The days are becoming longer, the air a little less brisk and people seem to be blinking a little more in the shining sunlight. That's right, spring is upon those of us in the Northern Hemisphere.

With this in mind, let's investigate exactly what is happening in the world and why.

Most people know the Earth revolves around the sun while it rotates on its axis. This revolution takes approximately 365 days and the rotation 24 hours. But because the Earth's axis is tilted and not perfectly vertical, interesting changes happen depending on which part of the Earth's surface is closest to the sun.

Imagine a Styrofoam ball with a pencil stuck through it at a 25-degree tilt, moving in an elliptical orbit around a center point. Winter occurs when the northern hemisphere of this ball tilts away from the sun, and summer occurs when the Northern Hemisphere leans into the sun. Spring and autumn occur when the Earth's axial tilt is in a liminal phase between the two extremes, so the sun is not overly cooling or warming the planet.

However, the start of spring has changed in recent years. While most of us learned back in grade school that the start of spring was March 21, "season creep" has lead to snow melting and birds chirping a little bit earlier, with the season now beginning March 20.

This is due to a combination of factors.

For one, the Gregorian calendar is not exactly 365 days, and each season isn't an even number of days; spring clocks in at roughly 92.758 days. Additionally, the Earth's elliptical orbit is constantly changing as a result of gravitational pull from the sun and other terrestrial bodies.

Seasons are defined as starting when the Earth's tilt is at a precisely defined 90-degree angle, and these intervals change as the orbit shifts. Spring officially begins at the Vernal Equinox, when the Earth's tilt is neither leaning away nor leaning towards the sun. The Equinox this year took place on March 20 at 7:44 A.M. Eastern Daylight Time (EDT).

In 2010, expect the Equinox on the same date at 1:32 P.M. EDT.

Spring Says Goodbye to SAD Symptoms
While a respite from the snow and cold of winter is very welcome, for some it can be a sea change of mood due to the high percentage of seasonal affective disorder (SAD) sufferers in the United States. For many, the beginning of spring is not only the signal for Easter candy and pesto season, it can also be a metaphorical crawling out of an emotional hole.

SAD is a type of winter depression that affects people who don't get enough sunlight. It is caused by an overproduction of melatonin, a hormone produced by the brain during nighttime hours. Melatonin is produced in higher quantities from December through February in some 14 million SAD sufferers due to decreased light hours.

SAD affects some six percent of the U.S. population, with another 14 percent coping with "winter blues." The symptoms of SAD usually start between September and November and continue until March or April, the Seasonal Affective Disorder Association says. For many people, SAD is a seriously disabling illness, preventing them from functioning normally without continuous medical treatment. Its peak season began in early January and ran through all of February, according to the American Psychiatric Association.

Whereas SAD symptoms disappear in spring, depression exists outside the rules of seasonal change.

According to health insurance company Value Options, citing a Rand Corporation study, "depression results in more days in bed than many other ailments." More than seasonal "blues," depression is a severe hindrance to interpersonal and professional performance. As such, more sick days are due to depression-related disorders than ulcers, diabetes, high blood pressure, arthritis and even the common cold.

Depression can also affect a company's productivity, morale and effectiveness, with absenteeism taking billions of dollars off the bottom line for U.S. businesses. The latest CCH Unscheduled Absence Survey revealed that large U.S. companies shell out $764,000 in direct payroll costs, and even more when "lower productivity, lost revenue and the effects of poor morale" are considered.

Fortunately for sufferers of depression, many companies are realizing the severity of the condition and supplying on-the-job training to recognize and cope with depression or company-sponsored counseling for sufferers.

Additionally, the U.S. Congress last year passed H.R. 6983 as part of the Troubled Assets Relief Program. The Paul Wellstone and Pete Domenici Mental Health Parity and Addiction Equity Act of 2008 requires most health coverage plans to treat patients seeking coverage for mental illness as equal to those suffering physical ailments. This means treatment is available at a more affordable cost to the millions who suffer from depression every year.

Earlier: Gloomy-Sky Moods of SAD, Winter Blues

Resources

The Science of Spring
by Robert Roy Britt
LiveScience, March 19, 2009

How Depression Affects the Workplace
Value Options

CCH 2007 Unscheduled Absence Survey
CCH / Wolters Kluwer Law & Business, Oct. 10, 2007

After 12 Years, Wellstone Mental Health Parity Act is Law
by Fred Frommer
The Associated Press, Oct. 3, 2008