Introduction to Ecliptic
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.
Conclusion:
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.
FAQs on Ecliptic
1. What is the ecliptic and how does it relate to the ecliptic plane?
The ecliptic is the apparent path that the Sun traces across the celestial sphere over the course of one year, as viewed from Earth. The ecliptic plane is the actual geometric plane containing Earth's orbit around the Sun. In essence, the ecliptic is the projection of the ecliptic plane onto the sky.
2. What is meant by the obliquity of the ecliptic?
The obliquity of the ecliptic is the angle between the ecliptic plane (Earth's orbital plane) and the celestial equator (the projection of Earth's equator into space). This angle is approximately 23.5 degrees and is the primary reason for the changing seasons on Earth.
3. What are the ecliptic constellations?
The ecliptic constellations are the group of constellations through which the Sun, Moon, and planets appear to move as seen from Earth. These are more commonly known as the constellations of the zodiac. Since the planets in our solar system orbit on nearly the same plane, they are always found in or near these specific constellations.
4. What is the fundamental difference between the ecliptic and the celestial equator?
The key difference lies in what they represent. The celestial equator is an extension of Earth's own equatorial plane into space. The ecliptic, however, is based on Earth's orbital path around the Sun. These two planes are not aligned; they are tilted at an angle of about 23.5 degrees to each other. This tilt is what defines the solstices and equinoxes.
5. Why is the ecliptic so important in the field of astronomy?
The ecliptic is fundamentally important because it serves as a primary reference plane for mapping the sky. Its main uses include:
- Defining Coordinate Systems: It is the basis for the ecliptic coordinate system, used to pinpoint the locations of celestial objects.
- Tracking Planets: The planets of our solar system have orbits that are only slightly inclined to the ecliptic, so they always appear close to this path in the sky.
- Predicting Eclipses: A solar or lunar eclipse can only occur when the Moon crosses the ecliptic plane during a new or full moon phase.
- Understanding Seasons: The tilt of Earth's axis relative to the ecliptic plane is directly responsible for the seasons.
6. Do all planets in our solar system orbit exactly on the ecliptic plane?
No, not exactly. While the ecliptic plane is defined by Earth's orbit, the other planets have their own orbital planes that are slightly inclined to it. However, the inclinations are very small for most planets (typically a few degrees). This is why all planets appear to travel within a narrow band in the sky centered on the ecliptic. Dwarf planets, like Pluto, can have much higher inclinations.
7. How does the concept of the ecliptic help explain the changing seasons on Earth?
The seasons are a direct consequence of Earth's axial tilt of 23.5 degrees relative to a line perpendicular to the ecliptic plane. As Earth orbits the Sun along this plane, the fixed tilt means that for part of the year, the Northern Hemisphere is tilted towards the Sun (experiencing summer), and six months later, it is tilted away (experiencing winter). The ecliptic provides the stable reference plane of orbit against which this constant tilt causes seasonal changes in direct sunlight.
