There’s a Certain Slant of Photons

Photo Dennis Vanderspek.
Lisa Stephaniak holds an anemone against a backdrop of the Hubble Deep Field.

A quiet, green astronomer lives in my garden. She’s about 7 feet tall, thin as a pencil, and says almost nothing that I can understand. One message, however, is clear enough: she (or he, for all I know) points one leafy finger precisely south all summer long.

This remarkable scholar is my compass plant (Silphium laciniatum), so named because its vertical leaves orient along a generally north-south axis, so that the leaves’ sides face consistently east and west. It’s an adaptation that helps the plant retain moisture by avoiding direct exposure to scorching southern sun. It’s also a pretty neat piece of applied astrophysics. How many of us can find south on a cloudy day without a cell phone app or a convenient mossy tree?

I’m not certain how the compass plant performs this feat, but on the other hand I’m less interested in the puzzle of its biology than in the profound mystery of its relationship with the sun. Most plants use sunlight in some way, and seem to pull off the trick without much apparent effort, but I think both gardeners and stargazers are diminished in their fields unless they attempt to understand the reality of both the very small and the very large with at least the same dedication the compass plant shows in each of its leaves.

My wife is the gardener in this scenario. I am the stargazer. So, we are both often in the back yard, but at different times.

Now, everybody knows that stars are very large. The sun, our parent star, could comfortably swallow over 1 million earths or 1,000 Jupiters. So is Jupiter very large or very small? Maybe we should ask the compass plant!

Then there is the garden. It is bathed in sunlight … which is to say tiny, massless photons thrown out by the fusion of hydrogen atoms into helium at the heart of the sun. In fact, all the twinkling light of the giant stars visible from your back yard comes from innumerable collisions of atoms inconceivably smaller than a grain of sand. The gift of a broad, sunny afternoon comes wrapped in the very tiniest of packages.

Where am I going with all this? To September! September is a reasonably quiet month in astronomy, but there is one event, an event both tiny and large, that is of interest to both gardeners and stargazers and occurs at exactly 3:21 PM on September 22: the Autumnal Equinox. If you think of the earth-sun system as the face of an analogue clock running backward, with midnight and 6:00 as the winter and summer solstices, then the Autumnal Equinox is located at 3:00. This is the moment when the earth’s tilt begins to point more directly away from the sun, bringing Spring to the southern hemisphere and Fall to the north. In short, winter is coming.

That is the astronomical perspective on the equinox, seen from an imaginary spot in deep space. For gardeners, of course, the results are more immediately visible: the days are getting shorter and there is a changed quality to the light. You may notice it yourself in odd afternoon slants and the shifting position of the sun on the horizon at sunrise and sunset. More on the latter phenomenon in next month’s column.

Have an astronomy question you’d like answered here? I take requests! Write me at

By Dennis Vanderspek

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