Normally folks mark March 21st as the first day of spring, also known as the vernal equinox. But this year spring came early on March 20, 2013 at 7:02 am EST.
Was spring really early? Well, it turns out in the last century March 20th was the standard date for spring for most years. Vernal equinox actually happened on March 21 in only 36 out of 100 years!
And for those who cannot wait for warmer weather, there is more good news. If you are living in Alaska, Hawaii, Pacific, Mountain and Central time zones, in 2016, spring will start on March 19. In 2020, March 19 will be the first day of spring in all of the United States.
Why Seasons Happen
Seasons are caused as the Earth, tilted on its axis, travels around the Sun. The part of the Earth tilted towards the Sun experiences summer, while the part that is tilted away, winter.
The day when every part of the Earth has an equal number of hours of day and night is known as Equinox. This occurs when the Earth’s tilt is exactly 90 degrees to the Sun [see Side Notes]. Equinox marks the change of seasons -- Vernal equinox is the start of spring in the Northern Hemisphere (autumn in the S.Hemisphere) and Autumnal equinox for the start of Autumn.
The shifting spring date has to do with our Earth's tilt which keeps changing.
Our Wobbly Earth
Have you noticed the movement of the head of a spinning toy top? If you look carefully, you will notice that though the top spins on its axis for a long time, its head keeps changing its position in a sort of a wobble? This happens when tops are not perfectly spherical.
The earth is not perfectly spherical either. Scientists have compared the head of the top to the north pole. Observing Earth's rotation, they have noticed that it too has a wobble and the northpole, traces the shape of a cone as it wobbles. Thankfully for us on Earth, this wobble is very slow and it takes about 26000 years - called an age, to trace one complete conical shape.
Known as precession of the equinoxes it even explains the ice ages. As you can see in the video below, the precession or wobbling changes the position of the earth's top, affecting the exact moment each year when Earth's tilt reaches 90 degree to the sun.
Our Earth is also being pulled by the gravity of other planets, which affects its location in the orbit as well as its tilt.
To account for all these changes, astronomers constantly make adjustments to the calendar dates. Now you know why the start of spring keeps changing!
Courtesy: NASA, others