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.