Autumnal Equinox

Fall has arrived, this past monday, Sept 23, but do you know why this season exists and occurs when it does, and that the reason for the season is actually scientific, since the Earth rotates on a tilted axis that sees different effects on daylight during it's orbit. 

Since the earth spins on a tilted axis of approx 23.5°, as it orbits the sun the surface faces different angles to the sun at different times of the year. In Canada we are fairly familiar with the longest day of the year, a.k.a. the Summer Solstice. Hard not to notice daylight hours that start as early as 4:30 a.m. ( in my home of Calgary, Alberta) and run through as late as 11:30 p.m. Go further north and it gets even longer, with the extreme being the sun not even dipping below the horizon. 

Conversely, in our northern hemisphere, we all know and dislike the opposite, that being the shortest day of the year, a.k.a. the Winter Solstice occuring on December 21st. In Calgary, that means the sun comes up at around 8:30 a.m. and goes down around 4:30 p.m.. Of course, in both cases, there is a slow buildup to these solstices, so you get a lot of days with either a lot of sunshine, or very little, and no-one particularly likes the latter. haha.

e·qui·nox

/ˈekwəˌnäks,ˈēkwəˌnäks/

Learn to pronounce

noun

1. the time or date (twice each year) at which the sun crosses the celestial equator, when day and night are of equal length 

So how about some balance? Enter the Equinox, when the sun sits on the plane of the earth's equator. As shown in the diagrams, the tilt of the earths' axis sees dramatic swings in daylight hours for the Summer and Winter Solstices but there are two points of time each year where the sun is directly atop the equator, meaning a balance between the hours of day and night, and a balance between the temperatures in the northern and southern hemispheres, relatively speaking. In the southern hemisphere at this time of year, they would be experiencing the Spring Equinox and we would experience our version in March. 

The Equinox occurs when the Earth's orbit takes it to the point where the sun crosses the Celestial Equator, an imaginary line that runs above the Earth's equator. At this juncture, there are an equal number of hours in the day and night (12 hours), so the Latin word Equinox literally means equal (equi) night (nox).

From here on in, we begin to see shorter and shorter daylight hours until we reach the Winter Solstice, which is the least amount of sunlight in our northern hemisphere. Maybe this is another reason for the ominous sounding phrase "Winter is coming!".