Happy Vernal Equinox Day
By my calendar, today is the first day of spring, even if this designation does seem a trifle arbitrary. While there is a gradual progression of the seasons, today doesn't seem that much different than yesterday. Or likely tomorrow. So what does all this "equinox" stuff mean?
Around here, things started looking increasing like spring some weeks ago. I usually date things based on what the cherry tree in my front yard tells me. If you've been keeping an eye on the weather and the happenings outdoors in your neighborhood, you may agree or disagree with my assessment. While we've had an unusually mild winter, other parts of the country are still under regular threat of winter storm warnings. Officially though, we all measure the start of spring on the same date.
The southern hemisphere sees things differently of course. Regardless of precisely when you want to date it from, it's the start of spring in the northern hemisphere while autumn if beginning down under. Everything is backwards in the southern hemisphere. Or vice versa, depending on which hemisphere you're reading this in.
Given this, it's clearly at least a tad confusing to refer to today as the vernal equinox. After all, the word vernal refers to spring and not autumn. How can it be the vernal equinox in the southern hemisphere when fall is just beginning? As such, today is more technically referred to simply as the March equinox rather than the vernal or autumnal equinox. That way, the reference is correct no matter which side of the equator you hail from.
So if "vernal" isn't really the right word for today's festivities, what about "equinox," is that still accurate? First off, while the "equi" part of the word does mean "equal," the date of the equinox is not equidistant between the winter and summer solstices. It's approximately in the middle, but only approximately. The word itself comes from the Latin and "nox" refers to the night. Thus, "equinox" is the date on which the day and night are of equal duration. But this too is somewhat misleading.
Technically, the date of the equinox is the date on which the plane of the earth's equator is exactly in line with the plane of its orbit around the sun. To put it another way, the line from north pole to south pole of the earth's rotation around its axis is exactly perpendicular to the plane of its orbit around the sun. But this is not the time at which we perceive the day and night to be of equal length.
The sun is a big thing, and you can use more than one definition as to what time sunrise and sunset occur. There's astronomical twilight, civil twilight, and so on. In order to agree upon the length of the day, we need to settle on an agreeable definition of sunrise and sunset. If you ask the common man, they would say that sunrise occurs when the limb of the sun's disc is first visible above the horizon, and sunset happens when the sun drops below the horizon in the west and is no longer visible. But these times occur when the center of the sun is fully 12 degrees below the horizon. The edge of the sun's disc may be at the horizon, but this means that it's center must be below the horizon to account for the sun's diameter and the refraction of light through the atmosphere. This is the point where we see it as crossing the horizon. If we waited until its center was right even with the horizon, it would already appear to be halfway up.
As such, on the date when, scientifically speaking, the plane of the earth's equator lines up with the plane of its rotation around the sun, and thus the date on which the equinox occurs, the average person will say that the day is slightly longer than the night, since they will judge that sunrise happened earlier than this mystical alignment of equatorial and orbital planes, and that sunset will happen slightly after this point.
The exact time of the equinox does vary slightly from year to year. The length of one rotation of the earth around the sun isn't evenly divisible by the time it takes to rotate around its axis. There's no reason to believe that it would be. The year isn't exactly 365 days long, and dividing the actual length by that of a day leaves us with a small remainder. Every four years we add a day to the calendar to make up for this and call it a leap year. The powers that be sneak in "leap seconds" every now and then too in order by keep things lined up as well as they can. Because of all this, it turns out that 2016 has the earliest March equinox since 1896. Call it the luck of the draw. The March equinox in 2020, and indeed every leap year for some while to come will be earlier still, until after the turn of the century when the equinox will start to drift later again. No, this doesn't have anything to do with global warming or that sort of thing. It's purely just an artifact of how we construct our calendar and measure time.
Solstice dates are easy, but equinox dates are more tricky. The winter solstice (December in the northern hemisphere) happens on the date with the longest night, and the summer solstice on the date with the longest day of the year. The summer and winter adjectives do have to be reversed for solstices in the southern hemisphere, but the exact dates for each aren't nearly as confusing as the dates of the two equinoxes that fall in between (not that they fall exactly halfway in between of course). The longest day or the longest night don't depend on how you judge the point of sunrise and sunset. Whatever definition you want to go with, you'll still get the right answer. All that matters are when the longest day and the shortest day happen.
Does any of this truly matter? Perhaps not too much, unless you're trying to impress your friends at a cocktail party without being in full possession of the facts. But as a photographer there is one significant way in which this is important. No, not the exact dates, but the very idea of days varying in length as the year progresses matters a lot. If the earth didn't wobble on its axis, there would be no seasons. And it's the changing of the seasons that adds spice to the craft of nature photography. To me, that makes all this solstice and equinox stuff worthwhile.
Regardless of which hemisphere you may be in, and regardless of exactly what name you call it, happy equinox everyone.