Explanation: Does the Sun set in the same direction every day? No, the direction of sunset depends on the time of the year. Although the Sun always sets approximately toward the west, on an equinox like today the Sun sets directly toward the west. After today's September equinox, the Sun will set increasingly toward the southwest, reaching its maximum displacement at the December solstice. Before today's September equinox, the Sun had set toward the northwest, reaching its maximum displacement at the June solstice. The featured time-lapse image shows seven bands of the Sun setting one day each month from 2019 December through 2020 June. These image sequences were taken from Alberta, Canada -- well north of the Earth's equator -- and feature the city of Edmonton in the foreground. The middle band shows the Sun setting during the last equinox -- in March. From this location, the Sun will set along this same equinox band again today.
This is a very useful demonstration of how the Sun moves across the northern hemisphere sky at different times of the year. This photo would be helpful in any elementary school class, when helping students develop a fundamental understanding of astronomy.
Re: APOD: Equinox in the Sky (2020 Sep 22)
Posted: Tue Sep 22, 2020 12:36 pm
by orin stepanek
Interesting! Though most know all this, it is interesting to see it spread out over time! Nicely done!
Re: APOD: Equinox in the Sky (2020 Sep 22)
Posted: Tue Sep 22, 2020 12:57 pm
by BobStein-VisiBone
Trying to infer how far apart the images were taken.
The sun images along the equinox path are spaced a little further than one diameter apart. By crude measurement of 49/21 pixels, or 2 and 1/3 diameters apart. The sun averages 32 minutes of arc in the sky so that's 74.7 minutes of arc between images, or 1/289 of a circuit or 4.98 minutes.
So I conclude the images were taken 5 minutes apart, or 1/288 of a day, well within the error of my crude measurement.
Re: APOD: Equinox in the Sky (2020 Sep 22)
Posted: Tue Sep 22, 2020 1:22 pm
by BobStein-VisiBone
PolishBear wrote: ↑Tue Sep 22, 2020 11:42 amThis photo would be helpful in any elementary school class, when helping students develop a fundamental understanding of astronomy.
Imagine a class making an image like this from scratch, mapping the sun from solstice to solstice. Since the sun moves 0.4 degrees a day at the equinoxes, which is less than the diameter of the disk, the photos should be taken every other day or perhaps rarer. This would space them apart in the N-S direction the way this photo spaces them in the E-W direction. If the timing were precise enough it might create patterns that evoke the analemma. A quantized solargraph as it were.
It would be fun to invent a gizmo anyone could fix to the roof that would show the image build over the year.
PolishBear wrote: ↑Tue Sep 22, 2020 11:42 amThis photo would be helpful in any elementary school class, when helping students develop a fundamental understanding of astronomy.
Imagine a class making an image like this from scratch, mapping the sun from solstice to solstice. Since the sun moves 0.4 degrees a day at the equinoxes, which is less than the diameter of the disk, the photos should be taken every other day or perhaps rarer. This would space them apart in the N-S direction the way this photo spaces them in the E-W direction. If the timing were precise enough it might create patterns that evoke the analemma. A quantized solargraph as it were.
It would be fun to invent a gizmo anyone could fix to the roof that would show the image build over the year.
I've done this in school with nothing more than a marker on a window whose shadow on the floor is marked at the same time every day. Do it correctly and you get an analemma. You can also demonstrate other patterns.
PolishBear wrote: ↑Tue Sep 22, 2020 11:42 am
This photo would be helpful in any elementary school class, when helping students develop a fundamental understanding of astronomy.
Imagine a class making an image like this from scratch, mapping the sun from solstice to solstice. Since the sun moves 0.4 degrees a day at the equinoxes, which is less than the diameter of the disk, the photos should be taken every other day or perhaps rarer.
"The middle band shows the Sun setting during the last equinox -- in March.
From this location, the Sun will set along this same equinox band again today."
PolishBear wrote: ↑Tue Sep 22, 2020 11:42 am
This photo would be helpful in any elementary school class,
It would be fun to invent a gizmo anyone could fix to the roof that would show the image build over the year.
I've done this in school with nothing more than a marker on a window whose shadow on the floor is marked at the same time every day. Do it correctly and you get an analemma. You can also demonstrate other patterns.
Re: APOD: Equinox in the Sky (2020 Sep 22)
Posted: Tue Sep 22, 2020 4:29 pm
by Grizzly
In 4th grade, I think, our teach had butcher paper on one of the back walls and at 9:30 precisely every morning would ask a child to put a dot where the sunlight through the window cast a shadow of the window corner on the wall. After we fell back it was at 8:30. We asked why but she said "We'll see.", or "What do you think". At some point the next spring we had a unit about seasons. The line on the wall connecting the dots was a really good teaching aid.
PolishBear wrote: ↑Tue Sep 22, 2020 11:42 am
This photo would be helpful in any elementary school class, when helping students develop a fundamental understanding of astronomy.
Imagine a class making an image like this from scratch, mapping the sun from solstice to solstice. Since the sun moves 0.4 degrees a day at the equinoxes, which is less than the diameter of the disk, the photos should be taken every other day or perhaps rarer.
since the sun moves 0.4 degrees a day at the equinoxes
the APOD is not quite exact when it states:
"The middle band shows the Sun setting during the last equinox -- in March.
From this location, the Sun will set along this same equinox band again today."
Is your point that the band from one equinox would overlap the band from the other equinox? But probably not precisely? That would be tacky. A solargraph of the sun with the aperture opened •briefly •every 5 minutes, •every other day, •from the December solstice to the June solstice, should show non-overlapping orbs throughout the middle, across a single equinox.
Or perhaps (scribble scribble scribble) every 5 minutes and 0.26 seconds. This would stagger the orbs from one bidiurnal band to the next, so they would "stack".
P.S. By "from scratch" I meant a class might have more fun capturing new photographs, as opposed to displaying a poster with today's APOD image. No new celestial bodies need be formed.
Re: APOD: Equinox in the Sky (2020 Sep 22)
Posted: Tue Sep 22, 2020 8:54 pm
by Cousin Ricky
Looking at the equinox sunset angle, it dawned on me that Edmonton is some serious north! I looked up some stats, and it turns out that Edmonton is farther north than London, UK, and is as far north as Punta Arenas, Chile is south. I once visited Punta Arenas a month prior to the summer solstice, and observing the path of the Sun through the course of the day was a wild experience.
I did spend a summer in Heidelberg, West Germany (yes, that long ago—15 years before the wall fell), about 4° more southerly than Edmonton; but although it felt strange to a child of the tropics like myself to see so much daylight so late in the evening, I did not pay attention to where the Sun was in the sky.
neufer wrote: ↑Tue Sep 22, 2020 2:01 pm since the sun moves 0.4 degrees a day at the equinoxes the APOD is not quite exact when it states: "The middle band shows the Sun setting during the last equinox -- in March. From this location, the Sun will set along this same equinox band again today."
Is your point that the band from one equinox would overlap the band from the other equinox? But probably not precisely? That would be tacky.
Equinox in the Sky