Once around the sun in a figure of eight

The spring equinox occurred Friday at 1.03pm. The moment the sun crosses the celestial equator heading south is always a cause for celebration in the astronomy-loving Griffin household. However, this year, it was extra special for me. That’s because the equinox marked the end of a project one year in the making.

In 2021 I mounted a pinhole camera on the west-facing deck of my Portobello home. Inside the camera was a single 4x5 inch glass plate coated with light-sensitive silver halide emulsion. Beginning on September 23, 2021, at 4pm precisely (5pm when summertime began), if the sun was visible in the sky, I opened the shutter for 20 seconds. At the winter solstice and autumn equinox, I opened the shutter for longer to record the apparent motion of the sun across the sky.

My project ended this week. I dismounted the camera and took it to a makeshift darkroom in the laundry. Under red light, I removed the plate, popping it into a chemical bath to develop the latent image. After a few minutes, I shouted with delight. My year-long project to capture the solar analemma was a success!

The elongated figure of eight curve you can see in this week’s picture is called a solar analemma. The shape of the analemma you see depends on your location, the tilt of our planet’s axis and Earth’s varying speed around the sun.

Thanks to Earth’s 23.5 degree axial tilt in relation to its orbit, the sun’s altitude and rate of motion in the sky change during the year. Also, because our home planet’s orbit is elliptical, its velocity changes as we go around the sun. We move fastest in January when closest and slowest in July when furthest away. Earth’s ever-changing speed is also a factor that affects the sun’s observed position.

This week’s picture illustrates the impact the combination of all these factors has on the observed position of the sun as seen from my garden. The analemma can be complicated to explain but it is a thing of great beauty.

 - Ian Griffin