The winter solstice is the shortest day of the year, in terms of the amount of sunlight in the northern hemisphere. In 2016, the winter solstice falls on December 21. To understand what the winter solstice is, and why it matters, you need to know some astronomy. Astronomy was part of the quadrivium (the arts of number, space, and time) from the ancient liberal arts curriculum.
- Mathematics is the study of number.
- Geometry is the study of number and space.
- Music is the study of number and time.
- Astronomy is the study of number, space, and time.
The study of astronomy helped you understand what the universe is like, and how and where and when you fit into it.
For thousands of years, people who lived in the northern hemisphere knew that the seasons of the year were the result of changes in the length and strength of daylight. The day and night are of equal length at around March 21, which is called the spring equinox (which means equal nights). For this reason, the vernal equinox is defined as the first day of spring, even if the weather is still wintery.
After the spring equinox, the sun seems to get a bit stronger every day. It rises a bit earlier every morning. It rises to a higher point in the sky at midday. It sets a bit later every evening. The lengthening and strengthening of the daylight continue until midsummer’s day, which is around June 21. Midsummer’s day is officially the first day of summer, even if the weather has been summery for several weeks.
On midsummer’s day, the northern hemisphere gets its longest and strongest daylight. The sun rises at its earliest point. The sun rises to the highest point in the sky for that year. The sun sets at the latest point in the year. If you go far enough North, the sun never actually sets on that day. The latitude at which you get a full day of daylight on midsummer’s day is called the Arctic Circle. At the North Pole, you get 6 months of daylight, starting on the vernal equinox.
As summer progresses, the weather stays warm, even though the sun gets a bit weaker each day. The sun rises a bit later. The sun reaches a slightly lower highpoint at midday. The sun sets a bit earlier. Its rays are not quite so bright and warm as they were the day before. Even so, the weather continues to get warmer at first. That is why July and August tend to be so warm. Yet eventually, we start to feel the effects of the shorter day length and weakening sunlight.
By around September 21, we reach a point at which the day and night are again of equal length. This is the autumnal equinox. After that, the nights in the Northern hemisphere are longer than the days, and the sunlight weakens further. The weather starts to get cold. The day length continues to get shorter and the sunlight weaker until around December 21, which is the winter solstice. At points north of the Arctic Circle, the sun does not rise at all for at least 1 day in the winter.
Equinox means equal day and night. At both the vernal equinox and the autumnal equinox, the day and night are of equal length. In the Northern Hemisphere, the summer solstice means the day on which the sunshine is longest and strongest, whereas the winter solstice is the day on which the sunshine is shortest and weakest. Although the sunshine starts to increase at that time, the weather continues to get colder because the days are still shorter than the nights. This pattern gave rise to the old adage, “As the days lengthen, the cold strengthens.”
Eventually, the sunlight lengthens and strengthens enough to produce warm weather. Then, the cycle of vernal equinox, summer solstice, autumnal equinox, and winter solstice repeat itself. Of course, winter and summer are switched in the southern hemisphere. The southern winter is the northern summer, and vice versa.
Ancient people could use sticks and stones to mark the position of the sunrise and sunset and moonrise and moonset at different points of the year. In 1720, archeologist William Stukeley noticed that Stonehenge, a prehistoric monument in Britain, was built along an axis that pointed to the midsummer sunrise.
In 1987, archeologists Clark and Marjorie Hardman noticed that the Serpent Mound, a prehistoric monument in Ohio, also marked the vernal and autumnal equinoxes and the summer and winter solstices, as well as information about the movements of the moon.
Prehistoric people knew that the sun traces the same path through the sky, relative to the stars, every year. Astronomers in ancient Babylon divided this path up into 12 portions. Each portion was then named after the constellation (group of stars) that it contained. Some of these constellations were named after animals, which is why that circular path was called the zodiac.
Many prehistoric Europeans understood about the sun’s circular path and the equinoxes and solstices. To represent this concept, they used a cross (representing the equinoxes and solstices) within a circle (representing the zodiac). If you extend the arms beyond the circle, with a relatively short arm to represent the winter solstice and a long arm to represent the summer solstice, you end up with a Celtic Cross, which became an important Christian symbol.
Prehistoric people could use sticks and stones to mark the direction of the sunrise and sunset, as well as the moonrise and moonset. However, they had no idea why the sun always followed the same path, year after year. They typically believed that the sun was a god. For this reason, primitive peoples in Europe organized their religious calendar around the sun. At the vernal equinox, they celebrated the victory of light over darkness, as the day became longer than the night. At midsummer’s day, they lit bonfires, to ward off the evil spirits that might take advantage of the sun’s change of direction. The autumnal equinox was usually celebrated with harvest festivals. The winter solstice was also a festival time. December 25 was chosen as a major festival because it was three days after the winter solstice—long enough for people to see that the days were getting longer again.
Prehistoric people believed that the earth stands still at the center of the universe, and that the sun revolves around it, once per day. Behind the sun, the stars supposedly revolved around the earth, once per year. Today, we know that this motion of the sun and the stars is an illusion. We know that the Earth is a ball that is spinning like a top. From your point of view, the sun seems to rise as your part of the Earth turns toward the sun. The sun seems to set as your spot on the Earth turns away from the Sun.
As the Earth is traveling around the sun, the north pole keeps pointing in practically the same direction, which is at an angle relative to earth’s path around the sun. As a result, the northern hemisphere is tipped away from the sun in the winter and toward the sun in the summer. That is why the North Pole gets six months of daylight between the spring and autumn equinoxes, and six months of nighttime between the autumnal and vernal equinoxes. The seasons are reversed in the southern hemisphere. The southern “summer” corresponds to the northern winter.
The sun’s circular path around the zodiac is also an illusion. It results from the fact that the earth is traveling in a nearly circular path around the sun. When viewed from earth, the sun seems to travel from one constellation to the next.
In 129 B.C., the Greek astronomer Hipparchus noticed that the stars seem to be shifted systematically from their positions, as described by older Babylonian astronomers. As a result of this shift, the vernal equinox now takes place in an earlier point of the year (as reckoned by the sun’s position in the zodiac) than it had in ancient times.
We now know the cause of this shift: the earth’s axis wobbles. As a result, the North pole traces out a complete circle once every 26,000 years. For this reason, each equinox comes at a time when the sun seems to be at a slightly earlier point in the zodiac, compared to the spot where it seemed to be the year before. However, this shift is so slight that you would not notice it in your lifetime.
The length of time that it takes for the vernal equinox to pass from one sign of the zodiac to the next was called an astrological age. Since there are 12 signs of the zodiac, the average length of an astrological age would be 2150 years.
Back in the 1960s, astrologers claimed that we were entering the Age of Aquarius, which supposedly meant that all sorts of wonderful things were about to happen. The group The Fifth Dimension expressed this hope in the song The Age of Aquarius, from the musical Hair:
Modern astronomers have taught us that astrology is nonsense, and that even if it weren’t, we won’t actually enter the “Age of Aquarius” for another 600 years. Meanwhile, we can still try to “Let the sun shine in!”
Photo by AlicePopkorn