What Is the Zodiac, Anyway?

By Peter Forrester |  December 20, 2018

Many people know the term “zodiac” from astrology, or fortune-telling based on stars and planets. But how many know the astronomical, scientific use of the word?

First of all, I need to introduce another term, the “ecliptic”. This can be imagined as an invisible line in the sky through which the Sun appears to move every year. I use the word “appears” because we are on a spinning Earth, which is the real source of the Sun moving across the sky every day. The Moon and planets always stay very close to this line. The ecliptic gets its name because eclipses only occur when the Moon is crossing it during full moon or new moon.

The ecliptic is inclined 23.4 degrees from the “celestial equator”, the line in the sky lying directly over the Earth’s equator. This corresponds to the well-known tilt in the Earth’s axis, in its orbit around the Sun.

From our vantage point in the Northern Hemisphere, the ecliptic rises from the horizon in the east, reaches its highest point to the south, and then descends to the west. So if you want to see a zodiac constellation, or a planet, or the Moon, your best bet is to start by looking south.

The concept behind assigning a Zodiac constellation to each month is a rough approximation of the scientific reality. The idea is that every month, the Sun occupies the space between the Earth and one of the twelve classic Zodiac constellations, and hence is said to be “located” in that constellation. Here the zodiac uses 12 divisions of the ecliptic into 30 degrees each, regardless of how big each of these constellations actually is.

However there are a couple of little catches here. There are actually 13 constellations through which the ecliptic passes. The one not used in the common 12-sign version of astrology is called Ophiuchus, the Serpent Bearer, and is located between and above Sagittarius and Scorpius. The constellations are different sizes and thus the astrological divisions reflect simplifications of where the Sun and planets are.

A further problem with the signs of the Zodiac is that they do not align properly with the current position of the Sun. For example, the astrological sign Aries is from March 21 to April 20, but the Sun actually passes in front of Aries between late April and mid-May. This is because of long-term fluctuations in the orbit and rotation of the Earth. The dates of the signs were assigned over 2000 years ago, and now are out of alignment.

I need say very little about the third major problem with astrology in general. There is no scientific evidence that a person’s life or personality, or future prospects have anything to do with the position of the stars or planets at the time of their birth, or at any other time. Astrology is a pseudo-science, a cool belief system that has very little basis in reality. I am here to explain the reality to you, as harsh as it may sound.

The zodiac, as used in astronomy, is a small band in the sky, about 8 degrees above and below the ecliptic, in which the Sun, Moon, and Solar system planets are usually located (there are as many as 10 other constellations in which planets can occasionally be found). Besides the 13 constellations mentioned above, which all cross the line of the ecliptic, there is a 14th one that nearly touches it and which the solar system objects more commonly occupy. This constellation is called Cetus, the Whale, and lies just below Pisces.

I’ll discuss the various zodiac constellations later, but for now I just want to list them, with the dates that the Sun is actually located there. You will notice these dates are about a month later than the traditional astrological  dates, which I will not give here. You can easily find them if you want to.

Aries: April 18 – May 13

Taurus: May 13 – June 21

Gemini: June 21 – July 20

Cancer: July 20 – August 10

Leo: August 10 – September 16

Virgo: September 16 – October 30

Libra: October 30 – November 23

Scorpio: November 23 – November 29

Ophiuchus: November 29 – December 17

Sagittarius: December 17 – January 21

Capricorn: January 20 – February 16

Aquarius: February 16 – March 11

Pisces: March 11 – April 18

Just one further point – it will be difficult to see the constellation in the month of its “sign”, since it is too close to the Sun, which means the sky will be too bright to see it when it is up.

Some of the zodiacs are dim, and others are very bright. We’ll get into the easiest ones to see on another occasion.

For now, your friendly Milton astronomer signing off.

See also: Skies Over Milton, December Edition.


Wikipedia. (2018, December 20). Cetus. Retrieved from https://en.wikipedia.org/wiki/Cetus.

Wikipedia. (2018, December 20). Ecliptic. Retrieved from https://en.wikipedia.org/wiki/Ecliptic#Plane_of_the_Solar_System.

Wikipedia. (2018, December 20). Ophiuchus. Retrieved fromhttps://en.wikipedia.org/wiki/Ophiuchus

Wikipedia. (2018, December 20). Sidereal and Tropical Astrology. Retrieved from https://en.wikipedia.org/wiki/Sidereal_and_tropical_astrology.

Wikipedia. (2018, December 20). Zodiac. Retrieved from https://en.wikipedia.org/wiki/Zodiac.

Author: Peter Forrester

I have been interested in astronomy and stargazing for many years, and now delight to offer some of my learning to others through my weekly blog posts.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: