Regulus and the "Standing Sun"
Unlike Spica, Antares and other bright stars of the zodiac, Alpha Leonis (or Regulus) is quite close to the ecliptic. Around 2,000 years ago, this may have enhanced the importance of Regulus. Since the sun, near the summer solstice, was headed right toward it, the star may have been a key indicator of solar speed at the time. How quickly, or slowly, did Regulus fade as sol approached? That would be a reflection of solar motion (actually Earth's). But was Regulus still visible at all then, i.e. near the longest day of the year 20 or so centuries ago ?
Back then the point of the sun's greatest ascent was not in western Gemini, where it is currently, but farther east in Cancer i.e. one constellation closer to Regulus. Therefore Regulus was overtaken by the sun in July instead of in August as it is now. Since the star disappeared a month earlier, in order to be observed near enough to the time of Earth's aphelion (July 4) for ancients to discern the effect (slowing or "standing" sun) it would still have to be visible in June.
To test whether Regulus could still be seen in June a couple of millennia ago, one would have to be able to see it in July recently. In both cases the star would be about a month away from its rendezvous with the sun. Lately I doubted Regulus was still observable in July. But I had forgotten my observations of half a century ago. An old diary dispels any doubt.
From my latitude, about 42 north, Regulus is still clearly visible, even conspicuous, on July 6. I had no trouble seeing it on that date in '73 (see above). Over two weeks later, on July 24, 1973, there was a conjunction of Venus and Regulus. The planet finally caught up to the star. The evening before then (July 23) I saw Venus but not Regulus. But the latter was still at least as high. I believe it can still be seen on that date by those with better eyesight (especially in the clearer skies and lower latitudes of the Near East). I saw it (near Venus again) eight years later, on July 23, 1981, albeit barely, and with optical assistance (60mm refractor). Those with good vision can probably see it at least to July 21.
When the solstice was in Cancer, the equivalent date of last viewing would've been June 21 or so. To the ancients, Regulus would've remained visible to the time of the summer solstice. That was just two weeks from Earth's aphelion passage, when Earth moved most slowly in its orbit and hence sol moved most slowly along the ecliptic. In fact sol was already moving relatively slowly at least a month prior to that. The result was that Regulus appeared to "linger" for a surprisingly long time in June (just as it does, to a theoretically even greater degree, now inasmuch as its last appearances are even closer to the time of aphelion). Unable to overtake Regulus for an inordinate period, it seemed that the sun had slowed or even halted.
posted by starman at 3:00 AM
18 Comments:
Regulus is a quadruple star system. It is comprised of two pairs of stars. Regulus A is a blue-white star; it is a companion to a star that may be white dwarf. Regulus BC is located about 5,000 AU (astronomical units) from Regulus A.
Thanks for the info on Regulus. I just changed the title to include the star's name, as the post is mainly about it (or at least the visible one).
I had half forgotten about what I wrote 46-47 years ago though, being vaguely aware, I searched my old records. Note the handwriting leaves something to be desired, though it's legible enough. :)
I wish I had had a good pair of binoculars in 1973. It would've been great to see Regulus on July 23, seventeen days after the last day I saw it that month. I wouldn't have had any difficulty knowing where to look, because Venus, then near a conjunction with Regulus, would've clearly indicated where to look.
Do you keep any diary or journal?
July 27, 2020
You're welcome; I'm taking notes on astronomy and putting them in a special file. I keep a journal; it is useful for retrieving information on past activities.
My old writings were almost exclusively about astronomy and space. More recent journal entries are far more eclectic. I assume the same is true of your journal.
July 27, 2020
Mine include plenty of things about astronomy and spaces. I also have some writings about my time in Turkey, South Korea, and Saudi Arabia. That includes telling about my visits to archaeological sites and museums in those countries.
Very interesting, Neal. Perhaps your journal can be the basis of an autobiography. :)
Back on topic: I wonder if the "standing sun" conclusion is much older than 2,000 years. Might it be 4,000 years old? That long ago, Leo was itself the site of the summer solstice. Is it possible the "standing sun" notion was based on ancient Egyptian (or Chinese?) observations of some star farther east, perhaps Spica or another star in Virgo at the start of summer (when the sun was in Leo)?
July 27, 2020
I think that it is possible that the "standing" sun idea might be older than 2,000 years. It could have been based on observations made by the Egyptian, Chinese, or another people.
Or all of them. It's noteworthy that different cultures represented the constellation of the solstice with different slow moving creatures--crab (Cancer), tortoise and beetle. Maybe they noted slow solar movement more or less concurrently. Of all the astronomers of antiquity, though, the Greeks were probably the most sophisticated. And I get the impression the names, although different, refer to the same constellation--Cancer--presumably named by the Greeks. In that case the observations are probably around 2,000 years old i.e. when the summer solstice was in that constellation.
July 28, 2020
The Greeks were very sophisticated in their study of astronomy. Their knowledge of it was very useful in navigation. The ancient Greek explorer Pytheas and his crew sailed through the strait of Gibraltar and into the Atlantic. They reached far northern parts of the Atlantic and observed icebergs. Pytheas even suggested that the moon affected tides.
Pytheas was not the only Greek to achieve a lot. Eratosthenes quite accurately measured the size of Earth. It wouldn't be surprising if the Greeks determined the sun's movement along the zodiac slowed near the summer solstice, even though Earth's orbit is nearly circular and the difference between aphelion and perihelion (hence movement at those times) isn't that great.
August 3, 2020
Positions of stars have shifted quite a lot. As we know, Polaris (Alpha Ursae Minoris) is the current north polar star, and Sigma Octantis is the southern one. Sigma Octantis has a magnitude of 5.45. From what I've read, there were two north polar stars called Kochab and Pherkad from about 1700 B.C. to 300 A.D.
Yes that's true. Shifting is due to proper motion as stars rotate around the galaxy at varying speeds, and also precession.
August 8, 2020
I note the sun reaches Regulus this year (or every year) around August 23, a little later than I thought it did. No wonder Regulus can be seen well into July. Presumably, a couple of millennia ago, it could be seen as late as the summer solstice (perhaps even a little later) and showed signs of "lingering" in the evening sky as the sun moved sluggishly toward it, when Earth neared aphelion.
August 8, 2020
As a diary entry indicates, back in 1987 I saw Antares as late as November 2. I needed 7x50 binoculars to see it, but those with good vision may not have needed them. The sun is in conjunction with Antares around November 30, or four weeks after the last day I was able to view it.
When the sun approaches, Regulus should last longer in the evening sky because unlike Antares it is not several degrees below the ecliptic. My guess is that someone with good eyesight can see Regulus around four weeks before its conjunction with the sun on August 23. As I suggested earlier, the star remains visible until about July 24. That suggests it was observable to the ancients to the date of the summer solstice or a little later.
August 9, 2020
Antares is a red supergiant; it's mass is estimated at about 12 times that of our sun. The distance between the Sun and Antares is about 550 light years. I have read that Antares is classified as spectral type M15lab-lb.
I'd like to add something. Regulus should persist longer than Antares in the evening sky (when conjunction with the sun approaches) not only because it is higher relative to the ecliptic; the sun itself is "standing" or approaching it relatively slowly in June/July. I guess for someone with good vision Antares vanishes by the end of October, 30 days before conjunction, whereas Regulus disappears 4 weeks before conjunction--roughly July 26 (or June 26 in ancient times). The difference isn't much but was perceptible to the ancients.
August 11, 2020
I just found a diary entry indicating I saw Regulus, as well as Venus, as late as July 23 (1981). I DID see Regulus that late, albeit barely, as it was very faint, even with a 60mm refractor.
June 11, 2022
Ergo, a revision.
June 11, 2022
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