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Winter 2011

Urban Jungle

The changing natural world at our doorsteps | Illustration and text by Patterson Clark      

January 18, 2011

Rounding the analemma on a fast track to longer days

The sun’s path across the sky reveals the Earth’s tilt and orbital eccentricity.

Commuters driving east in the mornings may have begun to notice that the sun has climbed slightly higher and is tracking to the north again.

If you take a photo of the sun from the same spot at the same time every week for a year and merge the photos, the sun will trace a figure 8 in the sky, a path known as an analemma.

That path is the result of two factors: Earth is tilted on its axis, and it orbits the sun on an elliptical path.

If our planet were in a perfectly circular orbit and had no tilt to its axis, we would always find the sun in the same spot at 8 a.m.. Yawn. (Figure 1)

Add the elliptical orbit -- where the Earth's orbital speed changes with its distance from the sun's

gravitational pull -- and the sun lags behind the clock for part of the year, then races ahead of the clock for another. The analemma for that would be a sun tracking back and forth on a straight line. (Figure 2)

It's our tilt that gives the analemma its figure. If Earth had a circular orbit but was tilted at its current angle, the sun's analemma would be a figure 8 with equal lobes. (Figure 3)