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December 21, 2004 > Winter Solstice

Winter Solstice

We can not help but notice that days have been getting shorter for a while now. From science lessons learned in school as well as annual reminders from the effects, we are well aware that this is a cyclic phenomenon and we will soon start to experience the opposite: progressively increasing daylight hours. Most living creatures appreciate sunlight. It is very common to have activity, liveliness, and general ebullience associated with sunlight and sleep, rest, or melancholy associated with darkness. It is not surprising that this seasonal phenomenon of gradually reducing daylight hours has a dampening effect on our spirits. This influence on our moods is now officially recognized by the medical community as a condition termed Seasonal Affective Disorder (SAD). The remedy for it is light, even artificial light. The turning point, when shortening days cease and give way to lengthening days is the Winter Solstice. Ancient and modern cultures alike celebrate this time of the year, often with a lot of lights, presumably to offset the lack of natural light.

Astronomical Explanation
Astronomically, there is a fairly straight-forward explanation of the phenomenon. Earth revolves around the Sun in space. The orbit that the Earth follows lies on a two-dimensional plane in space. In addition to this motion, the earth also spins around itself. A revolution of this spin defines an Earth day. At any given time, one 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 these conditions. However, this axis is tilted at an angle of about 23.5 degrees from the perpendicular. This tilt causes variations of sunshine in different parts of the Earth at different times of its orbit around the sun and gives rise to the 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 is connecting the centers of the Earth and the Sun. One such instance is around June 21 when the Northern Hemisphere of the Earth is facing the Sun, and the other is around December 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, being the harbinger of warm summer weather, and the December Solstice is known as the Winter Solstice for being 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 much warm rays of light from the Sun as 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.

The schematics shown that depict the movement of the Earth around the Sun illustrate the effect of the Earth's tilt during the solstices. Note that the solstices accentuate an imbalance of sunshine between the Northern and Southern Hemispheres whereas the Equinoxes represent times when the Hemispheres receive identical sunshine. The schematics further illustrate that the locations in the orbit of the Earth where the solstices occur need not coincide with the locations of perihelion and apohelion. In fact, it is understood that these do diverge gradually and after many thousands of years, there will be a time when the Winter Solstice will be close to the apohelion. However, since the orbit of the Earth is almost a circle, these changes are not expected to significantly influence seasons and weather.

In 2004, the Northern Hemispheric Winter Solstice is scheduled to happen at 4:42 AM Pacific Standard Time on December 21. So, by the time you read this article, the solstice will have occurred and Winter officially begun. 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, the orientation of the tilt etc. 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
There is allusion in literature to a sense of fear among mankind regarding Winter Solstice which led to various forms of celebration. Though there may have been an element of fear in ancient times that the 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 and their practical effects on life on Earth. Therefore, more than the prospect of completely losing sunshine, the driving force behind the 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 mark a day in broad strokes, 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 the harvest season and Winter, heralded by the Winter Solstice, is akin to a period of rest, sleep, or even death, with ensuing rejuvenation the next year. Noting that history has been dominated by Northern Hemispheric cultures, it is not surprising that Winter Solstice celebrations happen during or close to December. Although we believe the cyclical nature of the seasons had been understood by various civilizations, they nevertheless 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 get 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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