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.
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.
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.
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