Ecliptic . It is the projection of the orbital plane of the Earth on the celestial Sphere . The apparent path that the Sun makes in one year through the stars is also indicated with the name of ecliptic: more precisely, through the twelve well-known constellations of the Zodiac. Since the plane of the Earth’s orbit is inclined approximately 23.5º with respect to the Equator, the ecliptic is inclined at the same value with respect to the celestial equator. The ancients thus called the line of heaven in which eclipses occur. Logically, it coincides with the line that marks the plane of the Earth’s orbit around the Sun, which is practically the same as that of the rest of the planets and the same as the plane of rotation of the Moon around the Earth.


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  • 1 History
  • 2 Etymology
  • 3 Description
  • 4 Plane
  • 5 The ecliptic and the equator
    • 1 The celestial equator
    • 2 The equinoctial points
  • 6 The Path of the Sun
    • 1 Signs of the Zodiac
    • 2 The Planets and the Moon
  • 7 Sources


The ancients called the ecliptic the line of the sky in which eclipses occurred, which coincides with the line of the apparent annual path of the Sun through the constellations of the Zodiac. The cosmology of Antiquity described the movement of the animated Sun in two movements, one daily from East to West and the other retrograde, from 1 daily to the East, whose projection on the celestial sphere they called Ecliptic. The plane of the Ecliptic is inclined with respect to the plane of the equator. The obliqueness of the Ecliptic was measured by the Greek astronomer Eratosthenes in the 3rd century BC. C., giving it a value of 23º 51 ’19 “, although some historians suggest that its calculation was 24º, the data being due to subsequent observations by Claudio Ptolomeo .


The word “ecliptic” means “line of eclipses”. They named it because they discovered it by observing eclipses of the moon. If every time there is an eclipse of the moon, you measure the distance between the eclipsed moon and the stars, you can paint the position of that moon on the star map. After five or six eclipses you could see that the eclipsed moon is always close to the same line on the map. This is not the case with unexplained moons. If you occasionally paint the position of the unshadowed moons on the star map, you will see that they are scattered across a ring in the sky , half the width of your open hand.


In its journey around the Sun, the Earth maintains this inclination always in the same direction, causing the climatic Seasons, due to the different incidences of the beam of sunlight, on the Earth’s surface, a topic that we will delve into later. The projection of the Ecliptic on the Celestial Sphere, forms a maximum circle that is inclined with respect to it, 23 ° 27 ‘. The perpendicular incidence of sunlight beams sweep almost 47º (exactly 46º 54´) on the globe . When they hit 23º 27´ North Latitude, they reach the so-called Tropic of Cancer ( June 21). When they hit 23º 27´ South Latitude, the Tropic of Capricorn. These are the maximum and minimum points that the Sun will reach in its imaginary movement through the sky. These points are called Solstices , from the Latin Solstitium, which mean “the Sun furthest away”.

The names of the Tropics are determined by the constellations of Cancer, in the Northern Hemisphere of the Celestial Sphere and of Capricorn, in the South. Similarly, there are two points where the Celestial Ecuador and the Ecliptic intersect. These are the Vernal Point located in the constellation of Los Peces (Pisces) and the Autumn Point (d) located in the constellation of La Virgen (Virgo). The Vernal Point represents in the celestial coordinates what the Greenwich Meridianin the terrestrial coordinates, that is to say the origin point of the celestial coordinates. At these two points, the beams of sunlight strike the Earth’s Equator perpendicularly, illuminating the entire planet uniformly. These points are called Equinoxes, from the Latin Aequus Nox, which means “equal duration of the nights”. In its annual journey, the Earth reaches these points on March 21 and September 21 , respectively.


The plane of the ecliptic is called precisely the plane of that path which, with the plane of the Celestial Equator, that is (the projection of the Earth’s equator on the Earth’s sphere) forms an angle of 23º and 27 ‘. This angle is known as the obliqueness of the ecliptic and its constancy is firm for millions of years. It is known today, that it is decreasing in each century, about 48 arc seconds. This decrease will continue for a few thousand more years, until it reaches a minimum of 22º and 54 ‘until that angle increases again. The ecliptic cuts the plane of the celestial equator at two points. These two points, called equinoxes (equi-noccios = equal nights) or nodes. So we can say that the Sun is at the Spring Equinox, also called Vernal Point around March 21 and at the autumn equinox near September 23. In the middle of the route between both equinoxes, we will find the summer and winter solstices.

Those points, the solstices, are reached by the Sun approximately around June 21 and December 22respectively. Logically, the names of these four points are the same as the four seasons that begin successively on those dates in the Northern Hemisphere. Turning the plane of the Equator relative to the plane of the Ecliptic causes the equinoxes to be non-fixed. The duration of each turn of the equator is about 25,868 years. This movement of the equinoxes is called Precession and if you want to know the actual position of a star at any given time, you must proceed to correct that precession in the position of the celestial charts. In Astronomy, the Ecliptic is also used, as the essential circle used for a coordinate system called precisely “ecliptic coordinate system”. So that,

The ecliptic and the equator

The celestial equator

The celestial equator or equinoctial line is the plane perpendicular to the axis of rotation of the earth, which passes through the center of the earth. Geostationary satellites (such as television satellites, Meteosat …) are on this plane. Speaking of star maps, the celestial equator is the line on the map that passes through the exact east and exact west, and which follows the direction of the daily turn of the stars. On the map of the chapter that is the zodiac, I have painted the equator in red. The word “Ecuador” means “line of equality”. It is so called because the stars in this line spend as much time above the horizon as they do below. The same happens to the sun when you step on this line; that happens twice a year, at the equinoxes.

Equinoctial points

They are the two points on the star map where the equator and the ecliptic intersect. Are two; the one is called “Aries point” or “vernal point”, and the other “Libra point”. Go to the map of the page that is the zodiac and set the date to March 20 . On that date, the sun is near the Aries point, which is one of the two crossing points of the two yellow and red lines. On September 23 the sun is near the other crossing point, the so-called “Libra point”.

A curious thing is that if you count the days that pass from March 20 to September 23, and then the days that go from September 23 to the next March 20, you will see that the sun is not the same time in both halves of the circle. Spend a week longer in one half than the other. In order to realize that this does not take great technology , it is enough to plant a couple of sticks on the ground and look at the shadow of the sun for a year, so it should not surprise us that Greek astronomers were well aware of this and they look for an explanation to him.

The same two yellow and red lines can be seen drawn on the constellation map. In it you will see that neither the Aries point is in the constellation of Aries, nor the Libra point in that of Libra. That’s because when the points were named, the red line was elsewhere on the map. It has moved because of the precession of the equinoxes.

The Path of the Sun

Signs of the zodiac

Even thinking that the planets move in the celestial sphere, they do not roam all over it but are confined in a narrow strip that divides it in half. The stars are traditionally divided along this strip into the 12 zodiac constellations. This name, related to “zoo”, is due to the fact that most of these constellations have names of animals – Leo the lion, Aries the ram, Scorpio the scorpion, Cancer the crab, Pisces the fish, Capricorn the goat and Taurus the bull. Also the Sun is always somewhere in the celestial sphere and due to the rotation of the Earth, it rises and sets in the same way as the stars do.

Like planets, the Sun also moves through the zodiac making a complete circuit every year. Each month covers a different zodiac constellation, which is why there are 12 constellations. During that month that particular constellation is not seen, because the area of ​​the sky closest to the Sun is too bright for the stars to be seen (except, very briefly, during a total eclipse of the Sun).

However, you can calculate where the Sun is located in the zodiac (as the ancient astronomers did) by observing which is the last constellation of the zodiac to rise in front of the Sun or the first to get behind it. The Sun will be somewhere between the two. In this way, each month-long period throughout the year has its “zodiac sign.” Astrologers believe that stars mysteriously rule their lives and claim that there is a big difference depending on “under what sign” a person has been born. However, you should know that the “sign” assigned to each month in the horoscope is not the constellation where the Sun is that month, but where it has been in remote times. The difference is explained in the section on the precession of the equinoxes.

The Planets and the Moon

The planets seen in the sky are always close to the ecliptic, which means that their orbits are never very far from their plane. In other words, the solar system is fairly flat, with most moving almost on the same plane. The Moon’s orbit cuts the ecliptic at a small angle of about 5 degrees, which means that the Moon is also making its way through the celestial sphere through the zodiac. Half the time the Moon is to the north of the ecliptic and the other half to the south. If the Moon’s shadow meets Earth, the Sun is overshadowed by the shaded area; if, on the other hand, the shadow of the Earth covers the Moon, the Moon darkens and we have an eclipse of the Moon. Both eclipses can only occur when the Sun, Earth, and Moon are online.

When the Sun and Earth are on the ecliptic plane, the line is automatically on the same plane; If the Moon is also on the same line, it will also be on the plane of the ecliptic. It takes about a month for the Moon to make a complete turn around Earth, and during this time its orbit crosses the ecliptic twice, when it passes from one side of the orbit to the other. At the time of the crossing, the Sun can be anywhere along the ecliptic; it is not normally on the Earth-Moon line and therefore an eclipse does not take place. On certain occasions, however, she is in that line or close to it. If it then happens to occupy exactly the same point in the celestial sphere, we have an Eclipse of the Sun, since the Moon is between the Sun and us. On the other hand, if it occupies the exact opposite point to that of the Moon, the shadow of the Earth is cast on the Moon and we have a Moon Eclipse .


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