Vernal Equinox marks first day of spring
Posted March 14, 2014
Updated March 15, 2014
The Earth's 23.5º axial tilt creates the changing seasons. That tilt means that one of the hemispheres receives more sunlight most of the year, all but 2 days in fact. That pair of days provide a hand off of the seasons when neither the northern nor southern hemispheres receive more sunlight.
That day comes next on Thursday March 20 when the Vernal Equinox occurs.
The word “equinox” comes from the Latin words for “equal night”. With a name like that you’d expect 12 hours of sunlight (sunrise to sunset) that day. But the closest we’ll come to that is Sunday March 16. On that day, also called the “equilux” or “equal light”, we will receive just 11 hours, 59 minutes of sunlight. Why is this?
Much of the answer comes from our atmosphere. It refracts light as the sun rises and sets providing a periscope effect. The effect is so big that the sun appears to us nearly 2+ minutes before it is physically peaking over the horizon. Similarly it remains visible after it’s physically over the horizon providing an additional 4-5 minutes of sunlight to the day at our latitude.
Additionally, the equinox isn’t really a day on the calendar, but a very specific point in time (at 12:57 p.m. for this year’s Vernal Equinox). The equinox occurs as the center of the sun passes over Earth's equator. Once that happens the day is already getting slightly longer further dashing any hopes of a tidy 12 hours of sunlight.
This is a good week to get your bearings though. Note an object on the eastern horizon during sunrise and on the western horizon during the sunset. You can use these as a guide the rest of the year. The direction (or azimuth) of the rising and setting sun varies only a fraction of a degree this time of year so this will work any day this coming week.
Tony Rice is a volunteer in the NASA/JPL Solar System Ambassador program and software engineer at Cisco Systems. You can follow him on twitter @rtphokie.