Vernal Equinox Marks the First Day of Spring

Most people are aware that March 20th is officially the first day of spring for 2010 or the vernal equinox, whether or not the groundhog saw its shadow on Goundhog Day. The first day of spring is determined by the Earth's position in relation to the sun and vernal equinox is an astronomical term describing that position.

Miconceptions about the Equator

To understand the vernal or spring equinox, we have to first clear up a common misperception. Many people think that the Earth's equator describes the line around the Earth where the Earth is closest to the sun. In other words, the sun shines most directly on the equator and at an increasing angle as one goes further away from the equator. Yes and no. That's true on exactly two days each year, the vernal equinox and the autumnal equinox. Those two days are March 21^st (give or take depending upon the year) and September 23^rd (or thereabouts depending upon the year). Only on these two days each year the length of the day and the night are both exactly 12 hours everyplace all over the globe.

The Sun's Most Direct Rays

On every other day of the year, the sun's most direct, or vertical, rays fall somewhere other than the equator. During our spring and summer in the United States, the sun's vertical rays fall in the northern hemisphere, making it autumn and winter in the southern hemisphere. During our autumn and winter, this is reversed with the sun's vertical rays falling upon the southern hemisphere so that we receive only more indirect solar rays.

The Earth's Tilt is Responsible for the Seasons

The reason for this is the Earth's tilt. The Earth's axis, which runs from the North Pole to the South Pole, is not straight up and down relative to the plane of the Earth's orbit around the sun. It is tilted at roughly 23 degrees. The direction of the tilt doesn't change during the year so that on the shortest day of the year in the northern hemisphere's winter, also called the winter solstice, the North Pole is tilted directly away from the sun. On the longest day of the northern hemisphere's summer, the summer solstice, the North Pole is leaning toward the sun.

Which Way does the Earth Lean?

On both the vernal equinox and autumnal equinox the Earth's North and South Pole are both the same distance away from the sun because the Earth's tilt is sideways relative to the sun. That is to say that the Earth leans neither toward nor away from the sun on the vernal and autumnal equinox. Instead, the Earth's axis is exactly perpendicular to the sun's vertical rays falling on the equator for those two days. The sketch in the upper left hand corner of this article (click the thumbnail for an enlarged view) shows the position of the Earth's axis on the vernal equinox, summer solstice, autumnal equinox, winter solstice.