As our Sun, like all stars in our galaxy, circumambulates the Milky Way (at a brisk half million miles per hour), it also moves against the trend of its neighboring stars on a path toward the constellation Hercules. The Sun’s direction of motion in relation to the flow of local stellar traffic is called the
Solar Apex.
This target direction might be the natural phenomena that defines the boundaries of the zodiac.
I say it might, yet odds are against it. Space is big. Nonetheless, of all possible answers, this is the most engaging, interesting possibility, the most intuitively gratifying fantasy, hooked to the astrophysical structures of our surrounding space. As the plane of Earth’s orbit about the Sun defines the zodiac’s base plane, the direction of the aggregate forward motion of our solar system may define its boundaries.
During the entire course of human history, the direction of the Solar Apex has not changed a measurable fraction of a second of arc, consistent with the seeming invariability of the fixed zodiac itself.
Unfortunately, the position of the Solar Apex has been monstrously hard to calculate. The method is to note small differences in the positions of stars around us over time (much like looking out the window of an automobile at passing traffic) to calculate our movement compared to the rest. Astronomers’ tools have gotten better over the nearly two and a half centuries of looking. However, even today those tools still are not good enough.
Nearly every new journal article I have found on the Solar Apex over the last 30 years has reported testing or discussing how scientists should calculate it. Astronomers are still experimenting to learn how best to measure it.
William Herschel started the search: Two years after discovering Uranus (the first new planet discovered since ancient times), he first calculated the Solar Apex as being toward the star Lambda Herculis, which is at 25°10' Scorpio.
Inconsistencies in different astronomers’ determinations since Herschel are due not only to gradually improved tools, but also to which stars they used for comparison and how they observed those stars. Currently, astronomers give two Solar Apex positions several degrees apart, both known to be approximations.
One position, obtained by radio astronomy (RA 18:03:50.2, Dec 30N00'16", epoch J2000) converts to 6°39' Sagittarius. This is the position given most often in astrology references and is close to an earlier, long-standing estimate of R.A. 18:00 or 18:04.
The other contemporary “standard” position was obtained by visual observation of stars instead of radio astronomy. This value (RA 18:28:00, Dec 30N) converts to 15°25' Sagittarius.
Notice the range spanned by these different determinations. This last value (16° Sagittarius) is most of a sign-width different from Herschel’s original determination (26° Scorpio). The two contemporary standard determinations are about 9° apart.
Recent calculations of possible Solar Apex positions stretch across a wider span than this: A 2003 Cairo University paper modelled a new way of measuring the apex position against stars of different spectral classes. The main takeaway of the paper is that they found a wide range of possible positions and
only on average did these approximate the conventional positions. Depending on the spectral class of star used for comparison, the derived longitudes varied from as early as 14° Scorpio to as late as 3° Capricorn.
None of these values can be trusted to be exactly right. All are somewhere in Sagittarius – or a little earlier in late Scorpio – or a little later in early Capricorn. The simple truth is that astronomers do not yet know the correct location: They are still looking.
In 1959, at a time when most astronomers conventionally said the apex was about 18h of right ascension and +30° declination, Donald Bradley discovered the 1935 research of Professors A.N. Vyssotsky and Peter van de Kamp of the Leander McCormick Observatory (University of Virginia). Based on an analysis of the proper motions of 18,000 fainter (and mostly faster) stars, they summarized their findings:
The position of the apex differs considerably from the one derived from the proper motions of bright stars. In equatorial coordinates the position of the apex derived here is R.A. = 19.0h, Decl. = +36°, while the apex with respect to the bright stars is at R.A. = 18.0, Decl. = +30° which makes a difference [in R.A.] of 15°. In order to have an independent check on the R.A. of the apex, two additional solutions were made for the solar motion, using proper motions in R.A. of faint stars determined in the process of parallax determinations. The corresponding values of the R.A. of the apex are 18.8h (Allegheny and Yale-Johannesburg) and 18.6h (McCormick). A higher percentage of high velocity stars among the apparently faint stars may be the explanation of this well pronounced difference...
Vyssotsky and van de Kamp estimated that their calculations were accurate within plus or minus 1½°.
Writing as Garth Allen in the August 1960
American Astrology, Bradley calculated the celestial longitude equal to the Vyssotsky and van de Kamp determination (R.A. 19h, dec. 36° N, epoch B1950) and found that the new estimation was 29°24' Sagittarius – about half a degree from Sidereal 0°00' Capricorn!
It looked like – and some Sidereal astrologers still think it looks like – the Solar Apex defines 0° Capricorn.
To grasp the true import of this possibility, you need to know that three years earlier, in 1957, experiments in Sidereal mundane astrology disclosed 0° Capricorn to be a “master point” of the zodiac: Charts for Sun and Moon entry into Capricorn are the most accurate descriptors of events affecting masses of people collectively. (Mundane astrology is the primary topic of
Comprehensive Sidereal Astrology Volume III.)
Therefore, the Solar Apex might be the master point – and the defining point – of the Sidereal zodiac.
As astronomers continue to labor at resolving the question of the Solar Apex’s actual location, 21st century researchers have calculated apex locations ranging from mid-Scorpio to early Capricorn, depending on such things as which spectral class of star is used. It might be that Vyssotsky and van de Kamp had it pretty much correct after all.
Then again, space is big, and even the narrow corner of space where the apex is known to fall is big. Odds are against this being right.