Definition
The ecliptic is the path the Sun appears to follow across the sky over the course of a year, measured against the background stars. It is a great circle on the celestial sphere: the projection onto the sky of the plane of Earth's orbit. Seen from Earth, the Sun moves eastward along it by a little less than 1° per day — 360° in 365.24 days — and completes the circle once a year.
The name comes from eclipses. A solar or lunar eclipse can only happen when the Moon is new or full at the same time as it lies on, or very close to, this line.
Obliquity: the tilt between ecliptic and equator
Earth's rotation axis is tilted relative to its orbital plane, so the ecliptic is inclined to the celestial equator. The angle between the two circles is the obliquity of the ecliptic, ε. Its mean value is given by the formula adopted by the International Astronomical Union and used in Astronomical Algorithms, where T is the number of Julian centuries since J2000.0:
ε = 23°26′21.448″ − 46.8150″ × T − 0.00059″ × T² + 0.001813″ × T³For 2026 this gives ε ≈ 23.436°. The obliquity is currently decreasing by about 47″ per century. Over much longer periods it oscillates between roughly 22.1° and 24.5° with a period of about 41,000 years — one of the Milankovitch cycles that influence Earth's climate. Nutation adds a small periodic wobble of up to about 9″ on top of this slow trend.
Equinoxes and solstices
The ecliptic and the celestial equator cross at two points 180° apart: the equinoxes. When the Sun reaches the vernal equinox around 20 March it crosses the equator heading north; at the autumnal equinox around 22–23 September it crosses heading south. Halfway between them, at the solstices, the Sun reaches its greatest distance north or south of the equator, and its declination equals the obliquity.
| Event | Approximate date | Sun's ecliptic longitude | Sun's declination |
|---|---|---|---|
| March equinox | 20 March | 0° | 0° |
| June solstice | 21 June | 90° | +23.44° |
| September equinox | 22–23 September | 180° | 0° |
| December solstice | 21–22 December | 270° | −23.44° |
Ecliptic coordinates
Because the planets orbit close to the same plane, the ecliptic is the natural reference for the solar system. Ecliptic longitude λ is measured eastward along the ecliptic from the vernal equinox, from 0° to 360°; ecliptic latitude β is measured north or south of it. The Sun's ecliptic latitude never exceeds about one arcsecond. The Moon's orbit is inclined about 5.1° to the ecliptic, and the planets stay within a few degrees of it; Mercury has the largest orbital inclination of the eight planets, 7°.
Converting ecliptic coordinates to equatorial coordinates (right ascension α and declination δ) is a rotation by the obliquity ε around the line of the equinoxes:
tan α = (sin λ × cos ε − tan β × sin ε) / cos λsin δ = sin β × cos ε + cos β × sin ε × sin λWhen computing α from its tangent, take care to place the result in the same quadrant as λ; in a program, the two-argument arctangent function does this automatically.
The ecliptic and the zodiac
The constellations the ecliptic passes through form the zodiac. Astronomically the ecliptic crosses 13 constellations, not 12: between Scorpius and Sagittarius it runs through Ophiuchus, where the Sun spends about 18 days, roughly from 29 November to 17 December. Because the official constellation boundaries are unequal, the Sun spends anywhere from about 7 days in Scorpius to about 45 days in Virgo.
The astrological signs are something else: twelve equal 30° segments of ecliptic longitude counted from the vernal equinox. Because precession moves the equinox westward by about 50″ per year, the signs have drifted by more than 20° from the constellations that gave them their names.
Finding the ecliptic in the night sky
You can trace the ecliptic without a chart. The Moon and the bright planets always lie close to it, so a line drawn through Venus, Jupiter, Saturn and the Moon on a given evening follows it well.
Its angle to the horizon changes with the seasons. For northern-hemisphere observers the evening ecliptic stands steepest around the March equinox, and the morning ecliptic around the September equinox. These are the best times to catch Mercury low in the twilight, or to see the faint cone of zodiacal light after dusk in spring and before dawn in autumn.
In the 3D illustration on this page, the ecliptic plane is drawn as a translucent disk and the axis of the equinoxes marks the direction of the vernal equinox. On the sky disk, follow the Moon and the planets: they line up along the same great circle.
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Frequently asked questions
Is the ecliptic the same as the celestial equator?
No. The celestial equator is the projection of Earth's equator onto the sky, while the ecliptic is the projection of Earth's orbit. The two circles are tilted by about 23.4° and cross at the equinoxes.
Why do the planets follow the ecliptic?
The solar system formed from a flattened disk of gas and dust, so all the planets orbit in nearly the same plane as Earth. Seen from Earth, they therefore always appear close to the ecliptic.
Does the ecliptic move?
Slowly. Perturbations by the other planets rotate the plane of Earth's orbit by about 47″ per century, and the obliquity varies between about 22.1° and 24.5° over some 41,000 years. Precession also moves the equinoxes along the ecliptic by about 50″ per year.
