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Earth's rotation causes new zodiac sign assignments

January 14, 2011 11:23am
Astronomers from the Minnesota Planetarium Society have found that the moon's gravitational pull on the Earth changed our planet's position in relation to its axis, making the original alignment of the stars, the basis for the zodiac signs, "off" by about a month. Thus, a thirteenth zodiac sign, Ophiuchus, was added to the original 12.

With the addition of the 13th sign, Ophiuchus, astrology fans wonder how the new sign will fit in. Art by Analyn Perez
With the change, the birth days assigned to particular zodiac signs might not be accurate anymore.

Minnesota Planetarium Society board member Parke Kunkle said in an interview with Minnesota Star Tribune that the ancient Babylonians based zodiac signs on the constellations the Sun was "in" on the day a person was born.

Over time, the moon's gravitational pull has made the Earth "wobble" around its axis, creating a one-month bump in the stars' alignment.

"When [astrologers] say that the Sun is in Pisces, it's really not in Pisces," Kunkle said in the interview.

Because of additional one month in the cycle, the birthday classifications under each zodiac sign changed as follows:
  • Capricorn: January 20 - February 16
  • Aquarius: February 16 - March 11
  • Pisces: March 11 - April 18
  • Aries: April 18 - May 13
  • Taurus: May 13 - June 21
  • Gemini: June 21 - July 20
  • Cancer: July 20 - August 10
  • Leo: August 10 - September 16
  • Virgo: September 16 - October 30
  • Libra: October 30 - November 23
  • Scorpio: November 23 - November 29
  • Ophiuchus: November 29 - December 17
  • Sagittarius: December 17 - January 20
Science news website LiveScience explained that early astronomers calculated the old zodiac signs based on the position of the sun as it's perceived from the revolving earth. Based on this position, the sun passes through the constellations that formed the zodiac signs - Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces.

Originally, the zodiac signs were determined based on the constellation the sun was "in" on the day a person is born. Astronomers observed the sun's travel through the signs of the zodiac in the course of one year, spending about a month in each.

However, a movement of the Earth called "precession" causes the Earth to continually move around its axis. The gravitational attraction of the moon on the Earth causes the movement, causing the signs to slip one-tenth — equivalent to one month — of the way around the sky.

Because of the movement, the sun may no longer be within the constellation of a sign during a particular period. For example, those born between March 21 and April 19 consider themselves to be Aries, but the sun is actually in the constellation Pisces in that period.

The new sign, Ophiucus, is the constellation right after Scorpius.

According to a report on, the ancient Babylonians were believed to have removed the 13th sign because the 12 signs align more accurately with the calendar.

Ophiucus, the new addition to the zodiac sign family, is represented by a serpent tamer, holding a serpent with his two hands. The new sign is associated with higher education, healing and medicine.

Astrologist: Changes only apply to one type of zodiac

However, astrologer Jeff Jawer said in an interview with CNN that astrology fans need not worry because there are actually two types of zodiacs: the tropical zodiac, which Western astrology adheres to and is fixed to seasons; and the sidereal zodiac, which is fixed to constellations and is followed more in the East.

The "changes", he said, apply to the sidereal zodiac.

Jawer added that the alignments with constellations indeed changed compared to years ago, but this change is irrelevant for the tropical zodiac, which was codified for Western astrology by Ptolemy in the second century.

"When we look at the astrology used in the Western World, the seasonally based astrology has not changed [and] was never oriented to the constellations," he said in the CNN interview, adding he personally adheres to the tropical zodiac.

"Astrology is geocentric. It relates life on Earth to the Earth's environment, and seasons are the most dramatic effect, which is why we use the tropical zodiac," he said. — RSJ/LBG, GMANews.TV
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