How to determine the sign of the zodiac with absolute accuracy?

Published on September 20, 2016

How to determine the sign of the zodiac with absolute accuracy?

    In the wake of the hype about " panic in women's magazines, " I would like to dig deeper into such a frivolous question as the definition of a zodiac sign. Let us temporarily close our eyes to the fact that " astrology is not a science " and the sign of the zodiac is " not a constellation ". Just try with a serious look to figure out what factors need to be considered in order to determine exactly which constellation the Sun was in at the time of human birth.


    Cosmopolitan publishes a real table of horoscope signs amended for precession, as of 2016:

    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

    This is all great, but what about those who were born on a transition day, for example, September 16? Is he a lion or a maiden?

    The boundaries between the constellations are drawn along the circles of the celestial sphere of right ascensions and the declination arcs in the equatorial system of celestial coordinates. In this screenshot from Stellarium they are marked with red dotted lines.

    Of course, the sun crosses the boundaries of the constellations not exactly at midnight or noon. The exact time of transition varies from year to year, mainly due to the fact that there are not an integer number of days per year and leap years exist, and the precession does not sleep.

    For example, this year the center of the solar disk crossed the border of the constellations Leo and Virgo on September 16 at 15:40 Moscow time:


    And in 2017 this event will take place at 21:24 Moscow time:


    Moscow time ... Damn, we must take into account time zones ! Everything is harder and harder.

    And you can also remember that because of the parallax transition occurs at different times for observers in different hemispheres.

    The Earth moves around the Sun at an average speed of 30 km / s, which means a path equal to its diameter (12,742 km), it overcomes in 425 seconds. The difference can reach seven minutes!

    The Earth moves around the Sun not alone, it is accompanied by the Moon. In fact, not the Earth itself moves in orbit, but the center of mass of the system. The example of Pluto and Charon clearly shows how a large satellite affects the position of the planet:


    For the Earth-Moon pair, this effect is less, but it is and should be taken into account. On a new moon, the Earth moves faster than on a full moon. The moon is pushing, then slows down our planet in its orbital motion. Since for a year the moon makes a non-integral number of revolutions around the earth, its influence on each concrete day is different.

    It turns out, to determine the exact sign of the zodiac, you need to know, up to a minute, the time of birth according to UTC and the geographical coordinates of the maternity hospital! And how to determine the exact time of birth ? Childbirth - the process is not instant ... Seemed head? Has the whole child got out? Is the umbilical cord cut? Screamed? And if cesarean? The doctor will linger a little bit, and now we are no longer Leo, but Virgo.


    There is a version that the signs of the zodiac are determined "by the projection of the date of conception." Here, too, everything can be difficult c exact time.


    Due to the finite speed of sperm, the moment of ejaculation does not coincide with the moment of conception. It may take several days, until one of the sperm reaches the egg.

    Let's go back to heaven. The boundaries of the constellations are not tied to any real objects in the sky. They are where they are, because "so agreed." Clear and unambiguous boundaries between constellations were adopted in 1928 by the decision of the General Assembly of the International Astronomical Union. For several years, clarifications were made. In 1935, astronomers finally agreed that they would not be changed anymore. A person born in this troubled time will have to raise the archives of an astronomical union to determine which boundaries were in effect at the time of his birth.

    In general, I come to the conclusion that I can not give an unequivocal answer to the question in the title. It's time to finish the article, let's discuss in the comments what else can be considered.

    The article uses illustrations xkcd.comavailable under CC-By-Nc license .

    Update: in the comments suggest to take into account the angular diameter of the sun. Yes, the Sun really is in two constellations for a while. But in this case it is necessary to add in the zodiac not only Ophiuchus, but also Keith! No, the ecliptic does not cross this constellation. But the northwest corner of China is in dangerous proximity to it.

    We look bigger:

    Born on the night of March 27-28, you are not only fish, but also a little mammals.