Thursday, December 20, 2007 — Sunlight affects our world in many different ways. Winter solstice marks the first day of winter and is the shortest day of the year, because there is the least amount of daylight. Not only does this affect the weather, but it also can affect your health.

Sunlight gives your body Vitamin D, but during the winter months there's less daylight, so you have to find other ways to give your body that extra Vitamin D that it's lacking. Fortunately, there's a new book about Vitamin D at Bookends to help you out with this dilemma.

The amount of sunlight we get doesn't just magically change. Let me explain how it all works, so you can understand the science behind the winter solstice.

The earth orbits around the sun on its axis. The axis is really just an imaginary line that measures the earth's tilt. While orbiting, the tilt of the earth's axis changes, thus changing the seasons.

The winter solstice is when, because of the earth's tilt, our hemisphere is leaning farthest away from the sun, and because of that there is the least amount of daylight.

This tilting affects different areas of the earth in different ways. For example, during the summer in Alaska, there's daylight for three months at a time, and in the winter there's three months of darkness.

Australia is located in the southern hemisphere, and as a result, their seasons are reversed. They have winter when we're having summer. Because of this, their winter solstice also occurs during our summer.

After our winter solstice, which is December 22nd this year, the earth's axis continues to change the days begin to get gradually longer because our hemisphere is gradually leaning more towards the sun, and we get more daylight. Eventually, we'll get to the summer solstice, which is the longest day of the year.

It's a lot to take in, but the winter solstice marks an important turning point in our seasons. It will only get sunnier from here.