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Introduction to Ecliptic

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The plane of the Earth’s orbit around the sun is known as the ecliptic. From the observer’s perspective who is on the Earth, the movement of the sun around the celestial sphere with a period of one year can trace the path along the ecliptic that has a background of stars against it. It is important to reference the plane and acts as a basis for the ecliptic coordinate system. 

The ecliptic forms as one of the fundamental planes that are used as a reference for the positions these planes are the celestial sphere and the celestial equator. The ecliptic poles are perpendicular to the ecliptic, where the North ecliptic pole is the pole of the North equator. To one of the two fundamental planes, the ecliptic is closer to the unmoving background stars. The spherical coordinates are known as celestial latitude and longitude or ecliptic latitude and longitude. Longitude is measured along the ecliptic from the vernal equinox positively to the eastwards from  0° to 360°, the same direction in which the Sun appears to move. The Latitude can be measured perpendicularly with the ecliptic, which is about -900 to the southwards or northward -900 to the poles of the ecliptic. Where the ecliptic itself has 00 latitudes. To measure a complete spherical position, a distance parameter is required. 

The ecliptic coordinates are convenient and required for specifying the positions of the objects of the Solar System, as most of the orbits of the planets have small inclinations to the ecliptic. Because of the Earth's orbit, the ecliptic moves very little and it is relative to the fixed reference with respect to the stars.

Ecliptic Plane

The apparent path followed by the sun throughout the year is known as the ecliptic. Since Earth takes one year to orbit around the sun and the apparent position of the sun takes around one year to make a complete circuit along the ecliptic. If there are more than 365 days in a year, then the sun moves less than 10 eastward every day. Thus the small difference in the position of the sun against the stars on the Earth’s surface causes a particular spot so that the sun can be caught up about four minutes each day if Earth did not orbit. Hence a day on the Earth is longer than 24 hours instead of 23 hours 56 minutes a sidereal day. The actual speed at which the Earth orbits the sun will vary every year such that the sun moves along the ecliptic also varies. 

In the Solar System, most of the major bodies orbit the Sun in nearly the same plane. This is due to the way through which the Solar System is formed from a protoplanetary disk. Probably the current closest representation of the disk in the solar system is called the invariable plane of the Solar System. Earth's orbit, and thus the ecliptic, is inclined a little more than 10 to the invariable plane, Jupiter's orbit is within a little more than ​10/2 of it, and all other major planets are present within about 60. Due to this reason, most of the bodies of the Solar system appear very close to the ecliptic in the sky.

The invariable plane of the solar system is defined by the angular momentum of the entire Solar System, that is essentially the vector sum of all of the rotational and orbital angular momenta of all the bodies of the system that have more than 60% of the total angular momentum comes from the orbit of Jupiter. Due to the uncertainty about the exact location of the invariable planes, and since the ecliptic is well defined by the sun’s apparent motion, as the reference plane of the Solar System the ecliptic is used both for precision and convenience. Using the ecliptic instead of the invariable plane has a drawback that is over the geologic time scales, that will move against the fixed reference points that are present in the distant background of the sky.

Obliquity of the Ecliptic

The term used by the astronomers to describe the inclination of the equator of the Earth with respect to the ecliptic of the Earth’s rotation axis that is perpendicular to the ecliptic. This term is also known as the obliquity of the ecliptic. Because of the planetary perturbations, it is about 23.4 degrees and it is decreasing per hundred years for about 0.013 degrees. By the observation of the motions of other planets and the Earth, the angular value of the obliquity can be found over many years. As the understanding of the dynamics increases, the astronomers produce new fundamental ephemerides as the accuracy of observation improves, and from these ephemerides, various astronomical values that include obliquity are derived.

Until 1983, the obliquity for any date was calculated from the work of Newcomb, who analyzed positions of the planets until about 1895:

ε = 230 271 0811 .26 -4611 .845T - 011 .0059T2 + 011 .00181T3

Where ε is the obliquity and T is tropical centuries.

From the year 1984, the fundamental ephemeris of the Astronomical Almanac was found in the Jet Propulsion Laboratory's DE series of computer-generated ephemerides. This obliquity was based on DE200, which analyzed some of the observations from 1911 to 1979, it was calculated as:

ε = 230 261 2111 .45 - 4611 .815T - 011 .0006T2 + 011 .00181T3

These expressions for the obliquity are planned for over a relatively short time span for the high precision, and perhaps for several centuries. All of these expressions are used for the mean obliquity, which means that the obliquity was found without the nutation of the equator. Whereas the true or instantaneous obliquity has been included with the nutation. Nutation can be defined as a rocking, nodding, or swaying motion of the largely axially symmetric object in the axis of rotation. 

Eclipses always occur on or near it, because the orbit of the Moon is inclined only about 5.1450 to the ecliptic and the Sun is always very near the ecliptic. Due to the inclination of the orbit of the Moon, eclipses do not occur at every conjunction and opposition of the Sun and Moon, but it occurs only when the Moon is near an ascending or descending node at the same time it is a conjunction or new moon or opposition or new moon. The ecliptic is named so because the ancients noted that eclipses only occur when the Moon is crossing it.


The ecliptic plane is the reference plane or the imaginary plane that contains the orbit of the Earth where it rotates around the sun. The ecliptic forms the centre of the zodiac, a celestial belt that is about 200 wide in latitude through which the Moon, Sun, and planets always appear to move. Traditionally, this region is divided into 12 signs of 200 longitudes, each of which approximates the Sun's motion in one month. In ancient times, the signs correspond roughly to 12 of the constellations that straddle the ecliptic. Sometimes these signs are still used in modern terminology. The "First Point of Aries'' was named when the March equinox sun was actually in the constellation Aries, it has since moved into Pisces because of the precession of the equinoxes. 

FAQ (Frequently Asked Questions)

1. What is the Ecliptic Meaning?

Ans: The ecliptic meaning is as follows, it is the plane of the Earth’s orbit around the sun.

2. Define Plane of Ecliptic.

Ans: The plane of the orbit of the Earth is projected in all the directions combined forms a reference plane, this plane is known as the plane of the ecliptic.