The Winter Solstice, S.A.D. & the Leap Year.

What is a solstice?   

The earliest people on Earth knew that the sun’s path across the sky, the length of daylight, and the location of the sunrise and sunset all shifted in a regular way throughout the year. They built monuments such as Stonehenge in England – or, for example, at Machu Picchu in Peru – to follow the sun’s yearly progress.

But we today see the solstice differently. We can picture it from the vantage point of space. Today, we know that the solstice is an astronomical event, caused by Earth’s tilt on its axis, and its motion in orbit around the sun.Because Earth doesn’t orbit upright, but is instead tilted on its axis by 23-and-a-half degrees, Earth’s Northern and Southern Hemispheres trade places in receiving the sun’s light and warmth most directly. The tilt of the Earth – not our distance from the sun – is what causes winter and summer. At the December solstice, the Northern Hemisphere is leaning most away from the sun for the year.

At the December solstice, Earth is positioned in its orbit so that the sun stays below the north pole horizon. As seen from 23-and-a-half degrees south of the equator, at the imaginary line encircling the globe known as the Tropic of Capricorn, the sun shines directly overhead at noon. This is as far south as the sun ever gets. All locations south of the equator have day lengths greater than 12 hours at the December solstice. Meanwhile, all locations north of the equator have day lengths less than 12 hours.

For us on the northern part of Earth, the shortest day comes at the solstice. After the winter solstice, the days get longer, and the nights shorter. It’s a seasonal shift that nearly everyone notices.

What is seasonal affective disorder (SAD)?

Seasonal affective disorder, or SAD, is a type of depression that occurs during the same season each year. You may have SAD if you felt depressed during the last two winters but felt much better in spring and summer.

Anyone can get SAD, but it's more common in:

·         Women.

·         People who live far from the equator, where winter daylight hours are very short.

·         People between the ages of 15 and 55. The risk of getting SAD for the first time goes down as you age.

·         People who have a close relative with SAD.

Experts aren't sure what causes SAD. But they think it may be caused by a lack of sunlight. Lack of light may:

·         Upset your "biological clock," which controls your sleep-wake pattern and other circadian rhythms.

·         Cause problems with serotonin, a brain chemical that affects mood.

What are the symptoms?

If you have SAD, you may:

·         Feel sad, grumpy, moody, or anxious.

·         Lose interest in your usual activities.

·         Eat more and crave carbohydrates, such as bread and pasta.

·         Gain weight.

·         Sleep more but still feel tired.

·         Have trouble concentrating.

Symptoms come and go at about the same time each year. Most people with SAD start to have symptoms in September or October and feel better by April or May.

Treatment for SAD may include light therapy (phototherapy), psychotherapy and medications.

Why do we have a leap year?

Nearly every four years, we add an extra day to the calendar in the form of February 29, also known as Leap Day. Put simply, these additional 24 hours are built into the calendar to ensure that it stays in line with the Earth’s movement around the Sun. While the modern calendar contains 365 days, the actual time it takes for Earth to orbit its star is slightly longer—roughly 365.2421 days. The difference might seem negligible, but over decades and centuries that missing quarter of a day per year can add up. To ensure consistency with the true astronomical year, it is necessary to periodically add in an extra day to make up the lost time and get the calendar back in synch with the heavens.

The Egyptians were among the first to calculate the need for a leap year, but the practice didn’t arrive in Europe until the reign of the Roman dictator Julius Caesar. Before then, the Roman calendar had operated on a muddled lunar model that regularly required adding an extra month to maintain celestial consistency. Finally, in 46 B.C., Caesar and the astronomer Sosigenes revamped the Roman calendar to include 12 months and 365 days. This “Julian Calendar” also accounted for the slightly longer solar year by adding a leap day every four years.

Caesar’s model helped realign the Roman calendar, but it had one small problem. Since the solar year is only .242 days longer than the calendar year and not an even .25, adding a leap year every four years actually leaves an annual surplus of roughly 11 minutes. This minute discrepancy meant that the Julian calendar drifted off course by one day every 128 years, and by the 14th century it had strayed 10 days off the solar year. To fix the glitch, Pope Gregory XIII instituted a revised “Gregorian Calendar” in 1582. In this model, leap years occur every four years except for years evenly divisible by 100 and not by 400. For example, the year 1900 was not a leap year because it was divisible by 100, but not 400. The Pope’s updated calendar remains in use to this day, but it’s still not perfect—experts note that the remaining discrepancies will need to be addressed in around 10,000 years.

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