About the Ecliptic

Ecliptic is an astronomical term, not astrological, but it's fundamental for understanding many astrological concepts.

If you look up a definition of the ecliptic on the Internet, you'll find out that it is the apparent path of the Sun on the celestial sphere. However, I find this definition somewhat misleading, especially for non-technical people. When we hear about the path of the Sun, we usually imagine how the Sun rises in the morning in the East, culminates over the head at noon, and sets in the West in the evening, but that is not the movement that will help us to understand the ecliptic.

Why the sky is blue? Because the Earth has an atmosphere. If it didn't, we could see both the Sun and the stars in the daytime. So let's imagine for this lesson that there is no atmosphere and that today at noon we looked at the sky and noticed that the Sun was in a certain position, with the stars. If you have a spare sky map, you might have marked the position of the Sun on it.

Tomorrow at noon we'll look at the sky and notice that the position of the Sun against the stars has moved a little bit. We'll mark that new position on our map.

If we'll continue to monitor the position of the Sun against the stars like that for the whole year, we'll ultimately draw on the sky map a line, which on the real sky will be a circle. That's the path along which the Sun moves among the stars year after year. The Sun always moves along the same path, and it completes the circle in one year. This path is the ecliptic.

Of course, we know that it's the Earth rotating around the Sun, not the Sun rotating around the Earth, and this is why astronomers say that the ecliptic is the apparent path of the Sun. It appears to us, observers on the Earth, that the Sun moves around us.

All the other planets, as well as the Moon, move along the ecliptic but do not exactly follow it. If we mark in the sky the paths of all the planets over many years (as those paths somewhat change year after year), we'll get a road with the ecliptic as its centerline.

The Ecliptic and the Celestial Equator

Of course, you know what's the equator of the Earth, don't you? It's an imaginary line, or, a circle, surrounding the Earth and separating the Northern hemisphere from the Southern hemisphere. Everything to the North of the equator is in the Northern hemisphere, like Canada and the UK. Everything to the South of it is in the Southern hemisphere, like Argentina and Australia. This is easy, right?

Let's project the equator of the Earth onto the starry sky. It will be a big circle running around the whole sky and separating it into the Northern and Southern hemispheres. That will be the celestial equator. Everything in the sky to the North of it will be in the Northern hemisphere of the sky, and everything in the sky to the South of it will be in the Southern hemisphere.

We now know two big circles running around the whole sky: the ecliptic and the equator. How do they fit together, what's their relationship? As the image below shows, they intersect. The two points formed in the intersection are very important in astronomy and astrology, they are called the points of equinoxes. One of them is the point of the vernal equinox and the other one is the point of the autumnal equinox.

Now, as we just said, the celestial equator splits the whole sky and everything in it into two hemispheres, so it also splits the ecliptic into two halves. One half of the ecliptic, from the vernal equinox to the autumnal equinox, goes through the Northern hemisphere, and the other half, from the autumnal equinox to the vernal equinox, goes through the Southern hemisphere. You can also see in the picture that the angle between the ecliptic and the celestial equator is approximately 23 degrees.

The Travels of the Sun

So there is the ecliptic, which is the yearly path of the Sun among the stars, and there is the celestial equator, which is a projection onto the sky of the Earth's equator, and they intersect in two points, the Vernal Equinox and the Autumnal Equinox.

Now we are going to travel with the Sun along the ecliptic, which is a journey taking the whole year. One day, the Sun will arrive at the point of Vernal Equinox. You've probably guessed that this happens each year around the 21st of March. Why "around"? Because the precise moment when the Sun crosses the equator, which astronomers can define to a split of a second, is slightly different each year. What's typical for the day when this event takes place? The length of the day is equal to the length of the night in both hemispheres. That's because the Sun is exactly on the equator, and so sends equal amounts of warmth and light to the North and the South.

A similar situation occurs when the Sun reaches the point of Autumnal Equinox around the 22nd of September. But let's return to the 21st of April for now.

After crossing the celestial equator in the point of Vernal Equinox, the Sun continues to travel along the ecliptic, and so it enters the Northern Hemisphere of the sky. This means that the Sun is sending more of its energy to the North, and so in the Northern Hemisphere the days are becoming longer and longer while the nights are becoming shorter and shorter.

Around the 22nd of June, the Sun will reach the point that is marked as "Summer Solstice" in the picture. That's when the day in the Northern Hemisphere is the longest while the night is the shortest, while in the Southern Hemisphere it's the opposite.