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Igor Barani
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The first point of Aries now lies in Pisces. Why does it keep its name?

The answer

The name now marks a job, and the job has not changed. The point is still the zero from which the sky is measured, whatever stars stand behind it.

  1. The point was named for the stars behind it.

    Becausein Hipparchus’s day, about 130 BC, the Sun at the March equinox stood at the western edge of Aries, the Ram. Star charts still mark the point with the Ram’s sign, .

  2. The point slides west along the zodiac.

    BecauseEarth’s axis wobbles like a spinning top and traces a full circle in about 26,000 years. That carries the point about one degree every 72 years.

  3. It left Aries about sixty years after Hipparchus.

    Becauseby today’s constellation borders, it crossed into Pisces in 68 BC. The Sun now enters Pisces around March 12, before the equinox, and reaches Aries around April 19, after it. The point will cross into Aquarius in 2597.

  4. The name now stands for a definition, and the definition never mentions stars.

    Becausethe point is wherever the Sun crosses the celestial equator heading north. The sky’s longitude, right ascension, is counted from there. A new name would change no star’s address.

  5. Maps kept an old sky name the same way.

    Becausethe Tropic of Cancer was named when the Sun stood in Cancer at the June solstice. Since 1990 the solstice Sun has stood in Taurus, but the line still marks the farthest north the Sun can pass overhead.

  6. Astronomers have since changed the anchor and kept the zero.

    Becausea drifting point makes a poor reference. Since 1998 the zero has been tied to distant quasars, which barely drift. It sits as close as possible to the equinox of 2000. Addresses written for 2000 still work.

Another way

Rename the point for where it lies now, the first point of Pisces.

Becausethe new name would be true for about six more centuries. In 2597 it would need changing again, and every chart printed before then would carry a stale name. A name tied to a definition lasts longer than one tied to the view.

Check

Do the arithmetic. Along the Sun’s path, Pisces is about 37.5 degrees wide. At one degree every 72 years, the point needs about 2,700 years to cross it. From 68 BC to AD 2597 is 2,664 years.

KeepA name can outlive its scenery.

Correspondenceigor@barani.org