If you’re familiar with the word “equinox,” but unsure of its meaning, you're probably in the majority. Although an astronomy term, it has found its way into the vernacular--perhaps because it manages to sound simultaneously technical and mystical. The word has been used to brand such diverse offerings as an SUV from Chevrolet, a classic rock album by the band Styx, and a national chain of fitness clubs.
Pronounced either EE kwi nocks or EH kwi nocks, the word is derived from a Latin word meaning equal night, the significance of which we'll examine in a minute. An equinox is the moment in time, calculated to the minute, when the center of the Sun is above the Earth’s equator. This occurs only twice a year: in the spring around March 20 and in the fall around September 22-23. The Spring Equinox, also known as the Vernal Equinox, conventionally marks the official first day of spring in the United States. This year the Spring Equinox occurs on Thursday, March 20 at 1:48 a.m. Eastern Daylight Time (adjust as needed for your time zone). In the Mountain and Pacific time zones, because of the time difference, the Spring Equinox will actually fall on Wednesday, March 19, but calendars show the equinox and the first day of spring on March 20.
To understand why equinoxes occur only twice a year, you need to remember that Earth’s axis is tilted 23.5 degrees with respect to its orbit around the Sun. If it were not, then Earth’s equator would line up with the plane of Earth’s orbit around the Sun, and the center of the Sun would always be above the equator. Of course, we would have no seasons. The temperatures at a given location on the globe would not vary much over the course of a year. We would not see the Sun arcing high in the sky in the summer or arcing low--nearer the southern horizon--in the winter. It would pretty much follow the same course in the sky, sunrise to sunset, day after day, all year long. Wouldn’t that be boring!
Earth’s tilt with respect to its orbit around the Sun
Image courtesy of Tau’olunga
Image courtesy of Tau’olunga
But don't worry. Our 23.5-degrees-off-kilter planet ensures that here in the Northern Hemisphere we tilt toward the Sun in the summer and experience the heating effect of direct sunlight, and then tilt away from the Sun in the winter and get the chilling effect of indirect sunlight. In spring and fall, midway between the two temperature range (and tilt) extremes, we enjoy moderate temperatures.
Another way to think of the equinox is that it's when the ecliptic (the imaginary line that represents the path the Sun appears to take across the sky, as seen from Earth) intersects with the celestial equator (the imaginary line that represents where the plane of the Earth’s equator, were it to be extended out into space, would intersect with the sky). The diagram below shows the twice-yearly intersection of these two lines, with the Vernal or Spring Equinox marked. The other point of intersection shown corresponds to the Fall Equinox. The celestial equator is shown in blue and the ecliptic in red.
A Tale of Two Planes: Equator & Ecliptic
Diagram by Dr. Guy Worthey
The Earth’s sunward tilt in the summer puts the plane of the equator below the Sun’s center (which corresponds to the ecliptic), and the Earth’s sun-shunning tilt in the winter puts the plane of the equator above the Sun's center. Only during the in-between seasons of Earth’s orbit--spring and fall--can the Sun’s center intersect with the celestial equator and therefore be directly above the Earth’s equator.
The equal night reference from the Latin word origin refers to the fact that on the dates when the equinoxes occur, day and night are of approximately equal length everywhere on Earth. Note I said “approximately.” You must in fact identify the Spring Equilux if you wish to identify one of the two dates during the year when day and night are closest to being equal at your particular location. The Fall Equilux is the other date. The equiluxes occur quite close to the equinoxes; the Spring Equilux occurs a little before the Spring Equinox. Equilux, pronounced EE kwi lucks, means equal light.
The easiest way to pinpoint your Spring Equilux is to use the indispensable Sun and Moon Data calculator. Enter the date on which the Spring Equinox falls in your time zone, enter your location information, and click the Get Data button. Note the sunrise and sunset times, and then calculate the difference between them in minutes (for example, 7:18 p.m. sunset and 7:11 a.m. sunrise = difference of 7 minutes). This will be the time to beat. Now enter the date right before the first date you entered and check the difference in minutes between sunrise and sunset. Keep working your way back, one day at a time, until you identify the date with the smallest difference, that is, the date when sunrise and sunset are closest to being exactly 12 hours apart. Then you will know when to celebrate your Spring Equilux.
In my location in the Mountain time zone, the Spring Equinox falls on March 19. My Spring Equilux will fall on March 16, when sunrise and sunset will be exactly 12 hours apart. I rather like the balance of yin and yang embodied by the equilux. I think I’ll celebrate by consuming equal amounts of white and dark chocolate. For good measure, maybe I’ll burn a candle at both ends too.
Chinese symbol of yin and yang (dark and light)