Don’t be put off by the title of this blog, I’ll try to keep it simple 🙂
In Greek/Roman times it was thought that Earth was the center of the Universe. The Sun , the planets, the stars were all rotating around Earth.
Now we know that Earth itself is rotating, in 24 hours (although we keep saying that the sun rises in the East and sets in the West).
We also know that Earth is orbiting around the Sun in 365 days. As seen from Earth, the Sun seems to be moving across the sky. This apparent path of the Sun throughout the year is called the ecliptic.
If Earth would be rotating like in the picture below, then during the whole year, the Sun would be above the Equator and there would be no seasons. . Day and night would be both 12 hour, everywhere on Earth, throughout the year.
But everybody knows that this is not the case. In the Northern hemisphere. daylight will be longer than night in summer and shorter in winter, whereas in the Southern hemisphere it is just the other way around. The reason is that the axis of rotation of the Earth is slightly tilted, about 23.5° .
Notice that during Earth’s orbit, the direction of the rotation axis remains the same (pointing to the North Star).
In the picture below we can see what happens in a bit more detail. Let’s start with the 1st Day of Spring (20/21 March), when the tilt is not directed towards the Sun and the Sun is directly over the equator. Day and night are equally long all over Earth, it is called the March Equinox. The same happens on the 1st Day of Autumn (22/23 September), the September Equinox.
From March until June, the Sun moves North, days become longer and nights shorter until the 1st Day of Summer, 21/22 June, the Summer Solstice, after which the Sun moves back to the Equator. From September to December, the Sun moves South, days become shorter and nights longer, until the 1st Day of Winter, 21/22 December, the Winter Solstice, after which the Sun moves back again to the Equator. All this has been described from the perspective of the Northern Hemisphere. For the Southern Hemisphere everything is opposite.
So the axial tilt of Earth is responsible for the seasons and there are scientists who believe that this tilt has been critical for life: Axis Tilt is Critical for Life , but this is quite controversial.
Until now I have been rather vague about the apparent position of the Sun. “Above the Equator”, “moving North”, “moving South”. Actually, we can be much more precise nowadays, using a coordinate system similar to what we use on Earth, with latitude and longitude. But how did the Greek and Romans do that?
To describe the location of the Sun against the backdrop of stars, the Greeks used the concept of constellations, patterns of stars that were given a name, often based on their myths and religion. A constellation most of you will know is Orion, the mythological Greek hunter. The left photo was taken by the Hubble telescope and shows the characteristic shape of Orion. Right an antique star chart, published in 1824. To guide the eye, I have connected the main stars with blue lines
During a year, the Sun crosses twelve of these constellations , as indicated in the image below. These twelve constellations are called the Zodiac. The names of the constellations may be familiar to you, if you know your (Western) horoscope 😉 .
At the moment that this post has been published , the Sun has left the Leo constellation and entered Virgo.
Nowadays many more constellations have been defined (88!), covering the whole sky. Below you see a map of the celestial globe. The Celestial globe is similar to the Earth globe, but everything is projected to the “sky”.
Notice that the horizontal scale (Right Ascension) is not in degrees, like longitude on the Earth globe, but in hours and going from right to left. Same as on the Earth globe the choice of the origin (zero) is arbitrary. On the Earth globe it is the Greenwich meridian, on the Celestial Globe it is the March Equinox. The traditional name for this origin is the First Point of Aries.
The name was coined by the Greek astronomer Hipparchus, who lived from c 190 BC until c 120 BC. At that time the Sun had just entered the Aries constellation during the Spring Equinox. The March Equinox is indicated by the Aries symbol ♈︎ .
The ecliptic is shown, with the constellations (in white) which the Sun is passing throughout a year. Following the ecliptic from right to left, you will notice that it starts at 0h in Pisces, the Sun moves North until 6h (Gemini) then back to the equator at 12h (Virgo), going down South until 18h (Sagittarius) and back to Pisces for the next cycle.
Do you notice the contradiction? The First Point of Aries is now located in Pisces! How can that be? Time to talk about the Precession of Earth!
Above I wrote that the rotation axis of Earth, during its orbit around the Sun, is always pointing in the same direction (to the North Star). That is correct at the moment, but not forever, because there is a third movement of Earth.
Probably many of you have been playing with spinning tops. When a top (or a gyroscope) is spinning, it will often “wobble” under the influence of the force of gravity. The same happens with the Earth under the influence of the gravitational force of Sun and Moon. This wobble is called Precession.
Here is a short video about the precession of a gyroscope
The Precession of Earth is a very slow process. One round takes about 25800 year. Here is an animation of the process.
The red line connects the two equinoxes, the March Equinox is marked with the Aries symbol. Notice how the March Equinox passes all signs of the zodiac during one precession. 360° in ~ 25800 year, means a shift of 1° every 72 year. Since Hipparchus’ time, the First Point of Aries has shifted ~30° , it is no longer in Aries, but in Pisces and will cross over to Aquarius
around 2600 AD.
The second part of the video shows how the North pole points to different stars during a precession. In the picture you see that “today” it is pointing at Polaris (the North Star) , but in 3000 BC it pointed at the star Thuban, and in 14000 AD it will point close to the bright star Vega.
To summarise, here is a picture, with the three movements of Earth, the daily rotation (24 hours), the yearly orbit around the Sun (~365 days) and the precession of the rotation axis (~25800 years).
Notice that the rotation of Earth and its orbit around the Sun are counterclockwise , while the precession is clockwise! So the yearly motion of the Sun through the Zodiac goes from Pisces to Aries to Taurus etc. But the Spring Equinox has, since Hipparchus , shifted back from Aries to Pisces and will shift to Aquarius around 2600 AD.
Let me end this post with some explanation about why I decided to write it.
Everything written above belongs to the realm of astronomy. Unfortunately there exists also astrology, a pseudoscience. Basically astrology states that the position of the heavenly bodies affects our lives. For example, an important factor is the location of the Sun on the day that you were born. That becomes the sign of your horoscope. Here is an example:
I was born on 17 April, therefore I am a Ram. But that is nonsense, maybe true in Hipparcos’ time. Because of the precession I was actually born when the Sun was in Pisces (12 March – 19 April)
Of course astrologers are aware of the precession, see for example
Sidereal and Tropical Astrology. for various “solutions”.
Directly related to precession is the concept of Astrological Ages. As explained above, during one precession of 25800 year, the March Equinox traverses all 12 signs of the Zodiac, staying in one sign on average 2150 year. Such a period is called an Astrological age. At the moment we live in the Age of Pisces.
Astrologers claim that each age is characterised by certain properties. For example the Age of Pisces is the age of religion, the Age of Aries was the age of war and the coming Age of Aquarius will be the age of freedom.
You will not be surprised that in my opinion these Astrological Ages are even more nonsense than horoscopes.
A few months ago a friend told me about Matias de Stefano, who calls himself an Indigo Child and has memories from an earlier life in Atlantis. He is 29 year old and has many followers. I listened to his video lecture Total Recall. Here is a transcription. One quote to give you an impression of the lecture:
We are Beings (indigo/crystal) that come from the 6th to 13th dimension, to try to promote the 4th and 5th dimension inside the 3rd.
But the following statement is so ridiculous that I decided to write this post.
Earth spins around the sun in a process which lasts about 365 days, but at the same time, our Sun spins around another sun which is a lot bigger, called Syria, about every 26,000 years. As a year on Earth, the Sun’s year has its seasons, equinoxes, solstices and ages, too.
Apparently he has not the faintest idea what the Precession of the Equinox means.
I will end this post with a famous song from the 1967 musical Hair. ” This is the dawning of the age of Aquarius “