The "Standing Sun": A Martian Perspective
Artist's conception of the western sky on Mars just after sundown, with meteors and stars. Some of the latter are presumably near the solar trajectory, along the ecliptic.
As I've written, ancient astronomers were perceptive. A casual observer would not have noticed the slight decrease in the sun's movement along the ecliptic when Earth is at aphelion. Earth's orbit is nearly circular, which means there's little difference between perihelion and aphelion.
The mean Earth-sun distance is 93 million miles or 1 AU. When at perihelion Earth is 91.4 million miles from the sun. At aphelion the distance is 94.5 million miles. The latter distance is only 3.4% greater than the former. That means the difference in Earth's orbital speed (hence the sun's motion against the stellar background) is correspondingly minimal. It is discernible but hardly conspicuous.
Mars is a different matter. Whereas Earth's orbital eccentricity (degree of deviation from a circular orbit) is only 0.0167, that of Mars is 0.0935. Mars at perihelion is 128.4 million miles from
sol, but at aphelion the distance increases to 154.9 million miles. Whereas the difference between closest and farthest distance from the sun amounts to only 3.1 million miles or 3.4% here, Mars is 26.5 million miles, or over 20% farther from the sun at aphelion than at perihelion. Naturally this causes a more perceptible difference in the sun's speed along the ecliptic.
Since Mars is farther from sol than Earth, the sun always moves more slowly there. But the annual changes are relatively greater, hence more noticeable. Sol, as seen from Mars, traverses essentially the same zodiac constellations as it does here. Aphelion occurs before the summer solstice in the northern hemisphere. At the time of the solstice, the sun is on the border between Aquarius and Pisces. When Mars is at aphelion sol is presumably a little farther back, in Aquarius. Stars just east of the sun then, hence in the west after sundown, include those of Pisces and Pegasus, but none of first magnitude. Given the minimal atmosphere, however, even dim stars are visible--and for quite some time. Under aphelic conditions, stars 10-20 degrees east of the sun persist for many evenings. Their height in the western sky, just after sunset, remains little changed for weeks. The sun takes twice as long to reach them as it does here. Martian conditions certainly amplify the "standing sun" phenomenon noted by the ancients.
posted by starman at 5:32 AM
13 Comments:
That is an interesting essay on Mars. I would like to make astronomical observations from its surface. The moons of Mars, Phobos and Deimos, are very small. Phobos has a diameter of only 22.2 km. (13.8 miles). The diameter of Deimos is 12.6 km. (7.8 miles).
Indeed Mars would make a great observatory for many kinds of research. The atmosphere is so thin the stars don't scintillate (twinkle) and, with the exception of occasional dust storms and water-ice clouds, viewing is unhindered. In addition the small size of Phobos and Deimos means far less light to drown out faint celestial objects (The moon would also make a great observatory but the extreme brightness of Earth might hinder certain observations).
Btw it's been exactly 49 years since the close (perihelic) opposition of 1971. Do you remember Mars that summer?
I mentioned that when Mars is at aphelion (near the summer solstice) the sun is in the constellation Aquarius. That's a barren constellation. But just like our planet, Mars experiences precession. If precession moves the point of the solstice the same direction as here (westward) at some future date the point of the summer solstice (northern hemisphere) will appear near the star Delta Capricorni.
Lastly since Mars like Earth is at aphelion at approximately the start of northern summer, the slowness of the sun (actually planet) means that summer in that hemisphere lasts longer than winter. But aphelion also means northern summers are considerably colder than the shorter, southern ones.
August 11, 2020
Correction: It's been exactly 49 years since the closest approach of Mars to Earth in 1971. Opposition occurred the previous day, on the 10th. :)
August 11, 2020
Yes, I remember when that closest approach occurred in 1971. An interesting geological aspect of Mars is Arsia Mons, a volcano on the Tharsis bulge, which is close to the equator. Pavanis Mons and Ascraeus Mons are volcanoes to the north of Arsia Mons. There are seven holes on the flanks of Arsia Mons. They may be cave entrances. Arsia Mons was named after a legendary Roman forest. According to one story, the Romans heard the voice of the deity Silvanus prophesy in those woods that Rome would defeat the Etruscans.
That's very interesting, thank you. You remember the opposition of 1971, do you remember the first photos from Mariner 9 revealing the Martian volcanoes, including the big Nix Olympica? At first they were obscured by a planet-wide dust storm.
August 13, 2020
Yes, I remember those photos. Mars has some very ferocious dust storms. They can reach speeds of up to 160 km. (99.4 miles) per hour. A subglacial lake on Mars was identified in 2018. It is located about 1.5 km. (0.93 mi.) under the base of the southern polar cap. This lake has a width of about 20 km. (12.4 miles). Most of the terrain surrounding it is higher ground, but there is a depression on the eastern side.
Until now, I've overlooked one of the clearest indications of a "standing sun" in summer-- that season is the longest of the four. In the northern hemisphere summer lasts over 93 days whereas winter lasts only 89. February is the shortest month, whereas July and August have 31 days each. Since Earth slows near aphelion, it takes sol longer to pass from solstice to equinox in summer than in winter. The difference isn't much--a few days--but is noticeable, as is the slowing ("standing") of sol among the summer stars of the zodiac.
August 20, 2020
That is interesting information about the seasons of Mars. The Hellas impact basin on Mars is quite large. It has a depth of 7,152 m. (23,464.6 feet). That basin goes 2300 m. (7545.93 ft.) from east to west. The Hellas Basin is in the southern Hemisphere of Mars.
Actually I was referring above to Earth's seasons. Martian seasons are about twice as long. Hellas is said to be the deepest basin on Mars. No wonder carbon dioxide frost extends as far north as Hellas in wintertime. Cold air sinks and it can sink in the deep basin.
August 21, 2020
I'm sorry; I got in a hurry and accidentally typed it as Mars. You're right; Hellas is the deepest basin on Mars.
Mars has a substantial amount of seismic activity. From what I've read, more than 450 Marsquakes were detected during 2019.
I'm not sure but much seismic activity may stem from the enormous impact that created the crustal dichotomy. It may have caused vast internal as well as external disruption, the effects of which are still being felt, as Mars quakes.
By the way, getting back to the differences caused by aphelion and perihelion: One January day long ago, my father told a sibling: "The sun is strong this time of year." The old man had little education but he could discern the increased brightness of the sun when Earth is at near perihelion. It wouldn't be surprising if the ancients could also discern that, and other subtle differences such as changes in the sun's motion along the ecliptic.
August 22, 2020
I think it's very likely that the ancient people noticed the differences caused by aphelion and perihelion. They must have been perceptive in terms of observing a lot of things about the environment and what they saw.
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