sunlight, the greater the solar radiation, which means more heat and more energy. Trees actually grow north of the Arctic Circle in Norway, central Siberia, Alaska, and the Northwest Territories of Canada, but never further north than 70 degrees. No trees grow in Antarctica or surrounding islands. The southernmost tree growth is in Tierra del Fuego, approximately 55 degrees south.
Tierra del Fuego is sparsely populated, Antarctica not at all (permanent residents). Northern Canada, Alaska, Siberia, Greenland, and northern Scandinavia are among the least densely populated places on Earth. Even for modern man, these places are inhospitable. Few find the long, cold winters appealing (or even the perpetual summer sun). The northernmost city of more than one million people is St. Petersburg at 60 degrees north. The northernmost large city is Murmansk, Russia located at 69 degrees north and home to a little more than 300,000 people. Tellingly, that figure is down more than 30% from 1989 when Murmansk was an important Soviet seaport, the nation's only ice free port with open access to the ocean (and home the Northern Fleet).

Population density map - notice the "empty" high latitudes

Take a close look at a population map of the world and you'll see that humans don't settle more than 60 degrees north or south of the equator. It seems that the short winter days (and short summer nights) in places like Oslo and St. Petersburg are as extreme a people want to deal with.
It's a quirk of geography that that the polar regions are relatively inaccessible. The South Pole was only reached by Norwegian Roald Amundsen in 1911 and the North Pole may not have been reached until Amundsen made it in 1928. But even if it was easy to reach the poles, few would choose to go and none to live permanently.
A Few Thoughts on Twilight

A "white night" in St. Petersburg

While on the subject of day and night, I may as well touch on twilight. All places on Earth receive the same amount of annual daylight, its just distributed differently. But not every place gets the same amount of twilight. You've probably heard of the famously short tropical twilight. This is literally true. The sun sets (and rises) in the tropics at a more perpendicular angle to the horizon, which means it sets and rises faster than at higher latitudes where it rises and sets at a more oblique angle. The duration of evening twilight at Quito, Ecuador (right on the Equator) is a short 20 minutes at this time of year. That compares to over 30 minutes in Baltimore or over 40 minutes in Oslo. And as discussed above, the polar twilight is about a month long.
One final thought, there are actually three kinds of twilight: (1) civil twilight when the sun is 6 degrees or less below the horizon, and when outdoor activity is possible without artificial lighting; (2) nautical twilight when the sun is between 6 and 12 degrees below the horizon, named because there is enough light for sea navigation using the visible horizon, and; (3) astronomical twilight when the sun is between 12 and 18 degrees below the horizon, when the sky is still too light for all astronomical observations.
I was in Dublin a couple of years ago (latitude 53.3 degrees north) in early July and marveled that the sky still had faint hints of twilight at midnight. The European "white nights" and Alaskan "midnight sun" are terms for the long summer twilight.
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Earth's Other Movements
The Earth's axial tilt varies over a period of 41,000 years. But Earth's movements relative to sun vary cyclically in several other ways.

• Eccentricity, or the shape of the Earth's orbit around the sun. All orbits are elliptical but Earth's has very low eccentricity, meaning it is very circular. But the eccentricity varies on a 413,000 year cycle. Currently is closer to the minimum, but when eccentricity is higher the minimum distance from the sun (perihelion) and the maximum distance from the sun (aphelion) change, which means the amount of solar radiation reaching the Earth varies more throughout the year. This can have an affect on seasons and climate.

• Orbital precession, or how the orbit revolves around the sun.

• Axial precession, is the change in the direction of the Earth's axial tilt, think of the top of a rotating gyroscope. Currently the northern hemisphere tilts toward the sun at aphelion and the southern hemisphere faces the sun during perihelion. Meaning the southern hemisphere gets more radiation during summer and less during winter relative to the northern hemisphere. As the axis precesses over a period of 26,000 years, aphelion and perihelion occur closer to the equinoxes meaning both hemispheres get the same amount of radiation throughout the year. This would lead to more consistent seasonal variations for both hemispheres.

The combination of eccentricity, orbital precession, and axial precession affect the length, dates, and intensity of the seasons, resulting a 21,000 year cycle between seasons and orbits. As an example, in 5,000 years the summer and winter solstices will occur in September and March instead of June and December.

• Orbital inclination, or the angle of the ecliptic (Earth's orbital plane around the sun) relative the sun's equator or the solar system's invariable plane. The Earth has a fairly low orbital inclination (so do the other planets) but it drifts up and down over a period of 70,000 years.

• Nutation, or the frequent small perturbations in the axial tilt caused by the gravitational effects of the sun and Moon. These do not affect the Earth's overall 41,000 years axial tilt variation cycles, and amount to only a fraction of one minute of one degree.