Castor in the birth chart: the mortal twin of Gemini

Star data

sefstars catalogue, Swiss Ephemeris 2.10.03.

Castor is one of the two bright stars of the constellation Gemini, the twin of Pollux. In the tropical zodiac it no longer stands in Gemini: it sits at the start of the last third of Cancer, a few degrees before Pollux. The two stars look like a pair in the sky, but Ptolemy gives them different natures: Pollux is like Mars, Castor like Mercury.

In the sky

Castor and Pollux stand close together in the sky, and Castor is the fainter of the two, although Bayer gave it the first Greek letter. The two also differ in colour: Pollux is orange, Castor white.

Castor lies about 10° north of the ecliptic, further than Pollux, so the Moon never passes in front of it. A zodiacal conjunction with Castor is therefore a matter of degree rather than a close meeting in the sky. The two twins are about 3° apart in longitude, which is wider than the usual orb, so a planet within 1° of one of them is not conjunct the other.

What kind of star it is

To the naked eye Castor is a single star about 51 light-years away. A telescope splits it into two white stars, Castor A and Castor B, which circle each other in about 450 years. Each of them is in turn a close pair that can be detected only in the spectrum. A third, fainter pair, Castor C, lies a little further off and travels through space with the others. That makes six stars in all.

Castor C is a pair of red dwarfs that orbit each other in less than a day and eclipse each other on every turn, which makes it a useful test for models of small stars. Astronomers' estimates of the age of the whole system range from about 290 to 370 million years.

The double nature of Castor has a place in the history of astronomy. William Herschel established that its two bright stars really orbit each other: Allen dates this to 1802, and Herschel's paper on it was published in 1803. Allen adds that Herschel coined the word "binary" for such pairs.

Finding it from Ukraine

The Twins are a winter and spring sight. Castor reaches its highest point at midnight in mid-January, a couple of days before Pollux, and stays in the evening sky until late spring. From the latitude of Kyiv it climbs to about 71° above the southern horizon, a little higher than Pollux, and it is easy to see from anywhere in the country.

The two bright stars stand side by side to the upper left of Orion. Castor is the whiter one, a little to the north of Pollux. In summer the Sun passes through this part of the zodiac, and the Twins are lost in its glare.

The name

Castor is the Latin spelling of the Greek Kastor, one of the Dioscuri, the twin brothers of Greek myth. In the myth Castor was the mortal son of Tyndareus and Pollux the immortal son of Zeus, and Pollux chose to share his immortality with his brother. The story is told on the Pollux page. The IAU Working Group on Star Names notes that the old Greek name was taken up again in Renaissance times; the International Astronomical Union approved Castor as the official name in 2016.

Allen gives a few older names. In later Greek times the star was called Apollo, a name astronomers used until the time of Flamsteed. The Arabic name means "the head of the foremost twin", matching the Arabic name of Pollux, "the head of the hindmost twin". The Babylonians knew the star as the western one of the Twins.

Nature according to Ptolemy

In the first book of the Tetrabiblos Ptolemy describes the two bright stars in the heads of the Twins separately. The one in the leading head, Castor, has the nature of Mercury alone, and Ptolemy adds that "it is also called the star of Apollo". The following one, Pollux, has the nature of Mars. The overview explains how such comparisons are read.

Mercury stands for the mind, speech, writing, learning, trade and quickness of hand. A Mercurial star brings these themes to whatever it touches: curiosity, verbal skill, a talent for languages, teaching or argument. Mercury also has a restless and changeable side, and on a hard day the same quickness turns into scattered attention or a sharp tongue.

The difference from Pollux is the point of the pair. Pollux is pure Mars, the drive to act; Castor is pure Mercury, the drive to understand and to name. In a sky where the twins stand side by side, Ptolemy's formulas divide between them action and thought.

After Ptolemy

Later tradition was harsher on Castor than Ptolemy's description suggests. Allen, writing in 1899, sums up its reputation by saying that in astrology it was everywhere "a portent of mischief and violence", without naming his sources. This view is closer to the reputation of Pollux.

Conjunctions with planets and angles

Castor is read by conjunction within an orb of about 1° (orb); tighter contacts count for more. Two typical cases show how tradition applies a Mercurial nature.

• Mercury on Castor. A Mercurial star on Mercury itself doubles the planet's themes. Astrologers read it as a lively mind, a gift for words and a wish to learn and explain, with the matching risk of nervousness and scattering energy across too many interests.
• Castor on the MC. On the Midheaven the star's themes move into work and reputation: a career in writing, teaching, speaking, trade or anything that lives on information.

As always, a star refines what is already in the chart. A Mercury that is weak or under pressure in the chart will show the star's restless side more than its gifts.

In celebrity charts

Conjunctions within 1° to the planets from the Sun to Saturn and to the chart angles.

• Marine Le Pen: MC, orb 0°06′
• Lamine Yamal: Sun, orb 0°18′

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