Ecliptic

[1][2][a] It was a central concept in a number of ancient sciences, providing the framework for key measurements in astronomy, astrology and calendar-making.

From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic against the background of stars – specifically the Zodiac constellations.

Ancient scientists were able to calculate Earth's axial tilt by comparing the ecliptic plane to that of the equator.

[8] Because of the movement of Earth around the Earth–Moon center of mass, the apparent path of the Sun wobbles slightly, with a period of about one month.

[6] The crossing from south to north is known as the March equinox, also known as the first point of Aries and the ascending node of the ecliptic on the celestial equator.

The gravitational perturbations of the other bodies of the Solar System cause a much smaller motion of the plane of Earth's orbit, and hence of the ecliptic, known as planetary precession.

The combined action of these two motions is called general precession, and changes the position of the equinoxes by about 50 arc seconds (about 0.014°) per year.

[16] From 1984, the Jet Propulsion Laboratory's DE series of computer-generated ephemerides took over as the fundamental ephemeris of the Astronomical Almanac.

The Astronomical Almanac for 2010 specifies:[18] ε = 23°26′21.406″ − 46.836769″ T − 0.0001831″ T2 + 0.00200340″ T3 − 0.576×10−6″ T4 − 4.34×10−8″ T5 These expressions for the obliquity are intended for high precision over a relatively short time span, perhaps several centuries.

The only drawback of using the ecliptic instead of the invariable plane is that over geologic time scales, it will move against fixed reference points in the sky's distant background.

Of the two fundamental planes, the ecliptic is closer to unmoving against the background stars, its motion due to planetary precession being roughly 1/100 that of the celestial equator.

Longitude is measured positively eastward[6] 0° to 360° along the ecliptic from the March equinox, the same direction in which the Sun appears to move.

Because of the precessional motion of the equinox, the ecliptic coordinates of objects on the celestial sphere are continuously changing.

[4] The exact instants of equinoxes and solstices are the times when the apparent ecliptic longitude (including the effects of aberration and nutation) of the Sun is 0°, 90°, 180°, and 270°.

[31][32] The ecliptic forms the center of the zodiac, a celestial belt about 20° wide in latitude through which the Sun, Moon, and planets always appear to move.

As seen from the orbiting Earth , the Sun appears to move with respect to the fixed stars , and the ecliptic is the yearly path the Sun follows on the celestial sphere . This process repeats itself in a cycle lasting a little over 365 days .
The plane of Earth 's orbit projected in all directions forms the reference plane known as the ecliptic. Here, it is shown projected outward (gray) to the celestial sphere , along with Earth's equator and polar axis (green). The plane of the ecliptic intersects the celestial sphere along a great circle (black), the same circle on which the Sun seems to move as Earth orbits it. The intersections of the ecliptic and the equator on the celestial sphere are the equinoxes (red), where the Sun seems to cross the celestial equator.
Obliquity of the ecliptic for 20,000 years, from Laskar (1986). [ 15 ] Note that the obliquity varies only from 24.2° to 22.5° during this time. The red point represents the year 2000.
The apparent motion of the Sun along the ecliptic (red) as seen on the inside of the celestial sphere . Ecliptic coordinates appear in (red). The celestial equator (blue) and the equatorial coordinates (blue), being inclined to the ecliptic, appear to wobble as the Sun advances.
Inclination of the ecliptic over 200,000 years, from Dziobek (1892). [ 27 ] This is the inclination to the ecliptic of 101,800 CE. Note that the ecliptic rotates by only about 7° during this time, whereas the celestial equator makes several complete cycles around the ecliptic. The ecliptic is a relatively stable reference compared to the celestial equator.
As the Earth revolves around the Sun, approximate axial parallelism of the Moon's orbital plane ( tilted five degrees to the ecliptic) results in the revolution of the lunar nodes relative to the Earth. This causes an eclipse season approximately every six months, in which a solar eclipse can occur at the new moon phase and a lunar eclipse can occur at the full moon phase.
At Earth's poles the Sun appears at the horizon only and all day around equinox , marking the change between the half year long polar night and polar day . The picture shows the South Pole right before March equinox, with the Sun appearing through refraction despite being still below the horizon.
Equirectangular plot of declination vs right ascension of the modern constellations with a dotted line denoting the ecliptic. Constellations are colour-coded by family and year established.