The ecliptic is the apparent path that the Sun follows through the sky over the course of the year, relative to the stars, as seen from the vantage point of the Earth. It is traced on the imaginary celestial sphere, where that is intersected by the plane of the ecliptic, the geometric plane in which the Earth orbits the Sun. The name refers to the fact that eclipses occur when the full or new Moon meets the Sun on this path. Note that it does not refer to the hour-by-hour motion of the Sun as the Earth rotates, but the day-to-day motion of the Sun when observed at the same time each day.
Contents |
As the rotational axis of the Earth is not perpendicular to its orbital plane, the equatorial plane is not parallel to the ecliptic plane, but makes an angle of about 23°26', which is known as the axial tilt (or obliquity of the ecliptic).
The intersections of the equatorial and ecliptic planes with the celestial dome are great circles known as the celestial equator and the ecliptic respectively.
The intersection line of the two planes results in two diametrically opposite intersection points, known as the equinoxes. The equinox that the Sun passes from south to north is known as the vernal equinox or first point of Aries.
Ecliptic longitude, usually indicated with the letter ‹λ›, is measured from this point on 0° to 360° towards the east. Ecliptic latitude, usually indicated with the letter ‹φ› is measured +90° to the north or -90° to the south.
The same intersection point also defines the origin of the equatorial coordinate system, named right ascension measured from 0 to 24 hours also to the east and usually indicated with ‹α› or R.A., and declination, usually indicated with ‹δ› also measured +90° to the north or -90° to the south. Simple rotation formulas allow a conversion from α,δ to λ,β and back (see: ecliptic coordinate system).
The ecliptic serves as the center of a region called the zodiac, which constitutes a band of 9° on either side. Traditionally, this region is divided into 12 signs of 30° longitude each. By tradition, these signs are named after 12 of the 13 constellations straddling the ecliptic. The zodiac signs are very important to many astrologers.
Modern astronomers typically use other coordinate systems today.
The position of the vernal equinox is not fixed among the stars but due to the lunisolar precession slowly shifting westwards over the ecliptic with a speed of 1° per 72 years. A much smaller north/southwards shift can also be discerned (the planetary precession, along the instantaneous equator, which results in a rotation of the ecliptic plane). Said otherwise, the stars shift eastwards (increase their longitude) measured with respect to the equinoxes — in other words, as measured in ecliptic coordinates and (often) also in equatorial coordinates.
Using the current official IAU constellation boundaries — and taking into account the variable precession speed and the rotation of the ecliptic — the equinoxes shift through the constellations in the years (expressed in astronomical year numbering in which the year 0 = 1 BC, -1 = 2 BC, etc.) as follows:[1]
| UTC date and time of solstices and equinoxes[2] | ||||||||
|---|---|---|---|---|---|---|---|---|
| year | Equinox Mar |
Solstice June |
Equinox Sept |
Solstice Dec |
||||
| day | time | day | time | day | time | day | time | |
| 2007 | 21 | 00:07 | 21 | 18:06 | 23 | 09:51 | 22 | 06:08 |
| 2008 | 20 | 05:48 | 20 | 23:59 | 22 | 15:44 | 21 | 12:04 |
| 2009 | 20 | 11:44 | 21 | 05:45 | 22 | 21:18 | 21 | 17:47 |
| 2010 | 20 | 17:32 | 21 | 11:28 | 23 | 03:09 | 21 | 23:38 |
| 2011 | 20 | 23:21 | 21 | 17:16 | 23 | 09:04 | 22 | 05:30 |
| 2012 | 20 | 05:14 | 20 | 23:09 | 22 | 14:49 | 21 | 11:11 |
| 2013 | 20 | 11:02 | 21 | 05:04 | 22 | 20:44 | 21 | 17:11 |
| 2014 | 20 | 16:57 | 21 | 10:51 | 23 | 02:29 | 21 | 23:03 |
| 2015 | 20 | 22:45 | 21 | 16:38 | 23 | 08:20 | 22 | 04:48 |
| 2016 | 20 | 04:30 | 20 | 22:34 | 22 | 14:21 | 21 | 10:44 |
| 2017 | 20 | 10:28 | 21 | 04:24 | 22 | 20:02 | 21 | 16:28 |
Due to the inclination of the Moon's orbit and the resulting movement of Earth around the barycenter, and due as well to the perturbing influences on the Earth's orbit by the other planets, the true Sun is not always exactly on the ecliptic for a hypothetical observer at Earth's center, but may be some arcseconds north or south of it. It is therefore the centre of the mean Sun that outlines its path. As the Earth takes one year to make one complete revolution around the Sun, the apparent position of the Sun also takes the same length of time to make a complete circuit of the whole ecliptic. With slightly more than 365 days in the year, the Sun moves almost 1° eastwards every day (direction of increasing longitude). This annual motion should not be confused with the daily motion of the Sun (and the stars, and indeed the whole celestial sphere for that matter) towards the west along the equator every 24 hours. In fact, where the stars need about 23h 56m for one such rotation to complete the sidereal day, the Sun, which has shifted 1° eastwards during that time needs 4 minutes extra to complete its circle, making the solar day about 24 hours.
The distance between Sun and Earth varies slightly during the year, so the speed with which the Sun moves along the ecliptic also varies. For example, within one year, the Sun is north of the equator for about 186.40 days and south of the equator for about 178.24 days.
The mean Sun crosses the equator around 20 March at the time of the vernal equinox, when its declination, right ascension, and ecliptic longitude are all zero. (The mean sun's ecliptic latitude is always zero.) The March equinox marks the onset of spring in the northern hemisphere and autumn in the southern. The actual date and time varies from year to year because of the occurrence of leap years. It also shifts slowly over the centuries due to imperfections in the Gregorian calendar.
Ecliptic longitude 90°, at right ascension 6 hours and a northern declination equal to the obliquity of the ecliptic (23.44°), is reached around 21 June. This is the June solstice - or summer solstice in the northern hemisphere and winter solstice in the southern hemisphere. It is also the first point of Cancer and directly overhead on Earth on the tropic of Cancer so named because the Sun turns around in declination. Ecliptic longitude 180°, right ascension 12 hours is reached around 22 September and marks the second equinox or first point of Libra. Due to perturbations to the Earth's orbit, the moment the real Sun passes the equator might be several minutes earlier or later. The southernmost declination of the sun is reached at ecliptic longitude 270°, right ascension 18 hours at the first point of the sign of Capricorn around 21 December.
These traditional signs (in western tropical astrology) have given their names to the solstices and equinoxes, but in reality (as from the list in the previous chapter) the cardinal points are currently situated in the constellations of Pisces, Taurus, Virgo and Sagittarius respectively, due to the precession of the equinoxes.
Of the eight planets, the orbital plane of Mercury has the greatest difference from Earth's at 7° orbital inclination; other planets' inclinations range up to 3.39°. Pluto's, at 17°, was previously an exception until it was reclassified a dwarf planet, and other non-planetary bodies in the Solar System have even greater orbital inclinations (e.g. Eris at 44° and Pallas at 34°). Interestingly, the Earth has the most inclined orbit of all eight major planets relative to the Sun's equator, with the giant planets close behind.
| Inclination | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Name | Inclination to ecliptic |
Inclination to Sun's equator |
Inclination to invariable plane[3] |
||||||||
| Terrestrials | Mercury | 7.01° | 3.38° | 6.34° | |||||||
| Venus | 3.39° | 3.86° | 2.19° | ||||||||
| Earth | 0° | 7.155° | 1.57° | ||||||||
| Mars | 1.85° | 5.65° | 1.67° | ||||||||
| Gas giants | Jupiter | 1.31° | 6.09° | 0.32° | |||||||
| Saturn | 2.49° | 5.51° | 0.93° | ||||||||
| Uranus | 0.77° | 6.48° | 1.02° | ||||||||
| Neptune | 1.77° | 6.43° | 0.72° | ||||||||
The intersection line of the ecliptical plane and another planet's orbital plane is called the nodal line of that planet, and the nodal line's intersection points on the celestial sphere are the ascending node (where the planet crosses the ecliptic from south to north) and the diametrically opposite descending node. Only when an inferior planet passes through one of its nodes can a transit over the Sun take place. Transits, especially for Venus, are quite rare, because the Earth's orbit is more inclined than those of the inner two planets.
Inclination and nodal lines, as almost all other orbital elements, change slowly over the centuries due to perturbations from the other planets.
The orbit of the Moon is inclined by about 5° to the ecliptic. Its nodal line is not fixed, but regresses (moves towards the west) over a full circle every 18.6 years. This is the cause of nutation and lunar standstill. The moon crosses the ecliptic about twice per month. If this happens during new moon a solar eclipse occurs, during full moon a lunar eclipse. This was the way the ancients could trace the ecliptic along the sky; they marked the places where eclipses could occur.
Up to the 17th century in Europe, star maps and positions in star catalogues were always given in ecliptical coordinates, though in China, astronomers employed an equatorial system in their catalogues. It was not until astronomers started to use telescopes and mechanical clocks to measure star positions that equatorial coordinates came into use, which occurred so exclusively that nowadays ecliptical coordinates are no longer used. Nonetheless, this change is not always desirable, as a planetary conjunction would be much more illustratively described by ecliptic coordinates rather than equatorial.
Also see zodiacal coordinates.
