Monday, March 19, 2012

Earliest Spring in a Century Starts Tonight

For some parts of the country, spring officially starts early tomorrow morning for the East Coast, meaning Spring actually starts late tonight for the western part of the country. Now, the plants are blooming, the birds are chirping, and people across the world will try to balance eggs (although that can be done any other day of the year with equal amounts of patience).

Usually, spring starts on either March 20 or March 21. Spring hasn't started this early in over a century, but it's not due to a changing climate. Instead, this early astronomical spring stems from several factors related to the Earth's tilt and orbit.

A helpful video on the cause of the seasons. Video Courtesy of Ignite Learning.

Despite a somewhat pervasive myth -- one that many Harvard graduates have believed -- the Earth's seasons are not caused by the distance between the Earth and the Sun. Instead, seasons change based on the tilt of the earth.

The Earth rotates on its axis at a 23.5 degree angle. Because of this tilt, the northern hemisphere receives significantly less solar energy when it is tilted away from the sun, causing winter in the northern hemisphere. In contrast, the southern hemisphere receives more solar energy, producing largely warm weather conditions.

As the Earth rotates around the sun, the hemisphere receiving the most solar energy changes as the northern part of the Earth points toward the sun. Consequently, the seasons change during the Earth's revolution.

To be fair, the distance between the Earth and the Sun does have an impact on the Earth's temperature. However, the temperature difference between the farthest point (aphelion) on Earth's orbit and the closest point (perihelion) is relatively small compared to the seasonal changes caused by the Earth's tilt.

On the spring and autumn equinoxes, both hemispheres receive approximately the same amount of solar energy. On these dates, the center of the sun lies on the same plane as the Earth's equator.So why does the date and time change each year?

According to this article, there's a number of reasons:

1. A calendar year doesn't precisely equate to one entire orbit by the Earth. This is why we have leap years, and the equinox dates can change because of this.

2. The axis of Earth's rotation is changing, causing the date to change.

3. Although Earth's orbit is primarily determined by the Sun's gravitational pull, the other planets in our solar system tug on Earth as well, leading to changes in the equinoxes.

Now, the days will be getting longer in the northern hemisphere as more sunlight hits its surface. We won't have this solar energy equilibrium until September 22, the autumnal equinox.

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