Ever since I started this blog, I’ve been wanting to make you more aware of variations in sunlight throughout the year. As I wrote last spring, rooftop gardens give you nearly unobstructed access to sunlight, which is the most critical element in creating a successful garden. And since this week the autumnal equinox for the northern hemisphere occurs (September 22/23), I feel justified in belaboring the topic of sunshine.
What is the autumnal equinox? If you don’t remember from junior high science class, it’s one of two days during the year that due to the tilt of the earth vis-a-vis the sun, the hours of sunshine are just about equal to the hours of darkness. The term equinox is derived from the Latin for equal and night.
What the autumnal equinox presages, though, is that for the next three months sun conditions will only grow worse. The sun’s arc through the sky diminishes in both height and length until December 21, when we experience the shortest day of the year. The spring equinox for the northern hemisphere (March 20/21), of course, announces just the reverse–an increase in the sun’s arc until we enjoy the longest day of the year on June 21.
All of this is summarized in the graphic below which was provided to me when I bought a greenhouse window from Busy Beaver hardware in Fox Chapel many years ago.
I admit that this graphic is a bit tricky to figure out. But if you look at the top of the highest arc, you’ll see that you’re looking at the altitude angle of the sun at noon on June 21. The angle of the sun above the horizon at noon is approximately 73 degrees, the highest the sun ever appears in the sky above Pittsburgh. If you then follow the curve to its lowest points at each end, which actually runs off the graph, you’ll see that the sun rises (above the horizon) between 4 and 5 a.m. and sets sometime between 7 and 8 p.m., about 15 hours of sunshine.
If sunset at half past seven seems too early for the longest day of the year, then remember that our clocks are set an hour ahead for daylight savings time in the summer and the adoption of standard time means that sunrises and sunsets will be later and later as you move west through a time zone. That’s why in Pittsburgh you expect sunsets between 8:30 and 9:00 p.m. in the summer, using clock time rather than sundial time.
Perhaps the most difficult part of this graph, however, is to envision the compass bearings. To comprehend sun bearings, the equinox days for the start of fall and spring are the easiest to picture. On those days, if you’re looking due south at sunrise (0 degrees on the graph), the sun rises due east, 90 degrees to the left of due south and it sets due west, 90 degrees to the right of due south. Once we pass the autumnal equinox, sunrises and sunsets start moving progressively closer to due south, so on the shortest day of the year, December 21, the sun rises and sets less than 60 degrees east and west from due south. By June 21, the bearings move equally far to the north. The sun rises and sets more than 30 degrees north of due east and due west (ENE and WNW on your compass). Sunlight not only lasts longer in the summer, but it shines in places hidden by shadows much of the year.
Is your mind spinning yet? Well, there’s an interactive website I just discovered that may give you a feel for how the sun’s bearings change during the year. It’s www.suncalc.net. Once you enter a location and the date, it will superimpose a graph on a Google map or satellite photo, showing the bearing of the sun at that location at the current time. Once you get started, though, you can vary the both the date and the time and fine-tune the location, superimposing the graph on any portion of the Google map. In the case of the Dinette rooftoop, you can zoom in enough to identify the vegetable plant containers on the roof and see the angle of the sun for any time during any day of the year.
But you don’t have to be a vegetable gardener to appreciate this website. You can use it to see on which mornings of the year and at what time sunlight will come streaming into your bedroom windows, presuming they have a southeastern exposure. More sunlight is good for your mental health as well as plant growth. In fact, after playing with the website, you might want to change which room is your bedroom.