Friday, July 24, 2020

"Standing Sun"


















This is one of the "Planets and Stars" articles by the late Prof. Charles H. Smiley. From about 1970 to his death in 1977, Smiley's  articles appeared monthly in The Hartford Courant. I collected several and still have them. I'm revisiting this one as it seems relevant at this time of year (actually a bit earlier).
Smiley's newspaper column was for laymen and therefore he could not use technical jargon. Note the article doesn't mention the reason for the apparent slow motion of the sun against the stellar background. At this time of year, or more precisely around July 4, Earth is at aphelion, the point of its orbit farthest from the sun. Like all planets, Earth moves most slowly when at aphelion. Since the sun's movement against the stellar background is a direct reflection of Earth's orbital speed, in July the sun also moves relatively slowly. It covers less distance along the ecliptic, amidst zodiac constellations such as Gemini, than at other times of the year.
 Of course, the stellar background can't be observed when the sun is above the horizon. But soon after it set, the ancients inferred the sun had slowed, or even stopped, since it took longer for certain stars to be overtaken and become invisible (until reemerging in the morning sky, as the sun, still moving eastward, left them behind). Around 2,000 years ago, when the point of the sun's highest ascent in the sky was in Cancer (precession has since moved the point to Gemini) ancient astronomers presumably  observed that in June Regulus (or Denebola) was not "sinking into the rays of the setting sun" (as one recent writer put it). Leo remained visible (at least in part) after nightfall for a surprisingly long period in June. Because the constellation persisted for many evenings that month, the sun didn't seem to be getting closer and overtaking it. That was near the time of aphelion (July 4). Since the sun was at its slowest, the impression of stopping, while illusory, is understandable. Naturally the ancients came to equate the highest ascent of the sun in the sky with "standing" or apparent near-cessation of movement along the ecliptic.
I assume the ancients noted the opposite effect in January. Earth is at perihelion on January 4, just two weeks after the winter solstice. But whereas the term solstice (Latin for "sun standing") seems appropriate in July, at the start of the year the sun moves relatively rapidly along the ecliptic. The result, of course, is rapid overtaking of planets and stars just to its east at the time. I saw this once when Jupiter was in the evening sky late in December. Over the course of several days the planet's angular distance above the horizon (at nightfall) quickly shrank until it set too soon after sundown to be visible. That's quite a contrast with evening sky celestial objects early in July.

8 Comments:

Anonymous Neal Eugene Robbins said...


It is easy to see why the ancients came up with the idea of a standing sun. A lot of progress was made in cataloguing the various stars and constellations back then. A few ancient astronomers correctly deduced that Earth revolves around the sun. One is example is the Greek astronomer Aristarchus of Samos.

8:59 AM  
Blogger starman said...

There are always some brilliant people far ahead of their time. Eratosthenes, who measured the Earth c 200 BCE, was another. Basically, though, the ancients didn't have a clue what caused the sun to slow down when passing through Cancer. A heliocentric view is needed to explain it but the geocentric view prevailed e.g. ptolemy.


July 24, 2020

9:48 AM  
Anonymous Neal Eugene Robbins said...



Having a geocentric view was a major flaw in Ptolemy's thinking. It is unfortunate that so many people went along with it. Copernicus and Kepler had the heliocentric perspective but they came along much later. Fortunately, the heliocentric concept eventually prevailed.

2:46 PM  
Blogger starman said...

No doubt an observer on Mars (which has a more eccentric orbit than Earth) would see a more pronounced "standing sun" effect. Compared to Earth, with its nearly circular orbit, Mars is considerably farther from the sun at aphelion than at perihelion. The difference between the speed of the sun at those times is relatively much greater than observed here. In fact, since a near-circular orbit implies little difference, it attests to the observing skills of the ancients that they perceived a difference.



July 25, 2020

1:57 AM  
Anonymous Neal Eugene Robbins said...

I agree, it is proof of their skill. If humans had a research station on Mars, they would be able to gather some amazing data, not only about the environment of Mars, but also on the "standing effect" of the sun.

10:46 AM  
Blogger starman said...

I just revised the post--what else is new? :) Good to see your latest comment. I'll reply tomorrow.


July 25, 2020

11:20 AM  
Blogger starman said...

The "standing effect" of the sun is well-understood and needs no research. As I wrote it's clearly due to the diminished speed of the sun against the stellar background when Earth (or another planet) is at aphelion.
Already there are orbiters and landers on Mars which have greatly enhanced our understanding of the Martian environment. I don't know if humans can survive there for long periods, as low gravity may mean brittle bones.


July 26, 2020

2:22 AM  
Anonymous Neal Eugene Robbins said...

A key question as to do with whether artificial gravity could be created in the dwellings to increase the pull. That would make things normal indoors. However, when moving around outdoors, the low gravity could have detrimental effects. Colonizing a planet with gravity very similar to that of Earth would be less troublesome.

7:33 AM  

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