The Ides of Science V: Was Jesus a Capricorn?

No, this is not a blog about religion or about the religiosity or historicity of Jesus. It’s a blog about star signs and, since it is the festive season and the popularly accepted date of Jesus’ birth presents a useful location in a timeline from which we can make a comparison, it is a useful date to discuss. So I am not going to go into whether Jesus was actually born on this date, the accounts about him, or even whether he existed as a historic figure at all.

I also won’t be going into the fact that the dates themselves have changed over the last two thousand years. The Romans were using the Julian calendar in 1 AD, which consisted of 12 months and an extra day for leap years. Nowadays we use the Gregorian calendar, which was introduced in 1582 and is different to the Julian calendar by 1 day every 128 years. Other cultures at this time had their own ways of measuring the passage of time as well (the Mayan calendar has been one of interest in recent years). The winter solstice, however, is a fixed point in the calendar and corresponds to the popularly accepted date of the birth of Jesus. (It is probably the reason for why that date became accepted as the birthdate, since the Romans already celebrated this date in their festival to Sol Invictus.)

In summary, if the title of this blog offends, please rephrase it to be more accurate, but not as interesting. Would a person born around the winter solstice of 1 AD be a Capricorn?


Your Star Sign

My star sign is Pisces. What does that mean? To most people (in the cultures where star signs are popular) it means simply that I was born between 19 February and 20 March. But what’s important about those dates? For example, let’s say I was born on 19 February at 12:00 (noon). If I was born in 1982 I would be a Pisces, but if I was born in 1983 I would be an Aquarius. These transition dates are called “cusp dates” by astrologers, and the exact time at which the transition occurs is not a trivial thing to figure out. During the 1980s, for example, the time of transition varied from 00:20 to 20:00, and not in any easily recognisable order.

Essentially, your star sign is determined by the location of the Sun in the sky at the time of your birth. The Sun appears to move with the rotation of the Earth just as the stars do, but the tilt of the Earth’s rotational axis to its orbital axis causes the Sun to deviate from the paths of the stars, such that it makes a full passage across the ecliptic plane (the plane in which the Earth orbits the Sun and contains the 12 zodiac constellations) throughout the year. This means that, sticking with the Pisces example, the Sun would be in the sector of the sky designated to the Pisces constellation between 19 February and 20 March (the crossover between Pisces and Aries occurs at the Vernal Equinox). It isn’t really, but we will get to that.


Your Zodiac Sign

As an aside, I should point out that there is a difference between a Star Sign (Sun Sign) and a Zodiac Sign. The star sign considers the location of the Sun in the sky, while the zodiac sign specifically refers to the stars themselves. Your zodiac sign depends on what is known as the ascendant, i.e., which zodiac sign was rising on the eastern horizon at the time of your birth. Since the sky is divided into 12 segments, this means that a new zodiac sign will rise every two hours. Astrologers place meaning on not just the location of the Sun and horizon relative to the stars, but also the moon and planets. For the rest of this blog, we’ll focus on the star sign, since this is the one that is most commonly recognized in western society.

There is a difference between one’s star sign and one’s zodiac sign. The former is based on the relative location of the Sun in the sky, which changes constellations every three months. The latter is based on the ascendant, which is the zodiac that is on the eastern horizon at the time of one’s birth, which changes constellations every two hours.


Not a Pisces?

We have determined that I am a Pisces because the Sun was in the constellation Pisces at the time of my birth, right? Unfortunately not. When I was born, in March in the mid-1970s, the Sun was actually in the constellation Aquarius. It is almost certain, in fact, that the Sun was not in the constellation that you associate with at your time of birth either. You can find your real star sign here if you’re curious.

Why are we getting it wrong? We’ve been using zodiacal signs since the time of the Babylonians, right? That, actually, is the problem. The dates that are associated with the constellations have not changed over the centuries, but the location of the Sun in the sky during those dates has changed.

Let me explain. The Earth is tilted on its axis and so its orbit around the Sun resembles a top that is spinning at an angle. Additionally, the Earth is not a perfect sphere, but is slightly larger at the equator. Gravity from the moon and other planetary objects in the ecliptic plane cause an imbalance of the Earth’s orbit, creating what is known as precession. This movie provides an excellent example. Notice how the mass imbalance causes the rod supporting the mass to rotate? The Earth is subject to the same precession, which causes a rotation about this precession circle with a period of around 26,000 years. This means that the position of the stars relative to the Sun and planets will change over time, with their locations varying at the same rate.

In other words, the location of the Sun in the sky changes at a rate of about one constellation every 2200 years. So, if I was born between 19 February and 20 March in the year 1 AD I would be a Pisces. Today, however, I would be an Aquarius. Likewise a person born between these dates in the year 4000 AD would be a Capricorn, since that is where the Sun would be at the time of their birth.


And the Constellations Themselves?

We’ve established that your star sign changes depending on the year in which you were born. But the constellation themselves don’t change, right? Surely some pictures drawn by joining the dots between selected stars in the sky will always have the same appearance. What could be wrong with that? You can probably guess where this is going. No, the constellations are not constant either. The stars are all at different distances from the Earth, and the solar system orbits the center of its galaxy with a period of around 250 million years.

The Sun orbits the Milky Way Galaxy at a rate of once every 250 million years or so. Consequently, the locations of the stars in the night sky will change gradually over time. Thanks to Caltech for providing this image which was reproduced in the linked article from EarthSky.


Stars that are relatively near the Earth change their location in the sky as the Earth rotates around the Sun. Astronomers measure the angular change in the position of the star in six-month intervals and use simple trigonometry (knowing that the Earth have moved 300,000 km during those six months as it orbits the Sun) to work out the distance to the star. This is called parallax.

In a similar fashion, the relative location of each star to the Earth will change as it orbits the center of the galaxy. As a result, over long periods of time, the distribution of the stars in the sky changes. The constellations, then, will change their appearance. It takes tens of thousands of years for them to change to a point where they are no longer recognizable, but they are in a state of constant, gradual, change. The following picture shows a simulation of how some popular constellations will change over 150,000 years (thanks to Halycon Maps).

As the Earth orbits the galaxy its location relative to the stars will change over time. Consequently the constellations will eventually no longer resemble their current form. This image provide thanks to Halycon Maps; animated gifs of these are available in the linked article in Wired.



Was Jesus a Capricorn? Yes he would have been. If he were born today, however, he would be a Sagittarius. And the Sagittarius constellation would have looked a little different back in that time too. All of these orbital planes sure make things difficult.

Hope you enjoy the festival of Sol Invictus. All the very best from Howard Tutoring. See you next year.

Thanks to Cornell University for providing some of the material on precession. Thanks also to Wired magazine for their article on the changing appearances of the constellations.

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