Continuing on our weather related themes for this month…
I know that many of you have been posing the question, when does/did spring actually start? Well rest assured, the debate has been settled by my dear friend Helena Cavanagh (who by the way is not my neighbor, despite all those vicious rumors) She came up with the following brilliant and well researched explanation:
1) definition of Equinox:
either of two times of the year when the sun crosses the plane of the earth's equator and day and night are of equal length
vernal equinox: March 21
September equinox: September 22
(2) a more scientific explanation (http://scienceworld.wolfram.com/astronomy/Equinox.html):
As the Earth travels around the Sun in its orbit, the north-south position of the Sun changes over the course of the year due to the changing orientation of the Earth's tilted rotation axes with respect to the Sun. The tilt of the Earth's equatorial plane relative to the Sun is responsible for the seasons.
The Earth's rotational axis has a 23.5° obliquity to its orbital plane. The hemisphere of the Earth which is tilted toward the Sun receives a greater flux of solar energy ("flux" is just a fancy word for energy per unit area per unit time) than the hemisphere tilted away, resulting in higher temperatures. The effect of incidence angle on solar flux is well-known to everyone, since the day is warmest when the Sun is overhead (at which point the Earth's surface is nearly perpendicular to it) and then cools as the Sun nears the horizon (at which point the sunlight grazes the ground at an angle, resulting in a smaller amount of heating per unit area of the ground).
great explanation: http://www.analemma.com/Pages/Tilt/TiltEffect/TiltEffect.html (the last movie shows the tilt of the Earth)
The dates of maximum tilt of the Earth's equator correspond to the summer solstice and winter solstice, and the dates of zero tilt to the vernal equinox and autumnal equinox. The solstices and equinoxes shift through the Gregorian calendar according to the insertion of leap years.
"The modern definition of the equinoxes is the instant at which the center of the Sun crosses declination 0 (i.e., the celestial equator, which is the projection of the Earth's equator onto the sky).
With this definition, day and night are not quite the same length on the equinoxes due to:
(1) refraction of light from the sun as it passes through the Earth's atmosphere and
(2) the fact that sunrise and sunset are calculated from the limb (not the center).
Both of these effects slightly lengthen "day" relative to "night.""
In the southern hemisphere:
- the Winter solstice is the shortest day of the year (near December 22), when the Sun is farthest north. The declination of the Sun on the (southern) winter solstice is known as the tropic of cancer (23° 27').
- the Summer solstice is the longest day of the year (near June 22), when the Sun is farthest south. The declination of the Sun on the (southern) summer solstice is known as the tropic of capricorn (-23° 27').
Regarding our discussion about the date:
"The equinox occurs around March 20-22, varying slightly each year according to the 400 year cycle of leap years in the Gregorian Calendar"
(http://www.wordiq.com/definition/Vernal_equinox.html)