The ecliptic is the apparent path of the Sun on the celestial sphere, and is the basis
for the ecliptic coordinate system.
It also refers to the plane of this path, which is coplanar with both the orbit of the
Earth around the Sun and the apparent orbit of the Sun around the Earth.
The path of the Sun is not normally noticeable from the Earth's surface because the Earth
rotates, carrying the observer through the cycle of sunrise and sunset, obscuring the
apparent motion of the Sun with respect to the stars.
Sun's apparent motion The motions as described above are simplifications.
Due to the movement of the Earth around the Earth-Moon center of mass, the apparent path
of the Sun wobbles slightly, with a period of about one month.
Due to further perturbations by the other planets of the Solar System, the Earth-Moon
barycenter wobbles slightly around a mean position in a complex fashion.
The ecliptic is actually the apparent path of the Sun throughout the course of a year.
As the Earth takes one year to make a 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 ecliptic.
With slightly more than 365 days in one year, the Sun moves a little less than 1° eastward
every day.
This small difference in the Sun's position against the stars causes any particular spot
on the Earth's surface to catch up with the Sun about 4 minutes later each day than it
would if the Earth did not orbit; our day is 24 hours long rather than the approximately
23-hour 56-minute sidereal day.
Again, this is a simplification, based on a hypothetical Earth that orbits at uniform
speed around the Sun.
The actual speed with which the Earth orbits the Sun varies slightly during the year, so
the speed with which the Sun seems to move along the ecliptic also varies.
For example, the Sun is north of the celestial equator for about 185 days of each year, and
south of it for about 180 days.
The variation of orbital speed accounts for part of the equation of time.
Relationship to the celestial equator
As the rotational axis of the Earth is not perpendicular to its orbital plane, the Earth's
equatorial plane is not coplanar with the ecliptic plane, but is inclined to it by an
angle of about 23.4°, which is known as the obliquity of the ecliptic.
If the equator is projected outward to the celestial sphere, forming the celestial equator,
it crosses the ecliptic at two points known as the equinoxes.
The Sun, in its apparent motion along the ecliptic, crosses the celestial equator at
these points, one from south to north, the other from north to south.
The crossing from south to north is known as the vernal equinox, also known as the first
point of Aries and the ascending node of the ecliptic on the celestial equator.
The crossing from north to south is the autumnal equinox or descending node.
The orientation of the Earth's axis and equator are not fixed in space, but rotate about the
poles of the ecliptic with a period of about 26,000 years, a process known as lunisolar
precession, as it is due mostly to the gravitational effect of the Moon and Sun on the Earth's
equatorial bulge.
Likewise, the ecliptic itself is not fixed.
The gravitational perturbations of the other bodies of the Solar System cause a much smaller
motion of the plane of the 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 per year.
Once again, this is a simplification.
Periodic motions of the Moon and apparent periodic motions of the Sun cause short-term
small-amplitude periodic oscillations of the axis of the Earth, and hence the celestial
equator, known as nutation.
This adds a periodic component to the position of the equinoxes; the positions of the celestial
equator and equinox with fully updated precession and nutation are called the true equator and
equinox; the positions without nutation are the mean equator and equinox.
Obliquity of the ecliptic
Obliquity of the ecliptic is the term used by astronomers for the inclination of Earth's
equator to the ecliptic, or of Earth's rotation axis to a perpendicular to the ecliptic.
It is about 23.4° and is currently decreasing 0.013 degrees per hundred years due to planetary
perturbations.
The angular value of the obliquity is found by observation of the motions of the Earth
and planets over many years.
Astronomers produce new fundamental ephemerides as the accuracy of observation improves and
as the understanding of the dynamics increases, and from these ephemerides various astronomical
values, including the obliquity, are derived.
Until 1983 the obliquity for any date was calculated from work of Newcomb, who analyzed
positions of the planets until about 1895: ε = 23° 27′ 08″.26 − 46″.845 T − 0″.0059
T2 + 0″.00181 T3 where ε is the obliquity and T is tropical
centuries from B1900.0 to the date in question.
From 1984, the Jet Propulsion Laboratory's DE series of computer-generated ephemerides
took over as the fundamental ephemeris of the Astronomical Almanac.
Obliquity based on DE200, which analyzed observations from 1911 to 1979, was calculated:
ε = 23° 26′ 21″.45 − 46″.815 T − 0″.0006 T2 + 0″.00181 T3
where hereafter T is Julian centuries from J2000.0.
JPL's fundamental ephemerides have been continually updated.
The Astronomical Almanac for 2010 specifies: 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.
J. Laskar computed an expression to order T10 good to 0″.04/1000 years over 10,000
years.
All of these expressions are for the mean obliquity, that is, without the nutation of
the equator included.
The true or instantaneous obliquity includes the nutation.
Plane of the Solar System
Most of the bodies of the Solar System orbit the Sun in nearly the same plane.
This is likely due to the way in which the Solar System formed from a protoplanetary
disk.
Probably the closest current representation of the disk is known as the invariable plane
of the Solar System.
The Earth's orbit, and hence, the ecliptic, is inclined a little more than 1° to the
invariable plane, and the other major planets are also within about 6° of it.
Because of this, most Solar System bodies appear very close to the ecliptic in the sky.
The ecliptic is well defined by the motion of the Sun.
The invariable plane is defined by the angular momentum of the entire Solar System, essentially
the summation of all of the revolutions and rotations of all the bodies of the system,
a somewhat uncertain value which requires precise knowledge of every object in the system.
For these reasons, the ecliptic is used as the reference plane of the Solar System out
of convenience.
Celestial reference plane
The ecliptic forms one of the two fundamental planes used as reference for positions on
the celestial sphere, the other being the celestial equator.
Perpendicular to the ecliptic are the ecliptic poles, the north ecliptic pole being the one
north of the equator.
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.
Spherical coordinates, known as ecliptic longitude and latitude or celestial longitude and latitude,
are used to specify positions of bodies on the celestial sphere with respect to the ecliptic.
Longitude is measured positively eastward 0° to 360° along the ecliptic from the vernal
equinox, the same direction in which the Sun appears to move.
Latitude is measured perpendicular to the ecliptic, to +90° northward or -90° southward
to the poles of the ecliptic, the ecliptic itself being 0° latitude.
For a complete spherical position, a distance parameter is also necessary.
Different distance units are used for different objects.
Within the Solar System, astronomical units are used, and for objects near the Earth,
Earth radii or kilometers are used.
A corresponding right-handed rectangular coordinate system is also used occasionally; the x-axis
is directed toward the vernal equinox, the y-axis 90° to the east, and the z-axis toward
the north ecliptic pole; the astronomical unit is the unit of measure.
Symbols for ecliptic coordinates are somewhat standardized; see the table.
Ecliptic coordinates are convenient for specifying positions of Solar System objects, as most
of the planets' orbits have small inclinations to the ecliptic, and therefore always appear
relatively close to it on the sky.
Because the Earth's orbit, and hence the ecliptic, moves very little, it is a relatively fixed
reference with respect to the stars.
Because of the precessional motion of the equinox, the ecliptic coordinates of objects
on the celestial sphere are continuously changing.
Specifying a position in ecliptic coordinates requires specifying a particular equinox,
that is, the equinox of a particular date, known as an epoch; the coordinates are referred
to the direction of the equinox at that date.
For instance, the Astronomical Almanac lists the heliocentric position of Mars at 0h Terrestrial
Time, 4 Jan 2010 as: longitude 118° 09' 15".8, latitude +1° 43' 16".7, true heliocentric
distance 1.6302454 AU, mean equinox and ecliptic of date.
This specifies the mean equinox of 4 Jan 2010 0h TT as above, without the addition of nutation.
Eclipses
Because the orbit of the Moon is inclined only about 5° to the ecliptic and the Sun
is always very near the ecliptic, eclipses always occur on or near it.
Because of the inclination of the Moon's orbit, eclipses do not occur at every conjunction
and opposition of the Sun and Moon, but only when the Moon is near an ascending or descending
node at the same time it is at conjunction or opposition.
The ecliptic is so named because the ancients noted that eclipses only occurred when the
Moon crossed it.
Equinoxes and solstices
The exact instants of equinoxes or solstices are the times when the apparent ecliptic longitude
of the Sun is 0°, 90°, 180°, or 270°.
Because of perturbations of the Earth's orbit and peculiarities of the calendar, the dates
of these are not fixed.
In the constellations The ecliptic currently passes through the
following constellations:
Astrology
The ecliptic forms the center of a band about 20° wide called the zodiac, on which the
Sun, Moon, and planets are seen always to move.
Traditionally, this region is divided into 12 signs of 30° longitude, each of which
approximates the Sun's motion through one month.
In ancient times the signs corresponded roughly to 12 of the constellations which straddle
the ecliptic.
These signs give us some of the terminology used today.
The first point of Aries was named when the vernal equinox was actually in the constellation
Aries; it has since moved into Pisces.
See also Formation and evolution of the Solar System
Protoplanetary disc Invariable plane
Notes and references
External links The Ecliptic: the Sun's Annual Path on the
Celestial Sphere Durham University Department of Physics
Seasons and Ecliptic Simulator University of Nebraska-Lincoln
MEASURING THE SKY A Quick Guide to the Celestial Sphere James B. Kaler, University of Illinois
Earth's Seasons U.S. Naval Observatory The Basics - the Ecliptic, the Equator, and
Coordinate Systems AstrologyClub.Org Kinoshita, H.; Aoki, S..
"The definition of the ecliptic".
Celestial Mechanics 31: 329–338.
Bibcode:1983CeMec..31..329K.
doi:10.1007/BF01230290. ;
comparison of the definitions of LeVerrier, Newcomb, and Standish.