Blog: Spring is here - more about the vernal equinox

NEW ORLEANS (WVUE) - We are trading out winter for spring this week.

Monday is the beginning of astronomical spring in the Northern Hemisphere, marked by the spring equinox. The spring, or vernal, equinox this year falls on March 20 at 4:24 CST. But what is an equinox?

The word equinox is defined by Merriam Webster as one of the two points on the celestial sphere where the celestial equator intersects the ecliptic. In simpler terms, one of the points where the sun appears to pass directly over the equator, allowing for the Northern Hemisphere and Southern Hemisphere to have approximately the same amount of light - one hemisphere is not more tilted towards the sun than the other. The celestial equator is an imaginary line in the sky that is on the same plane as the earth’s equator. The ecliptic is the apparent path of the sun among the constellations in a year. The word equinox was derived from this occurrence, coming from the words “equal night” in Latin - aequus (equal) and nox (night). In reality, the day and night are not exactly equal but fairly close.

The spring and autumnal equinoxes are the points at which this phenomena occurs each year, marking the start of the new astrological season. These two days are also the only two days in the year that the sun rises due east and sets due west.

You may have noticed meteorologists emphasizing the difference between astronomical spring and meteorological spring. Astronomical spring is based on the the position of Earth in relation to the sun whereas meteorological spring is related to the annual temperature trend.

Unlike meteorological spring, which falls on March 1 every year, astronomical spring can vary in date. The change comes due to the earth’s rotation around the sun actually taking 365.45 as opposed to 365, which leads to the observance of a Leap Year every four years.

The length of astronomical seasons also varies due to the elliptical pattern of the earth’s rotation around the sun. This causes the seasons to vary between 89 and 93 days. In order to make observance of climatological data easier, the meteorological seasons were created.

Meteorological seasons are broken up into groups of three, based on the temperature trend of the time period. The coldest months fall during December, January and February - making up winter. March, April and May are grouped together as spring. June, July and August are summer while September, October and November are autumn.

This grouping of months reduces the variability of seasonal length and allows meteorologists and climatologists to easily analyze and collect climate data. The varying astronomical seasons would make climate data comparison difficult as the dates change each year.

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