Astrology to Astronomy

FAS Astronomers Blog, Volume 30, Number 4.

I don’t know how many times I talk to someone who knows that I have an interest in Astronomy, but they refer to it as Astrology. Well Astrology does have something to do with the night sky and, from an historical standpoint, it is related to astronomy. Both deal with the path the Sun takes over the course of a year and the position of the Sun in the sky.

When looking up at the night sky, we, of course, can see stars, and we often see patterns in the stars. In ancient times, folks connected the dots and they saw animals, people, and objects to which they assigned names. These names and groupings of stars became the constellations. There are 48 ancient constellations that were probably known for centuries and were listed in Ptolemy’s Almagest around the year 150.

As the Europeans began to sail about the globe, they observed the stars of the southern hemisphere and saw more patterns in the night sky. So, additional constellations were identified. In 1922, the International Astronomical Union (IAU) settled on an official list of 88 constellations.

The way we connect the dots to draw pictures of the constellations is somewhat arbitrary. Some such as Orion and Scorpius are obvious. Other such as Canis Major, Virgo, and Sagittarius are a bit harder to see and are often drawn in different ways.

However, the boundaries between the constellations are not arbitrary. In 1928, the IAU divided the sky into an official list of the 88 constellations. They defined specific boundaries between them so that every object in the sky belongs to one and only one constellation. In fact, the brighter stars are all assigned names based on the constellations within which they are located. For more on star names, see Observing the Stars.

The ancients also identified special constellations, which marked the path the Sun takes throughout the year. It turns out there are twelve of these (Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius, and Pisces). The Sun spends around a month in each. They became known as the constellations of the zodiac and formed the basis for astrology.

I don’t know too much about astrology, so let’s just say the date of your birthday determines your astrological sign, which is where the Sun was in the sky when you were born (almost). The original assignment of astrological signs to dates was done a few thousand years ago and at the time correctly mirrored the Sun’s path and position.

However, things have changed a bit. The Earth wobbles through a 25,722-year cycle as its north pole traces a circular path in the sky. We call this the Precession of the Equinoxes because this wobble causes the timing of the Equinoxes to change over the years.

Today the north pole points toward the star Polaris (aka the North Star). However, this wasn’t the case in the past and won’t be in the future. Around 6,000 years ago the Earth pointed to somewhere near the tail of Draco the Dragon. In 12,000 years, the star Vega will be the closest we have to a north star.

The Earth’s wobble has shifted the timing of when the Sun passes through the twelve constellations. Today, the Sun appears in each constellation around a month later than what the astrological signs say. For example, the sign Virgo is for folks born between August 23 to September 22. However, the Sun is found in the constellation Leo from August 11 to September 16, and Virgo from September 17 to October 31. So, if you were born on September 10, are you a Virgo or a Leo? Astrology says you are a Virgo and astronomy says you are a Leo.

Well, in any event, the Sun does appear to move across the sky throughout the year. Because the IAU has precisely defined the boundaries of the constellations, we know when the Sun will appear in each constellation. This leads to another slight difficulty. The Sun passes through the lower part of the constellation Ophiuchus, which is between Scorpius and Sagittarius, from November 30 to December 17. So, if you were born on December 1, are you really an Ophiuchus?

Therefore, while astrology has twelve constellations of the zodiac, astronomy has thirteen constellations of the ecliptic. And, yes, astronomers call the path of the Sun the ecliptic and not the zodiac. And you may have noticed that the official name of two of the constellations is Scorpius (not Scorpio) and Capricornus (and not Capricorn). Astronomers like to use Latin names for things.

So far, I’ve focused on when the Sun passes through each of the constellations. However, when the Sun is in a specific constellation, we can’t see it. Therefore, astronomers tend to think about constellations, not based on the position of the Sun, but when they appear in the early evening night sky, which is six months earlier or later. As an example, the Sun appears in Sagittarius during the winter, but we can see Sagittarius in the night sky during the summer. So, astronomers consider it to be a summer constellation.

Because seasons are caused by the tilt in the Earth, the path of the ecliptic is high in the sky during the summer and low in the sky during the winter (almost). This is true for the ecliptic in the daytime, but we can’t see stars during the day. It is the opposite at night. During the summer, the night side of the Earth tilts down and the ecliptic is lower. During the winter, the night side tilts up, and the ecliptic is higher. So, the winter constellations (defined by astronomers) will appear higher in the night sky, while the summer constellations (defined by astronomers) will appear lower in the night sky.