A tale of two sunsets

TWICE per year, on April 22 and again on August 22, the sun sets directly behind the Mt Cargill television transmitter as viewed from my garden. I must confess to being obsessed about this biannual phenomenon, which I first witnessed while enjoying a beer on my deck back in 2014. Most years, if the forecast is good, I always book the afternoon off and rush home to set up an array of cameras and telescopes to record the spectacle.

Thanks to Covid19, this year, I didn’t need to hotfoot it from my workplace in Dunedin to Portobello. Like many others, I have been working from home inside my bubble. This gave me plenty of opportunities to really plan to record this year’s passage of the sun in some detail.

I wanted to find a way of showing the motion of the sun during the day and how the sun’s position changes in the sky from one day to the next, if possible, in a single photo.

I photographed the location of the sun every few minutes for 30 minutes before sunset on as many days as possible last week. I managed to get a good run of images on two days: April 20, when the sun passed behind the upper part of the transmitter, and April 25, when the sun missed the tower and set a few degrees north.

This week’s composite image is the fruit of my labour. The individual pictures of the sun are taken 23 minutes apart on each night, creating two separate daily sun trails.

The apparent change in the position of sunset is actually caused by the Earth’s motion around the sun. It turns out that the degree of the sun’s movement along the horizon depends on the time of year and your latitude. The sun’s progress along the horizon changes fastest at the equinoxes and is much slower from one night to the next around the solstices. Also, the sun’s daily change of position along the horizon is more substantial the farther north or south you are from the equator.