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Astronomy

Dennis Mammana: Analemma, and the Long and the Short of the Solstices

Analemma
View Analemma and the Solstices after dark this week. (Dennis Mammana illustration / creators.com)

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 (this year the solstice occurs at 3:07 a.m. PDT June 21) 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?”

And here’s another: “Why doesn’t the sun appear due south at noon every day like my school books said it should?”

No need to panic; the Earth hasn’t spun out of its orbit. The discrepancy originates from our use of mechanically simple clocks to measure the sun’s rather 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.

Now, this doesn’t refer to our star’s nasty disposition but rather to its average or constant behavior throughout the year.

So why, then, do our clocks often differ from reality? To explain, let’s begin at the beginning.

We all know that 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 the morning hours, the sun appears ante (before) meridian, or a.m. In the afternoon, the sun is post (after) meridian, or p.m.

So, if you were to go outdoors every day for a year and plot the sun’s position at exactly 12 noon (ignoring daylight-saving time, to keep things simple), you might expect to see it migrate up and down along the meridian, reaching its highest point, of course, 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 not in a circular path but along an ellipse, and it 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 appears 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.”

Equally interesting is that every object in our solar system displays a different analemma, some very simple and some quite complex. Click here to learn more about this phenomenon from some great text, illustrations and animations.

So what does this all mean to us? Well, it answers the questions of those who pay close attention to such matters; you will find that the longest and shortest days of the year — and the earliest and latest sunsets and sunrises — never occur when you might expect them!

Dennis Mammana is an astronomy writer, author, lecturer and photographer working from under the clear dark skies of the Anza-Borrego Desert in the San Diego County backcountry. Contact him at [email protected] and follow him on Twitter: @dennismammana. Click here to read previous columns. The opinions expressed are his own.

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