The equinoctial and solstitial points and the constellations

It is a well-known fact that the so-called 'signs' of the zodiac — those empty, rectangular areas of the astrologers — no longer coincide with the constellations of the same name. The reason is the precession, which I already mentioned in the previous chapter. Due to the precession, the shift is approximately one sign every 2000 years. Nowadays, the difference is almost one sign. On September 23, for instance, when the Sun enters the 'sign' of Scorpius (which the astrologers call Scorpio), it is still in the constellation Virgo, and about to enter that of Libra.

Two thousand years ago the constellations and the signs did match approximately, but in fact exact coincidence can never occur. The reason is obvious. While by definition each sign is 30 degrees long, the actual constellations occupy different lengths along the ecliptic. The official constellation boundaries adopted by the International Astronomical Union show that the Sun travels 44 degrees in Virgo, but only seven degrees in Scorpius. Moreover, each year the Sun spends 18 days in Ophiuchus, a constellation for which there is no corresponding sign!

So it is clear that no exact instant can be quoted when signs and constellations would have coincided. But of course it is possible to calculate the times at which the March equinox enters successive constellations along the ecliptic. The March (vernal) equinox is one of the two intersections of the ecliptic and the celestial equator; its celestial longitude is 0°. Therefore, the vernal equinox defines the beginning of the 'sign' of Aries.

I have made the calculations taking into account the variable speed of the precession (50.2687 seconds of arc per year in the year 1900, but growing to 50.2910 by 2000) and the slow rotation (47" per century) of the plane of the ecliptic itself. In fact, use was made of the precession formulae by J. H. Lieske e.a. (1977) which presently are officially adopted by the International Astronomical Union.

In the year -1865 the March equinox passed from Taurus into Aries, and in —67 it entered the constellation Pisces. That year, the beginning of the sign of Aries coincided, on the ecliptic, with the western boundary of the like-named constellation. Not until A.D. 2597 will the March equinox cross the boundary into Aquarius. In A.D. 4312 this equinox will enter Capricornus. See Figure 49,a.

The September equinox — the other ecliptic-equator intersection — passed from Libra into Virgo in the year -729, and it remains there until A.D. 2439, when it will enter Leo.

Now in Sagittarius (since the year —130), the December solstice will cross into Ophiuchus in the year 2269, and into Scorpius in 3597. The June solstice (longitude 90°) passed in —1458 from Leo into Cancer, in —10 into Gemini, and in December 1989 into Taurus, as inspection of any good star atlas shows — see also Figure 48.a. In A.D. 4609 it will pass from Taurus into Aries.

Giza From The South

Fig. 49.a: The successive positions of the vernal (March) equinox with respect to the official (IAU) constellation boundaries (the dashed lines). These boundaries are defined as arcs of equal right ascension or declination with respect to the celestial equator and the equinox of the epoch 1875.0. The position of the vernal equinox is indicated at intervals of1000 years. In A.D. 1489 the equinox passed only 0° 10' from the 'corner' of Cetus, the Whale, but it did not enter this constellation.

Fig. 49.a: The successive positions of the vernal (March) equinox with respect to the official (IAU) constellation boundaries (the dashed lines). These boundaries are defined as arcs of equal right ascension or declination with respect to the celestial equator and the equinox of the epoch 1875.0. The position of the vernal equinox is indicated at intervals of1000 years. In A.D. 1489 the equinox passed only 0° 10' from the 'corner' of Cetus, the Whale, but it did not enter this constellation.

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