What’s A Solstice?
For many of us, December 21, 2016, marks this year’s Winter Solstice. But for plenty of others, it’s also the Summer Solstice. How can this be? What is actually unique about today from an astronomical sense? What is a solstice anyhow?
First, let’s be considerate of all people regardless of whether they live in the northern or southern hemisphere. Let’s refer to today’s event as the December solstice. We do this because while those of us that live in the northern hemisphere consider it to be Winter, our friends south of the equator are in the middle of their Summer. This, of course, is because the Earth is tilted on its axis–which means that for part of the year, the northern part of our planet is tilted towards the Sun (and thus receiving more exposure to solar radiation i.e. Summer), while the southern half is tilted away (less solar radiation i.e. Winter). The other half of the year, this is reversed. (If you live directly on the equator, you can call it whatever you want because you get to have Summer 365 days a year.)
A solstice occurs on the day when the planet’s hemisphere has its maximum (or minimum) amount of solar exposure (in December, it’s the northern; in June it’s the southern’s turn), which in turn gives us the longest and shortest days of the year. Contrast that to the equinoctes (yes, that really is the plural form of equinox), in which the length of day is equal to the length of night.
Now, you might have heard the solstices being referred to as the “First Day of Spring/Winter”, which if you live in a geographic area that has noticeable seasons you know this isn’t true. It would be more appropriate to consider a solstice as the middle of a season. Starting today, the daylight hours get longer in the northern hemisphere and shorter in the southern.
So where did the “First Day of Spring/Winter” idea come from?
I really don’t know.
If you find out, please let me know.
Image Credit: NASA
Today is the March Equinox. You’ve probably already heard it a few times today; people running around proclaiming with utmost exuberance how today is the first day of Spring. After the long winters that some of us endure, the arrival of Spring is welcome news. But what is really going on today? After all, where I live it still feels like the middle of Winter, but flowers were already blooming on a trip I took to California a couple of weeks ago. If we based “The First Day of Spring” on climate patterns, regions across the globe would be recognizing a wide variety of days throughout the year.
When someone says today is the first day of Spring, what they really mean (whether they know it or not) is that today represents an equinox; specifically, the March Equinox. On Earth, an equinox is the point in its orbit around the Sun when both hemispheres are equally illuminated; our tilted Earth lines up to a point in which the Sun passes directly over the equator. This happens twice a year, on the March and September equinoctes (which I learned today is the proper plural form of the word equinox).
Contrary to popular belief, the day of the equinox does not represent the day where daylight and darkness are equal. You can thank geometry, the atmosphere, and the Sun’s angular diameter to cause that equality to happen at different times geographically. What today does mean though, is that the equinoctes are the only two days in which the Sun rises due-East and sets due-West, and which the Sun would pass directly overhead from an observer on the equator.
One other very important thing that you must know if you don’t learn anything else today. Way too many people believe that the equinoctes are the only day of the year that an egg can be balanced on its end. While its true that on the equinox an egg can be balanced, it’s also true of every other day of the year; it makes no difference!
There are other times during the year (read: our orbit around the Sun) that we recognize Earth residing at a special place. There’s Perihelion (which we went over in January) and Aphelion, and then the widely-celebrated solstices; but I’ll save that for another time.
Happy March Equinox!
This article originally posted on March 20, 2012.