Luca Vanzella
Sunset Azimuth Sweep
Update: This image is featured on NASA Astronomy Picture of the Day at apod.nasa.gov/apod/ap200922.html
The two major contributors to the Sun's apparent motion in the sky throughout the year are (a) Earth orbits the Sun while tilted on its axis of rotation and (b) Earth's orbit around the Sun is elliptical (not circular). These factors mean that the position (azimuth) where the Sun sets on the horizon changes slightly from day to day. In the northern hemisphere, the sunset azimuth moves northward from the first day of winter until the first day of summer, then returns southward until the first day of winter. The change in azimuth is most perceptible around the equinoxes (about Mar 21 and Sep 21) and least perceptible around the solstices (about Jun 21 and Dec 21). The farther north or south from Earth's equator, the greater the total change in the sunset azimuth. Here in Edmonton, Alberta at latitude 53.5 N, the change in sunset azimuth in the six months between solstices is quite large, some 84 degrees, from 229.4 (SW) to 313.5 (NW).
A very good place in Edmonton to see sunset along a familiar horizon is Rowland Park Lookout, west of McNally High School, on the bank of the North Saskatchewan River valley. From there, one can see a very wide swath of the horizon, with a skyline from Saskatchewan Drive in the southwest, to the downtown towers directly west, and the apartment buildings on Jasper Avenue in the northeast. To illustrate the sweep of the sunset azimuth, I photographed sunset as near as possible to the 21st of each month from Dec 2019 to Jun 2020, at a fixed location near one of the park benches.
This composite image is the final result of blending seven sunset sequences across three sunset foreground images representing winter, spring and early summer. The image shows several effects on the sunset azimuth due to Earth's axial tilt and elliptical orbit: azimuth change across six months (perihelion to aphelion), azimuth change from month to month, and changes in angle and curvature of the sunset arcs.
All images were taken with a Canon Rebel T3i camera at a focal length of 10 mm, sitting on a Manfrotto tripod, powered by a 12 volt battery buck-converted to the camera's 8.4 volts and activated by a Neewer intervalometer. The solar images, filtered with a Baader solar filter, are spaced 5 minutes apart. The foreground images were taken just after sunset on three of the same dates as the solar images (2019 12 26, 2020 04 18, 2020 05 23).
Here's the essential data from the 7 shoots. Each shoot was planned to start when the Sun was at about 24 degrees altitude and end shortly after sunset.
Date, Sunset time, Az, Chg az, Az Delta 21st Num Delta days
2019 12 26, 16:18, 229.6, +0.2
2020 01 22, 16:56, 236.9, 7.3 +0.4 27 27
2020 02 19, 17:51, 252.1, 15.2 -1.3 55 28
2020 03 20, 19:48, 271.7, 19.6 -0.7 85 30
2020 04 18, 20:41, 290.4, 18.7 -1.8 114 29
2020 05 23, 21:41, 308.2, 17.8 +0.8 149 35
2020 06 20, 22:06, 313.5, 5.3 0.0 177 28
Image Acquisition
Camera: Canon Rebel T3i
Lens: Canon EF-S 10-18mm, at f/11, 10mm FL
Filter: Baader Solar
Mount: Manfrotto tripod with geared head
Power: 12v battery buck-converted camera 8.4v
Timer: Neewer intervalometer
Interval: 10 seconds. Even though I wanted 5 minute spacing for the solar images in the final composite, I shot every 10 seconds as insurance against passing clouds and for flexibility in selecting the lowest altitude solar image above the horizon which varies from flat to sprinkled with buildings of various heights.
Time on site acquiring images: About 17 hours
Image Processing
Processed with GIMP and Paint Shop Pro.
Time processing images: About 34 hours (2 hrs per sunset sequence, plus 20 hours on the final image).
Acknowledgements
Alister Ling built the buck converter for me so I could run the camera for several hours - the camera's battery just wouldn't cut it during the colder months. Although I ultimately didn't use the techniques, Alister helped me explore using Hugin to create panoramas and seamlessly blend skies, and a luminance, flat field divide technique in GIMP. Thank you Alister!
Footnote
For comparison, see my Sunrise Azimuth Sweep image from 2017.
Sunset Azimuth Sweep
Update: This image is featured on NASA Astronomy Picture of the Day at apod.nasa.gov/apod/ap200922.html
The two major contributors to the Sun's apparent motion in the sky throughout the year are (a) Earth orbits the Sun while tilted on its axis of rotation and (b) Earth's orbit around the Sun is elliptical (not circular). These factors mean that the position (azimuth) where the Sun sets on the horizon changes slightly from day to day. In the northern hemisphere, the sunset azimuth moves northward from the first day of winter until the first day of summer, then returns southward until the first day of winter. The change in azimuth is most perceptible around the equinoxes (about Mar 21 and Sep 21) and least perceptible around the solstices (about Jun 21 and Dec 21). The farther north or south from Earth's equator, the greater the total change in the sunset azimuth. Here in Edmonton, Alberta at latitude 53.5 N, the change in sunset azimuth in the six months between solstices is quite large, some 84 degrees, from 229.4 (SW) to 313.5 (NW).
A very good place in Edmonton to see sunset along a familiar horizon is Rowland Park Lookout, west of McNally High School, on the bank of the North Saskatchewan River valley. From there, one can see a very wide swath of the horizon, with a skyline from Saskatchewan Drive in the southwest, to the downtown towers directly west, and the apartment buildings on Jasper Avenue in the northeast. To illustrate the sweep of the sunset azimuth, I photographed sunset as near as possible to the 21st of each month from Dec 2019 to Jun 2020, at a fixed location near one of the park benches.
This composite image is the final result of blending seven sunset sequences across three sunset foreground images representing winter, spring and early summer. The image shows several effects on the sunset azimuth due to Earth's axial tilt and elliptical orbit: azimuth change across six months (perihelion to aphelion), azimuth change from month to month, and changes in angle and curvature of the sunset arcs.
All images were taken with a Canon Rebel T3i camera at a focal length of 10 mm, sitting on a Manfrotto tripod, powered by a 12 volt battery buck-converted to the camera's 8.4 volts and activated by a Neewer intervalometer. The solar images, filtered with a Baader solar filter, are spaced 5 minutes apart. The foreground images were taken just after sunset on three of the same dates as the solar images (2019 12 26, 2020 04 18, 2020 05 23).
Here's the essential data from the 7 shoots. Each shoot was planned to start when the Sun was at about 24 degrees altitude and end shortly after sunset.
Date, Sunset time, Az, Chg az, Az Delta 21st Num Delta days
2019 12 26, 16:18, 229.6, +0.2
2020 01 22, 16:56, 236.9, 7.3 +0.4 27 27
2020 02 19, 17:51, 252.1, 15.2 -1.3 55 28
2020 03 20, 19:48, 271.7, 19.6 -0.7 85 30
2020 04 18, 20:41, 290.4, 18.7 -1.8 114 29
2020 05 23, 21:41, 308.2, 17.8 +0.8 149 35
2020 06 20, 22:06, 313.5, 5.3 0.0 177 28
Image Acquisition
Camera: Canon Rebel T3i
Lens: Canon EF-S 10-18mm, at f/11, 10mm FL
Filter: Baader Solar
Mount: Manfrotto tripod with geared head
Power: 12v battery buck-converted camera 8.4v
Timer: Neewer intervalometer
Interval: 10 seconds. Even though I wanted 5 minute spacing for the solar images in the final composite, I shot every 10 seconds as insurance against passing clouds and for flexibility in selecting the lowest altitude solar image above the horizon which varies from flat to sprinkled with buildings of various heights.
Time on site acquiring images: About 17 hours
Image Processing
Processed with GIMP and Paint Shop Pro.
Time processing images: About 34 hours (2 hrs per sunset sequence, plus 20 hours on the final image).
Acknowledgements
Alister Ling built the buck converter for me so I could run the camera for several hours - the camera's battery just wouldn't cut it during the colder months. Although I ultimately didn't use the techniques, Alister helped me explore using Hugin to create panoramas and seamlessly blend skies, and a luminance, flat field divide technique in GIMP. Thank you Alister!
Footnote
For comparison, see my Sunrise Azimuth Sweep image from 2017.