The astronomy of astrology

It’s been a while since I last posted so instead of talking about the details of using telescopes or taking astrophotographs, I will discuss what for many people are two interchangeable terms: astronomy and astrology. More specifically, this post shows how some interesting oddities of astrology can also interest astronomers.

As a reminder, astrology is the study of the movements of celestial objects with the goal of predicting or justifying life events while astronomy is the scientific study of the properties, interactions and evolution of celestial objects. While I feel that our life choices are not influenced by the movement of the Sun across the zodiac constellations, or the alignment of certain planets I do think that people who believe in astrology may find significant interest in the evidence-based view of the universe astronomers take.

Lets begin by discussing an interesting tenet of astrology: star signs. The 12 signs of the western zodiac are based on the constellations which lay along the path on which the Sun appears to travel over the course of a year. Your star sign should ideally relate to the constellation which the Sun passes through on the day of your birth. But, most the time things are not this simple.

Every year, as the Earth orbits the Sun, the line of sight between us and one zodiac constellation is blocked by the Sun, this means that, as viewed from Earth, the Sun will appear to be sitting within this constellation. However, the dates associated with the star signs have not been technically correct for around 2000 years, i.e. the constellation in which the Sun appears the day of your birth may not fit with your star sign.

This is caused by a discrepancy between the way we define a year and changes in the movement of the Earth. As a society, we define a year as starting on a set date (the 1st of January) and running for a given number of days (365 – or 366 on a leap year). However, a year in astronomical terms is defined as the time it takes for the Earth to revolve once around the Sun, and these two measurements do not necessarily match up.

Figure 1. The position of the vernal equinox in the night sky. The Sun currently passes through this point on the ecliptic (red line) on 20th March. Graphic taken from Stellarium.
Figure 1. The position of the vernal equinox in the night sky. The Sun currently passes through this point on the ecliptic (red line) on 20th March. Graphic taken from Stellarium.

Specifically, in astronomical terms a year consists of the time period between vernal equinoxes. The vernal equinox occurs when the ecliptic (the line the Sun makes across the sky in one year) crosses the celestial equator (an imaginary line projecting out from Earth’s equator into space), see the blue arrow in Figure 1.

However, due to the fact that the Earth slowly wobbles on its axis every 26000 years (called Precession), the date of the vernal equinox slowly shifts by about 1 day every 70 years. The effect of this shifting equinox is that the position of the Sun within different constellations slowly changes, therefore, the Sun may be in a different constellation during the month of your birth than it was when astrological charts were first drawn up. Below is a table comparing traditional star sign dates with the actual position of the sun on these dates.

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The table also includes the length of time the Sun spends in each constellation. As a convenience in astrology, the star signs are given equal lengths. However, in reality, constellations have different sizes and cut across the ecliptic at different angles. The Sun spends 44 days in the constellation Virgo but only 8 days in Scopius (making true solar Scorpios a rare breed!). You may also notice an unfamiliar constellation: Ophiuchus (the serpent barer), it’s a large constellation but only a small part of it is actually crossed by the Sun in early December. Strangely this was known in ancient times but not included as a sign of the zodiac.

Gemini - 2015 Gemini - 28015Figure 2. The motion of the stars over thousands of years changes the constellations. Nearby stars (such as Pollux) appear to move faster. Graphic taken from Stellarium.
Gemini – 2015
Gemini – 28015Figure 2. The motion of the stars over thousands of years changes the constellations. Nearby stars (such as Pollux) appear to move faster. Graphic taken from Stellarium.

So when will the traditional star sign dates once again match up with the position of the sun within these constellations? Well, this could happen in about 24000 years, after the Earth has completed a full precession rotation. However, by then the stars themselves will have moved relative to each other, changing the shape of the constellations forever. In Figure 2, the constellation Gemini (my true solar star sign) is shown as it appears now (2015), and how it is projected to appear after 26000 years of star movement (date 26000 + 2015).

All of this serves as a reminder that the universe does not neatly fit into equally spaced constellations or fixed calendars, instead it is amazingly complex and constantly changing. So perhaps astrology can be a starting point for peoples’ interest in the universe, or at least to getting a better understanding of the science behind the horoscopes.

Post by Daniel Elijah

8 thoughts on “The astronomy of astrology

  1. This article is the epitome of unscientific. He is simply parroting the misguided, but popular theme of the day. He is attempting to use sham science to validate his opinion. Here are some undebatable facts; (I am an attorney, and I know the difference.)
    First, it is certain that the author has never studied a lick of astrology. If he had studied astrology, he would know astrology is valid… At it’s simplest- Do you not sleep when the Sun sets? And do you not wake when the Sun rises? Do we not choose to sew seeds in spring? Harvest in Fall?
    Second, Astrology is based on the astronomical year, not the sidereal zodiac (fixed stars) which the author assumed astrology to use. If astrology used the sidereal zodiac, the author would be correct about ‘signs’ not lining up due to the Procession of the Equinoxes, but that is not the case.
    Astrology uses the geocentric zodiac (which is unaffected by Procession) which always has, and always will begin on the spring equinox which occurs in our calendar on or about March 21st every year. Ancient mythology shows us that this has been going on unchanged for thousands of years, ie. the Jesus story, the Exodus story, the story of Hercules and his 12 labors, etc, etc.
    One of the hardest things a person can do is learn to think for themselves, and learn to discover truth rather than parrot the wrong ideas of others.

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  3. Indian astrology, called jyotish, used a fixed zodiac, that is the first point of aries is in the same place it was 2000 years ago whereas western astrology uses the zodiac that moves with the spring equinox. The two have moved about 23 degrees apart today and that figure,called ayanamsha is one of the most important calculations made in an Indian chart

  4. This article was written with the best of intentions, and it is clear the authors are trying to help foster greater understanding of the world around us. Unfortunately, the information they provide is inaccurate. In western astrology, the signs are NOT the same as constellations, and this understanding has been known by astrologers/astronomers (they were one in the same throughout most of history) for over 2400 years. I would encourage the authors to take a closer look at both the history of science and the fundamentals of astrology, and then rewrite the above article.

  5. Astrology divides the annual season cycle, as experienced from Earth’s northern hemisphere, into twelfths, with the solstices and equinoxes being the fixed quarterly reference points. When the system of astrology was formally codified, convenient groupings of stars contained within each of these twelve zones were designated as a sign of the zodiac. Apparently, this was done with some intent to mythologically reflect nature’s specific seasonal action upon life during each of the twelve timeframes. History, archaeology, and astronomy all point to this having been formalized sometime around 300 BC, under Hellenistic influences, probably in Alexandria, Egypt. The Greeks noticed the procession of the equinoxes within a couple of centuries, and it was immediately understood that, not the constellations (sidereal zodiac), but the season cycle (tropical zodiac) is what must be retained as the frame of reference in order to maintain the system’s original intent. Fundamentally, astrology seems to hypothesize that the solar system itself is an organism, of which human beings are a vital part. However accurate or erroneous this hypothesis may be, the precession of the equinoxes, and the constellations not matching up with their original seasonal placement, is absolutely of no help whatsoever to modern scientists’ attempts to disprove astrology.

  6. As noted previously, astrologers have known about the precession of the equinox for centuries. Tropical astrologers do not care. Their map is based on the equinox and solstice points of the sun. Sidereal astrology (such as Jyotish in India) does adjust the chart based on the fixed stars (appox. 1 degree every 72 years).

    Like all maps, the astrological chart is an abstraction. The astrological zodiac does borrow the names of the constellations for their coordinate system. But it does not use the astronomical boundaries.

    You can argue against astrology all you like. But you sound foolish rather than wise when you select an argument that is not relevant.

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