The
ecliptic is the apparent path that the Sun
traces out in the
sky during the year, appearing
to move eastwards on an imaginary spherical surface, the
celestial sphere, relative to the (almost)
fixed stars. More accurately, it is the intersection of the
celestial sphere with the
ecliptic
plane, which is the geometric
plane containing the mean
orbit of the
Earth around the
Sun. (The ecliptic plane should be distinguished
from the
invariable plane of the solar
system, which is perpendicular to the vector sum of the
angular momenta of all planetary
orbital planes, to which
Jupiter is the main
contributor. The present ecliptic plane is inclined to the
invariable plane by about 1.5°.)
The name ecliptic arises because
eclipses
occur when the full or new Moon is very close to this path of the
Sun.
Ecliptic and equator
As the rotation 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
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 which 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
β 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).
Ecliptic and stars
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 (see below).
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
Astronomical Julian calendar
years (in which the year 0 = 1 BC, -1 = 2 BC, etc.) as follows:
- The March equinox passed from Taurus into Aries in year -1865, passed into
Pisces in year -67, will pass
into Aquarius in year 2597,
will pass into Capricornus in year 4312.
It passed along (but not into) a 'corner' of Cetus on 0°10' distance in year 1489.
- The June solstice passed from Leo into Cancer in year -1458, passed into
Gemini in year -10, passed
into Taurus in December year
1989, will pass into Aries in
year 4609.
- The September equinox passed from Libra into Virgo in year -729, will pass into
Leo in year 2439.
- The December solstice passed from Capricornus into Sagittarius in year -130, will
pass into Ophiuchus in year 2269, and will
pass into Scorpius in year 3597.
Ecliptic and Sun
Due to perturbing influences on the Earth's orbit by the other
planets, the
true Sun is not always exactly on the
ecliptic, but may be some arcseconds north or south of it. It is
therefore the centre of the
mean Sun which 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, the whole celestial sphere for that matter) towards the west
along the equator every 24 hours. In fact, where the stars need
about 23h56m 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 just 24
hours.
Because the distance between Sun and Earth varies slightly around
the year, the speed with which the Sun moves around the ecliptic is
also variable. 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 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
orbit, the moment the real Sun passes the equator might be several
minutes earlier or later. The southern most 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.
In any case it must be stressed that although these traditional
signs (in western
tropical
astrology) have given their names to the solstices and
equinoxes, 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.
Ecliptic and planets
Most planets go in orbits around the sun which are almost in the
same plane as the Earth's orbital plane, differing by a few degrees
at most. As such they always appear close to the ecliptic when seen
in the sky.
Mercury with an orbital
inclination of 7° is an exception.
Pluto, at 17°, was previously the exception
until it was reclassified a
dwarf
planet, but other 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.
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.
Ecliptic and Moon
The orbit of the
Moon is inclined by about 5°
on the ecliptic. Its nodal line is not fixed either, 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.
Ecliptic and star coordinates
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
zodiac
coordinates.
References
- J. Meeus; Mathematical Astronomical Morsels; ISBN
0-943396-51-4
- (produced with Solex
10 written by Aldo Vitagliano)
External links