Created By: Brady Arthurs, Brittany Erickson, Josh Piermantier and McKenna Szautner

The motivation of our research stemmed from a curiosity surrounding how the creation of the zodiac constellations informed the ideas and beliefs of ancient peoples and their understanding of the night sky. It is important to understand the role of the Earth’s precession in exemplifying the difference between a sidereal year and a tropical one.  To the layman, this is the difference between the time it takes for the Earth to make a full rotation around the sun and the time the seasons take to begin repeating. Axial precession is the slow change in the orientation of the Earth’s rotational axis due to the gravitational forces the Earth is subject to as a result of other massive bodies.1 The Earth’s wobble over time has meant that the zodiacs as they were first created no longer coincide with the constellations they were initially named.2 It is clear that the precession of the equinoxes is a natural phenomenon that was not understood by those who first used the zodiac for spiritual and time-telling purposes.3 The consequences of these errors as our research will show is that unless these calendars are re-calculated using modern astronomy seasonal and spiritual rituals will occur out of their correct place simply because the precession of the Earth was not taken into account.4 The importance of this research can also be found in the fact that it makes people aware of actual astronomical facts and challenges the validity of astrology. As Hipparchus first noted the gradual precession of Earth’s axis has a great impact on stellar coordinates over time. We will examine the history of the discovery of precession, how we understand precession today and how this phenomenon undermines the creation of the zodiacs. The Earth’s precession and its conflict with the creation of the zodiacs underlines the importance of adhering to empirical science and separating fact from fiction when it comes to understanding the night sky.

The History of Precession:
Greek astronomer Hipparchus of Nicaea (190 BC – 120 BC) is credited with discovering the phenomenon of the Earth’s precession. In Ptomely’s Algamest it was reported that Hipparchus had written a (now lost) book titled On the Length of a Year.5 Italian astrophysicist and archaeoastronomer  Giulio Magli  works primarily on the relationship between ancient cultures and the night sky. He notes that due to the extremely slow nature of the phenomenon compared to the length of human life it is impossible for a naked-eye astronomer to discover this phenomenon simply through their own observations.6 Thus Hipparchus had to rely on the previously existing data that went back two to three centuries. He had to trust the previously existing data as he collected a great quantity of astronomical data of over more than 800 celestial objects coming from the Alexandria observatory and compared this data to his existing data.6 As the first to practice astronomical photometry at his observatory on the island of Rhodes, the Alexandrian data was compared with the catalog he had created of nearly 1000 stars that included their positions, celestial coordinates and their apparent brightness.7 In trusting the observations of scientists past Hippharchus deduced that the equinoctial points moved over a distance of one degree per century along the ecliptic, in a direction opposite to that of the annual revolution of the Sun.5 This discovery allowed the differentiation between a sidereal and a tropical year in which Hipparchus wrote a book about the length of a year where he notes the length in time needed for the Sun to travel from one tropic or equinox to the same one again. He found that this time was equal to 365.25 days minus 1/300 of a day and night and not to a fourth of a day as was previously believed.5 While Hipparchus was correct that the Earth does have a slow and continuous precession  it is now known that the tropical year is not equal to the time interval between two successive spring equinoxes but instead is defined by the time needed for the mean tropical longitude to the Sun to increase by 360 degrees. These longitudes are measured from the vernal equinox which is in motion due to precession. The length of the tropical year is decreasing at the amount of 0.53 seconds per century.5 Nonetheless, Hipparchus’ findings of discrepancy between the observations he made and those of times before him are crucial in our understanding of precession and show that the zodiacs as they were created are out of place. This discovery was a great contribution to astronomy and has implications for records and calendars made before his findings.

Precession As We Understand It Today:
While many people today know that there is a “north” star, called Polaris, few of us know that the north star, as well as the entire night sky, changes over time. This is due to axial precession, which alters the position of every object in our night sky over long periods of time.1 Today, we understand that precession has a period of 25,772 years and that the Sun precesses about 50.3 arcseconds every year, or one degree every 71.6 years.8 This means that the tropical year, which is measured from solstice to solstice, is about 20 minutes shorter than the sidereal year, which is measured by the Sun’s position relative to the rest of the celestial sphere.9  This means that the time it takes for the seasons to repeat is shorter than the time it takes for the Earth to actually circle the Sun. Due to this fact, the Sun’s position in the zodiac also changes a small amount every year, so after several millennia, the Sun will be in a completely different zodiac than what is predicted by the seasonal zodiac calendar. This means that even if keeping track of the zodiac signs was once a way of describing a person based on the stars, they are no longer accurate and will not be useful for this purpose until the cycle repeats itself again.

The Reason the Earth Precesses:

Figure 1:“Earth Precession” by NASA. This image shows how the axis rotates, or precesses, around a central point at the same time the Earth rotates around its central axis.

An important fact to make note of is the reason we must account for precession in calculating the zodiac in the first place. Precession occurs due to Earth’s oblong shape, which is not a perfect sphere but rather an oblate spheroid.8 This shape is created by rotating an ellipse on one of its two principal axes, which to the common reader is the shape of a three-dimensional oval. As a result of the spin of the Earth itself, the mass that makes up the planet is subject to centrifugal force.10 Therefore the mass around the equator, which is perpendicular to the axis of rotation of the planet, is constantly trying to escape Earth’s gravity. This phenomenon is very similar to the pull we experience when holding on to the outside of a merry-go-round that is being spun very fast. However, while we are able to hold on with our arms, the matter around the equator is held to the Earth by a much stronger force: gravity. Gravity works against the centrifugal force acting on the equatorial matter, and the two work to create a balance.10 The balance is found when the mass around the equator is stretched slightly. This means that Earth is about 43 kilometers longer in its equatorial axis than it is in its polar axis, which gives the Earth a “bulge” around the equator.8

How Gravity Impacts Precession:

Figure 2:“Precession Torque” by Tauʻolunga on Wikimedia. This image shows the precession of Earth’s axis and how this occurs due to the Sun’s gravitational pull on the Earth

Due to the tilt of Earth’s axis and gravity being a distance and mass dependent force, the gravitational pull of the Sun and moon are not felt uniformly all around the Earth. The aforementioned bulge around the equator experiences a stronger force of gravity than the poles do, which causes a torque to be applied to the Earth.8 This torque is what causes the poles to precess about the celestial sphere. If you imagine that the poles make a circle while they precess about the celestial sphere, the torque is in the direction of the centre of the circle on average, so Earth’s axial tilt is unaffected by it.8 The strength of the torque also changes over time, being weakest at the time of the equinoxes because both the northern and southern hemispheres are equally close to the Sun.8 In fact, on the day of the equinox, the Earth experiences no torque from the Sun at all.

What Precession Means for Astrology:
As a result of analyzing all these factors, we know that the celestial sphere appears much differently to us than it did to our ancestors. While the positions of the stars relative to each other are not noticeably different, they appear to all be shifted relative to us. For example, as mentioned above, many people are aware that the north star is Polaris. This will no longer be true in 12,000 years, as the Earth’s axis will have precessed and the north end will point toward the star Vega.11 Precession also completely invalidates the notion that the astrological sign a person is born under is in any way connected to their destiny, as astrology today is still based on a person’s birth month and does not account for precession whatsoever.

The Hindu: Calendars, Ancient Rituals and Precession:
The precession of the equinoxes has some real implications for people of traditional religious and cultural groups who used the created zodiac system for their rituals. One such example is the Hindu who uses the ‘Panchang’ or Indian almanac. The system used depends on an old astronomical text known as ‘Suryasiddhanta’.4 Astronomers during this period believed that one tropical year was equal to the time taken by the Sun to make one complete revolution and did not take into account the effects of precession. Astronomers of this time who noticed processional motion merely thought that precession is an oscillatory motion and correction in astronomical calculations was not deemed necessary.4 At the time of the making of the Suryasiddhanta the Vernal equinox was close to the star Piscium of Pisces but due to precession the vernal equinox has shifted and yet ritualistic calendars are still being based off of original composition.4 As Mukhopadhyay makes clear, while precession is slight it does have a significant impact as the calendar still uses the sidereal year, the length of the year being taken as 365.25875648 days as fixed from the findings by Aryabhata in about 505 AD.4 He compares this with the length of a tropical year being 365.24219789 days so that the Hindu civil year exceeds the correct length by 0.01656 days.4 Thus the result of 14000 years of accumulated error have meant that solar months which are now altered which has lead to the beginning of the year now being wrong by nearly twenty-three days.4 There is reluctance to change the calendar, but the author notes that if no alterations are made and erroneous calculations are followed for 1500 years more, then the festivals will be totally out of season and the festival of spring will be observed in winter.4 Thus it is clear the importance of relying on the current science to help uphold these important rituals and practices that are observed by around 250 million Hindus. This case study helps emphasize the importance of empirical science to correct the errors made in ancient calendar making. If we can base these festivals on the facts we know today, taking into account the effects of precession, then the rituals can occur in their proper seasons as they were first intended.

About The Constellations and Zodiac Signs:

Figure 3: Picture of different zodiac constellations.Source: by Till Credner.Permission: Public Domain. Courtesy of Wikimedia

The first thing that we must clarify about the constellations and zodiac signs is that astrology is not astronomy. Astronomy is the scientific study of everything in outer space. It requires using the scientific method to prove theories about the universe and the study of the stars, space and the physical universe as a whole.12 Astrology is not science, it is not necessarily predictive and has no scientific warrant or basis on its findings. Astrology is the study of the movements and relative positions of celestial bodies interpreted as having an influence on human affairs and the natural world.13 This section explains the difference between astrology and astronomy, and how the definition of the zodiac signs are built about 8° either side of the ecliptic. It will also include all apparent positions of the Sun and Moon and discuss familiar planets that have changed since the discovery of the constellations that created them.14

In ancient times astronomers did not fully understand how Earth, the Sun, and the stars moved. People imagined that the constellations in the sky might be important symbols and would often tell stories of gods and other myths, this began the creation of the zodiac signs. Over 3,000 years ago the Babylonians divided the zodiac into 12 equal parts so as the earth orbits the Sun it would appear to pass through each of them.15 In earlier times, when most celestial measurements were designed to give the position of the Sun, Moon, or a planet, it was more convenient to measure from the path these bodies actually follow. The zodiac originated, then, as a time measuring device, without any notion of people being born under different signs.16 The zodiac was not created in an endeavour to establish in heaven signs for supposedly superstitious purposes, but was simply an early attempt to create a calendar based on observation.16 It is obvious that the zodiac would have been much more useful to early civilizations as a calendar incorporating allusions to climate conditions than as a scheme designed to facilitate “fortune telling.”16

Picture 4: The zodiac signs symbols.
Source: by Tavmjong
Permission: Public Domain. Courtesy of Wikimedia

An example of what it means to be “born under” a sign is that when they first created the 12 signs of the zodiac, a birthday between January 20th and February 18th would make you an Aquarius. Now 3,000 years later the sky has shifted because Earth’s axis doesn’t point in quite the same direction. Not being a physical body the zodiac has no gravitational pull; hence the alarm and discomfort felt by scientists at the idea of people believing in it, and their strong desire to disprove any ‘mysterious influence of the stars”. Yet the influence interpreted by astrologers in calculating horoscopes is quite simply gravitational attraction.16 This is important today because astrology has shifted into this belief that the zodiac signs are an accurate prediction of someone’s future, with magazines and other forms of media marketing them as horoscopes to prey on the naive qualities of their readers. The development of this phenomena has completely abandoned the science behind the stars and their constellations as a method of time measuring, and has since become a foundation for which people generate views about themselves based on their horoscopes. What began as a discovery of different constellations has become embedded as a widespread feature in all media forms.

The problem with this is that the zodiac changes have been going on for a long time. The path that the Sun travels on day by day is known as the ‘ecliptic’ and the rising position of the Sun changes each day so the path has changed over the past 20,000 years. The ancient Greeks knew about it, this means that the constellation the Sun is in on the day you are born no longer matches up with where it was when the zodiacs were first configured. The idea of being ‘born under’ a sign is often repugnant to scientists.16 So it is necessary to differentiate between the science of astronomy and the practice of astrology. This is also why it is important to popularize the study of the science behind the Earth’s wobble instead of trying to use the zodiac signs to interpret the future.

Figure 6: The zodiac signs as constellations placed in the sky.Source: by WolfgangRiegerPermission: Public Domain. Courtesy of Wikimedia

The zodiac is a circle of twelve constellations. Each of exactly 30 degrees extent, lying along the path of the Sun, and by means of which the Sun, moon, and planets can be measured.16 Each day as the Sun passes through the sky it passes through the zodiac, the path of which is called the ecliptic. This means the line on which eclipses may occur and does not alter at all from year to year.16 An eclipse can only happen when the Sun and moon are both situated upon it. The Sun, moon, and planets confine their movements to a narrow band some 8 degrees to either side of the ecliptic, which is why it is called the zodiac.17 Astrology began around 3,000 years ago and the Babylonians used this knowledge to guide actions and tell mythical stories. When the Babylonian culture was absorbed by the Greeks, astrology gradually came to influence the entire Western world and eventually spread to Asia as well.7 The zodiac derives its name from a Greek word meaning ‘living creatures’. Most of these constellations are named after persons or animals, whose shapes they are fancied to resemble.17 While Astrology and the discovery of the zodiac signs is

Figure 5: The zodiac signs symbols in pictures.Source: by Micheletb Permission: Public Domain. Courtesy of Wikimedia

different than the study of Astronomy, it cannot be denied that they astrology and zodiac signs are connected because astrology uses the position of the Sun on the date that you were born to determine what your zodiac sign is.

All of the zodiac signs are based on the seasons and not constellations, and all twelve of the signs have names and dates associated with them.18 Along with the names and the dates of the signs, they also have a spirit animal associated with it.19 Also, all the signs have a different symbol that is associated with it.20 Finally, all the signs are associated with a different element of the earth: fire, Earth, air, or water.21 This relates back to the thesis because due to the precession of the Earth the babylonians divided the sky in twelve sections and created the zodiac signs out of the constellations and gave the meaning behind each sign an importance.

Aries Taurus Gemini Cancer Leo Virgo Libra Scorpio Sagittarius Capricorn Aquarius Pisces
Dates Mar 21- Apr. 19 Apr 20- May 20 May 21- June 20 June 21 – July 22 July 23- Aug 22 Aug 23- Sept. 22 Sept 23- Oct 22 Oct 23- Nov 21 Nov 22 – Dec 21 Dec 22 – Jan 19 Jan 20- Feb 18 Feb 19- Mar 20
Spirit Animals Hawk Beaver Deer Woodpecker Salmon Bear Raven Snake Owl Goose Otter Wolf
Symbols Ram Bull Twins Crab Lion Virgin Scale Scorpion Archer Mountain Goat Waterbearer Pisces
Elements Fire Earth Air Water Fire Earth Air Water Fire Earth Air Water


The precession of the equinoxes underlines the inaccuracies of the created zodiacs, places holes in ancient traditions, and stresses the importance of relying on empirical science and current data for information about the night sky. While inaccurate in some of his findings, Hipparchus made major strides for the science of astronomy by discovering this slow and gradual phenomena. His discovery revealed the difference between a sideridal or fixed year, compared to the tropical year which was altered by the earth’s precession. Because ancient stargazers did not account for this phenomenon the zodiacs which they created, and many ancient and seasonal traditions are out of place if they are not altered. While astrology may be prominent with some people in the modern age who enjoy looking at their horoscope it is paramount that when looking for accurate information they turn to empirically based astronomy. The phenomenon of the Earth’s precession is a great event to emphasize this exact fact. Despite ancient spiritual and ritualistic beliefs centering around the zodiac, it is clear that in the realm of science we cannot be stuck in the past and must test existing hypotheses to build off of our existing knowledge base just as Hipparchus did. Our research shows the importance of using the current empirically based science and shows how the science of astronomy stands in direct contrast with the myths of astrology. The Earth’s wobble is an interesting phenomenon that should continue to be examined so that more can be known regarding its impact on Earth. 


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