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December 13, 2005 > Days are getting shorter...

Days are getting shorter...

by Tina Cuccia

You may have noticed that days get shorter this time of the year, with the shortest day and the longest night in the Northern Hemisphere occurring on Winter Solstice, this year at 10:35 p.m., Pacific Standard Time on Wednesday, Dec. 21. Winter Solstice marks the beginning of winter in the Northern Hemisphere and summer in the Southern Hemisphere. As we are breaking out our winter coats, scarves and mittens, those "down under" are reaching for suntan oil, bathing suits and beach umbrellas.

Most are aware of this cyclic phenomenon, when the sun appears at its lowest point in the sky, and its elevation time at noon appears to be the same for several days before and after the solstice. The origin of the word "solstice," is derived from the Latin word solstitium, from sol "sun" and stitium, meaning "a stoppage." After Winter Solstice, days begin to grow longer and the nights become shorter.

Living creatures need and appreciate sunlight, which is associated with a feeing of liveliness and ebullience. Sleep, rest or feelings of melancholy are associated with darkness. It's not surprising that this seasonal phenomenon of gradually reducing daylight hours can have a dampening effect on our spirits. The influence of sunlight on our moods has been officially recognized by the medical community as a condition called Seasonal Affective Disorder (SAD). The remedy for SAD is light -- even artificial light will do.

Astronomically speaking, there is a fairly straight-forward explanation of the phenomenon of winter solstice. Planet Earth revolves around the Sun. The orbit that the Earth follows lies on a two-dimensional plane in space. As Earth revolves around the sun, it spins around. One revolution of that spin defines an Earth day.

At any given time, half of the Earth's surface receives sunlight and the other half is in darkness (except for reflected light from the moon). If the axis of spin of the Earth was perpendicular to the plane of orbit, then daylight hours would be exactly one half of the whole day on all parts of the Earth during the entire year. It is generally acknowledged that there would be no distinct seasons under those conditions. However, the axis is tilted at an angle of about 23.5 degrees. This tilt causes variations of sunshine in different parts of the Earth at different times of its orbit around the sun and provides seasons as we know them. Only at the Earth's Equator is daylight exactly half of a day.

The orbit of the earth around the sun is technically elliptical, but it is very close to being a circle. Since it is an ellipse, there is one point on the orbit called "perihelion," where the earth is closest to the sun. The opposite point on the orbit, called "aphelion," is when the Earth is farthest from the Sun.

As the Earth revolves around the Sun, the axis of spin maintains its orientation with respect to the orbital plane. So, twice a year, this axis lies on a plane that is both perpendicular to the orbital plane and connects to the centers of the Earth and the Sun. One such instance is around June 21 or 22, when the Northern Hemisphere of the Earth is facing the Sun, and the other is December 21 or 22, when it is the Southern Hemisphere's turn to face the Sun. These are termed Solstices and in the Northern Hemisphere, the June Solstice is known as the Summer Solstice, a harbinger of warm summer weather. December Solstice is known as the Winter Solstice, the precursor of winter.

Winter weather following the Winter Solstice is the consequence of the Northern Hemisphere facing away from the Sun and not receiving as many warm rays of light from the Sun as it receives in June. In the Southern Hemisphere, the seasons are reversed and hence the Winter Solstice will occur in June and the Summer Solstice in December.

Astronomers who study the Earth's motion around the Sun need to consider the effect of the Moon that orbits the Earth as well as other gravitational forces among the planets and the Sun. Although the shape and size of the orbit, the amount of tilt, and the orientation of the tilt of the Earth remain practically fixed during the lifetime of any individual, these do change over long periods of time (measured in thousands of years) and affect the seasons and climates of the Earth.

Cultural Celebrations:

A sense of fear among mankind regarding Winter Solstice that often appears in literature has led to various forms of celebrations. Ancient and modern cultures alike celebrate this time of the year, often with many lights, presumably to offset the lack of natural light. While there may have been an element of fear in ancient times that waning sunshine would continue to the point when the Sun completely vanished, there is evidence that as early as 5,000 years ago, contemporary but distant cultures such as the Anasazi (Southwestern U.S.) Indians and Irish Celts had a clear understanding of the movements of celestial bodies such as the Sun, the Moon and the stars as well as their practical impact on life on Earth.

However, more than the prospect of completely losing sunshine, the driving force behind Winter Solstice celebrations may have been the urge to mark the end of one year and the beginning of the next.

Just as sunrise, activity, sunset and a period of rest define a day in a rather broad stroke, a year is also divisible into seasons that mirror these periods: Spring is the birth of a new year, Summer is bountiful with crops, Autumn is the slowing down and the end of harvest and Winter, heralded by the Winter Solstice, is akin to a period of rest, sleep, or even death, with ensuing rejuvenation to occur the following year.

Winter Solstice celebrations usually take place during or close to December. It is believed that the cyclical nature of the seasons had been understood by various ancient civilizations and they practiced rituals to encourage continuation of these cycles.

Solstices were very important to Chumash tribe of Native Americans along the coast of California. Their Winter Solstice celebration would last many days. Chinese celebrate Winter Solstice, called Dong Zhi, with a feast. Iranians celebrate the Zoroastrian festival of Sada when a huge bonfire is ignited at sunset to help the Sun become more powerful and increase sunshine. Hopi Indians celebrate Soyal with rituals to help light defeat darkness.

Before Christ, Romans had their major festival on Winter Solstice. When Julius Caesar established a new calendar in 46 B.C., Winter Solstice was on December 25. This date became a tradition for Winter Solstice celebrations and remained constant through calendar revisions though it no longer fell on the day of Winter Solstice. It was later adopted by Christians as the date of birth of Jesus Christ. This has established itself as the most widespread Winter Solstice celebration around the world today.

 
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