Spica

Spica is Alpha Virginis, the brightest star in Virgo, apparent magnitude 0.97, at about 250 light years. The Latin name means the ear of wheat, which is what the Virgin holds in the constellation figure.

Its physical nature is not what the eye suggests. Spica is a close binary of two hot blue stars, spectral types around B1 and B2, orbiting each other in four days at a separation so small that they are distorted into ellipsoids by mutual gravity. The system varies slightly in brightness as these distorted shapes present different areas to us, a rotating ellipsoidal variable, and the primary is also a Beta Cephei pulsator. Neither component is resolvable; everything known about the pair comes from spectroscopy and photometry.

Spica's historical importance is out of proportion even to its brightness. Hipparchus, in the second century BC, compared his own measurement of the star's position relative to the autumn equinox with earlier Babylonian and Greek records and found a discrepancy of about two degrees. From that discrepancy he inferred the precession of the equinoxes. It is one of the foundational discoveries of positional astronomy, it was made using this star, and it is the reason every zodiacal position quoted in an old text has to be dated before it can be used.

Spica lies close to the ecliptic, about two degrees south, and is occasionally occulted by the Moon and rarely by planets.

Spica is among the stars proposed as an alignment target for Egyptian temples, and Norman Lockyer argued in the late nineteenth century that a temple at Thebes was oriented to its setting and readjusted over centuries as precession shifted it. That reconstruction is contested, and archaeoastronomical alignment claims generally need more caution than they receive.

Ptolemy gives Spica the nature of Venus with a lesser Mars component, and the tradition reads it as one of the two or three most fortunate stars in the sky: wealth, honour, renown, refinement, and a marked gift for the arts and sciences.

The Venus half carries essentially the whole delineation. The Mars component does very little work, and unusually for a mixed nature it is not used to introduce a caveat, most fixed stars with a martial element acquire a warning about violence or a fall, and Spica largely does not. It is read as almost unqualified good fortune, which is rare in a literature that generally prefers omens of disaster.

There is a plausible reason for the benign reading that has nothing to do with the star's physics. Spica is the ear of grain in the hand of the Virgin, and the constellation is associated across Mesopotamian, Greek and later traditions with harvest, agricultural abundance and grain goddesses: Nisaba, Demeter, Ceres, Astraea in various identifications. A star embedded in a harvest figure acquires a harvest character. The astrological nature and the constellation's iconography reinforce each other, and it is not possible to say which came first.

Spica is one of Agrippa's Behenian fifteen, paired with emerald and with sage, another complete star, stone and plant set from a named Renaissance source.

The detailed modern delineations descend, as usual in English, from Robson in 1923. His entry for Spica is among the most positive in his book, and it should be read as a 1923 compilation of earlier material rather than as an ancient text.