I remember saying that Serpentis is located at the 19th degree of Scorpio, which is actually 18.00-18.59 Scorpio. It's not a fixed star, but rather just a special degree of the zodiac.
My current explanation on the rationale behind this assignement is that it has something to do with the golden ratio applied to astrological signs.
You know about that very special ratio, found almost everywhere in nature, which embeds a certain harmony - the golden ratio: 1.61803399, known also as phi.
square phi = phi +1
If applied to zodiacal signs we may find the exact point where a 30 degrees long zodiacal sign may be split in two segments related to each other by the golden ratio, phi. That exact point is located at 18deg33min in any signs. This means that the 19th degree of any sign holds a very special meaning.
In this thread http://www.astrologyweekly.com/forum/showthread.php?t=1953 I tried to find some correlations between what we already know and all these special degrees.
As for your question "Does it mean that we can't use the degree of fixed stars directly in considering a natal chart?", my answer is "Sure you can. Forget about sidereal astrology, it only confuses you."
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