Understanding analemma, solstices
I’m always impressed by my readers’ attention to details. It frequently shows up around this time of year with some great questions.
For example: “If the first day of the Northern Hemisphere summer (June 21, this year) is supposed to be the longest day of the year, why aren’t its sunrise and sunset times the earliest and latest of the year?”
No need to panic. The discrepancy originates from our use of mechanically simple clocks to measure the sun’s complex apparent motions.
In other words, our timepieces measure not the position and motion of the actual sun, but rather those of the mean sun. This refers to our star’s average or constant behavior throughout the year.
So why, then, do our clocks often differ from reality?
The sun rises in the eastern sky and sets in the west. At midday, it crosses an imaginary line that runs through the sky from due north to due south: the meridian. In highest point on the summer solstice.
But nature is rarely that accommodating. First, the Earth’s axis is tipped by
23.4 degrees to the plane of its orbit. That makes the sun appear at its highest point in the sky near the first day of summer and at its lowest near the first day of winter.
Second, our planet orbits the sun along an ellipse and moves at different speeds throughout the year. This occasionally causes the sun to appear ahead of and behind schedule. In other words, the actual sun seems to run slightly faster or slower than our clocks.
So, recording the sun’s position at the same time for a year shows that not only does the sun’s annual movement not migrate north and south along a straight line; it actually traces out a distorted figure 8 pattern called an “analemma.”
So what does this all mean to us? It means that the longest and shortest days of the year — and the earliest and latest sunsets and sunrises — never occur when you might expect them!