When is the official start of Spring 2010?

Answer:

Saturday, March 20

In the US and Canada:
Saturday, March 20, at approximately midnight is the official first day of spring for 2010 in the Northern Hemisphere (Vernal Equinox).

On that day, the Southern Hemisphere observes the Autumnal Equinox, which is the equivalent of the start of Autumn.

 

First day of spring in the Northern Hemisphere

In general, the four seasons correspond to the relative position of the sun to the earth. Meteorological determination of spring is calculated according to when the sun passes through the equatorial plane. When going from winter to spring, the sun is moving north; as soon as the sun crosses the equator, we call it spring. (This applies to places north of the equator.)

 

First day of spring in the Southern Hemisphere

"Spring" - a time of melting and new growth - occurs about September-November in places south of the equator, e.g., Australia, New Zealand, Africa, Brazil).

Countries such as Australia and New Zealand, however, designate the first day of September as the official first day of spring.

The official date of spring south of the equator (corresponding to the first official day of fall in places north of the equator) would be around September 20/21, depending on when the sun crosses the equator.


Background information regarding the seasons:
Spring comes in between the 19th to the 23rd of March and at different times. It changes on a yearly basis because the first official day of spring is the (Spring) Vernal Equinox. This is when the sun is directly above the equator. It rises due East and sets due West and does not do so on the exact same day every year; this is because the calendar is not exactly 365 precise days every single year.

In 2009, spring arrived on March 20; the sun was above the equator, and crossed to the Northern Hemisphere at approximately 11:47 p.m.