Ecliptic

The ecliptic is the mean plane of the apparent path in the Earth's sky that the Sun follows over the course of one year; it is the basis of the ecliptic coordinate system. This plane of reference is coplanar with Earth's orbit around the Sun (and hence the Sun's apparent path around Earth).[1] The ecliptic is not normally noticeable from Earth's surface because the planet's rotation carries the observer through the daily cycles of sunrise and sunset, which obscure the Sun's apparent motion against the background of stars during the year.

Ecliptic with earth and sun animation
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.

Sun's apparent motion

The motions as described above are simplifications. Due to 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. 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.[2]

Because Earth takes one year to orbit the Sun, the apparent position of the Sun takes one year 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[3] every day. This small difference in the Sun's position against the stars causes any particular spot on Earth's surface to catch up with (and stand directly north or south of) the Sun about four minutes later each day than it would if Earth would not orbit; a day on Earth is therefore 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 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.[4] The variation of orbital speed accounts for part of the equation of time.[5]

Relationship to the celestial equator

Earths orbit and ecliptic
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 vernal and autumnal equinoxes (red), where the Sun seems to cross the celestial equator.

Because Earth's rotational axis is not perpendicular to its orbital plane, 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.[6] 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.[3] 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.[7] The crossing from north to south is the autumnal equinox or descending node.

The orientation of 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 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 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.[8]

Once again, this is a simplification. Periodic motions of the Moon and apparent periodic motions of the Sun (actually of Earth in its orbit) cause short-term small-amplitude periodic oscillations of Earth's axis, and hence the celestial equator, known as nutation.[9] This adds a periodic component to the position of the equinoxes; the positions of the celestial equator and (vernal) equinox with fully updated precession and nutation are called the true equator and equinox; the positions without nutation are the mean equator and equinox.[10]

Obliquity of the ecliptic

Obliquity of the ecliptic is the term used by astronomers for the inclination of Earth's equator with respect 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 (47 arcseconds) per hundred years due to planetary perturbations.[11]

The angular value of the obliquity is found by observation of the motions of Earth and other 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.

Obliquity of the ecliptic laskar
Obliquity of the ecliptic for 20,000 years, from Laskar (1986).[12] Note that the obliquity varies only from 24.2° to 22.5° during this time. The red point represents the year 2000.

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.[13]

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.[14]

JPL's fundamental ephemerides have been continually updated. The Astronomical Almanac for 2010 specifies:[15]

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.[16] J. Laskar computed an expression to order T10 good to 0″.04/1000 years over 10,000 years.[12]

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.[17]

Plane of the Solar System

Ecliptic plane top view Ecliptic plane side view FourPlanetSunset hao annotated
Top and side views of the plane of the ecliptic, showing planets Mercury, Venus, Earth, and Mars. Most of the planets orbit the Sun very nearly in the same plane in which Earth orbits, the ecliptic. Four planets lined up along the ecliptic in July 2010, illustrating how the planets orbit the Sun in nearly the same plane. Photo taken at sunset, looking west over Surakarta, Java, Indonesia.

Most of the major 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. Earth's orbit, and hence, the ecliptic, is inclined a little more than 1° to the invariable plane, Jupiter's orbit is within a little more than ½° of it, and the other major planets are all within about 6°. Because of this, most Solar System bodies appear very close to the ecliptic in the sky.

The invariable plane is defined by the angular momentum of the entire Solar System, essentially the vector sum of all of the orbital and rotational angular momenta of all the bodies of the system; more than 60% of the total comes from the orbit of Jupiter.[18] That sum requires precise knowledge of every object in the system, making it a somewhat uncertain value. Because of the uncertainty regarding the exact location of the invariable plane, and because the ecliptic is well defined by the apparent motion of the Sun, the ecliptic is used as the reference plane of the Solar System both for precision and convenience. 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.[19][20]

Celestial reference plane

Ecliptic vs equator small
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.

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 pole 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.[21]

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[3] 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 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.[22]


Summary of notation for ecliptic coordinates[23]
  spherical rectangular
longitude latitude distance
geocentric λ β Δ  
heliocentric l b r x, y, z[note 1]
  1. ^ Occasional use; x, y, z are usually reserved for equatorial coordinates.
  1. ^ Occasional use; x, y, z are usually reserved for equatorial coordinates.

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 Earth's orbit, and hence the ecliptic, moves very little, it is a relatively fixed reference with respect to the stars.

Ecliptic inclination dziobek
Inclination of the ecliptic over 200,000 years, from Dziobek (1892).[24] 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.

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[25] lists the heliocentric position of Mars at 0h Terrestrial Time, 4 January 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 January 2010 0h TT as above, without the addition of nutation.

Eclipses

Because the orbit of the Moon is inclined only about 5.145° 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.[26]

Equinoxes and solstices

Positions of equinoxes and solstices
  ecliptic equatorial
longitude right ascension
March equinox 0h
June solstice 90° 6h
September equinox 180° 12h
December solstice 270° 18h

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°. Because of perturbations of Earth's orbit and anomalies of the calendar, the dates of these are not fixed.[27]

In the constellations

Constellations ecliptic equirectangular plot
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. (detailed view)

The ecliptic currently passes through the following constellations:

Astrology

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.[29] Traditionally, this region is divided into 12 signs of 30° longitude, each of which approximates the Sun's motion in one month.[30] In ancient times, the signs corresponded roughly to 12 of the constellations that straddle the ecliptic.[31] These signs are sometimes still used in modern terminology. The "First Point of Aries" was named when the March equinox Sun was actually in the constellation Aries; it has since moved into Pisces due to precession of the equinoxes.[32]

See also

Notes and references

  1. ^ USNO Nautical Almanac Office; UK Hydrographic Office, HM Nautical Almanac Office (2008). The Astronomical Almanac for the Year 2010. GPO. p. M5. ISBN 978-0-7077-4082-9.
  2. ^ U.S. Naval Observatory Nautical Almanac Office (1992). P. Kenneth Seidelmann (ed.). Explanatory Supplement to the Astronomical Almanac. University Science Books, Mill Valley, CA. ISBN 0-935702-68-7., p. 11
  3. ^ a b c The directions north and south on the celestial sphere are in the sense toward the north celestial pole and toward the south celestial pole. East is the direction toward which Earth rotates, west is opposite that.
  4. ^ Astronomical Almanac 2010, sec. C
  5. ^ Explanatory Supplement (1992), sec. 1.233
  6. ^ Explanatory Supplement (1992), p. 733
  7. ^ Astronomical Almanac 2010, p. M2 and M6
  8. ^ Explanatory Supplement (1992), sec. 1.322 and 3.21
  9. ^ U.S. Naval Observatory Nautical Almanac Office; H.M. Nautical Almanac Office (1961). Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac. H.M. Stationery Office, London. , sec. 2C
  10. ^ Explanatory Supplement (1992), p. 731 and 737
  11. ^ Chauvenet, William (1906). A Manual of Spherical and Practical Astronomy. I. J.B. Lippincott Co., Philadelphia. , art. 365–367, p. 694–695, at Google books
  12. ^ a b Laskar, J. (1986). "Secular Terms of Classical Planetary Theories Using the Results of General Relativity". Bibcode:1986A&A...157...59L. , table 8, at SAO/NASA ADS
  13. ^ Explanatory Supplement (1961), sec. 2B
  14. ^ U.S. Naval Observatory, Nautical Almanac Office; H.M. Nautical Almanac Office (1989). The Astronomical Almanac for the Year 1990. U.S. Govt. Printing Office. ISBN 0-11-886934-5. , p. B18
  15. ^ Astronomical Almanac 2010, p. B52
  16. ^ Newcomb, Simon (1906). A Compendium of Spherical Astronomy. MacMillan Co., New York. , p. 226-227, at Google books
  17. ^ Meeus, Jean (1991). Astronomical Algorithms. Willmann-Bell, Inc., Richmond, VA. ISBN 0-943396-35-2. , chap. 21
  18. ^ "The Mean Plane (Invariable Plane) of the Solar System passing through the barycenter". 3 April 2009. Archived from the original on 3 June 2013. Retrieved 10 April 2009. produced with Vitagliano, Aldo. "Solex 10" (computer program).
  19. ^ Danby, J.M.A. (1988). Fundamentals of Celestial Mechanics. Willmann-Bell, Inc., Richmond, VA. section 9.1. ISBN 0-943396-20-4.
  20. ^ Roy, A.E. (1988). Orbital Motion (third ed.). Institute of Physics Publishing. section 5.3. ISBN 0-85274-229-0.
  21. ^ Montenbruck, Oliver (1989). Practical Ephemeris Calculations. Springer-Verlag. ISBN 0-387-50704-3. , sec 1.4
  22. ^ Explanatory Supplement (1961), sec. 2A
  23. ^ Explanatory Supplement (1961), sec. 1G
  24. ^ Dziobek, Otto (1892). Mathematical Theories of Planetary Motions. Register Publishing Co., Ann Arbor, Michigan., p. 294, at Google books
  25. ^ Astronomical Almanac 2010, p. E14
  26. ^ Ball, Robert S. (1908). A Treatise on Spherical Astronomy. Cambridge University Press. p. 83.
  27. ^ Meeus (1991), chap. 26
  28. ^ Serviss, Garrett P. (1908). Astronomy With the Naked Eye. Harper & Brothers, New York and London. pp. 105, 106.
  29. ^ Bryant, Walter W. (1907). A History of Astronomy. p. 3. ISBN 9781440057922.
  30. ^ Bryant (1907), p. 4.
  31. ^ See, for instance, Leo, Alan (1899). Astrology for All. p. 8.
  32. ^ Vallado, David A. (2001). Fundamentals of Astrodynamics and Applications (2nd ed.). El Segundo, CA: Microcosm Press. p. 153. ISBN 1-881883-12-4.

External links

Astrological sign

In Western astrology, astrological signs are the twelve 30° sectors of the ecliptic, starting at the vernal equinox (one of the intersections of the ecliptic with the celestial equator), also known as the First Point of Aries. The order of the astrological signs is Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces. Each sector is named for a constellation it passes through.

The concept of the zodiac originated in Babylonian astrology, and was later influenced by Hellenistic culture. According to astrology, celestial phenomena relate to human activity on the principle of "as above, so below", so that the signs are held to represent characteristic modes of expression. Modern discoveries about the true nature of celestial objects have undermined the theoretical basis for assigning meaning to astrological signs, and empirical scientific investigation has shown that predictions and recommendations based on these systems are not accurate. Astrology is generally regarded as pseudoscience.

The twelve sector division of the ecliptic constitutes astrology's primary frame of reference when considering the positions of celestial bodies, from a geocentric point of view, so that we may find, for instance, the Sun in 23° Aries (23° longitude), the Moon in 7° Scorpio (217° longitude), or Jupiter in 29° Pisces (359° longitude). Beyond the celestial bodies, other astrological points that are dependent on geographical location and time (namely, the Ascendant, the Midheaven, the Vertex and the houses' cusps) are also referenced within this ecliptic coordinate system.Various approaches to measuring and dividing the sky are currently used by differing systems of astrology, although the tradition of the Zodiac's names and symbols remain consistent. Western astrology measures from Equinox and Solstice points (points relating to equal, longest and shortest days of the tropical year), while Jyotiṣa or Vedic astrology measures along the equatorial plane (sidereal year). Precession results in Western astrology's zodiacal divisions not corresponding in the current era to the constellations that carry similar names, while Jyotiṣa measurements still correspond with the background constellations.In Western and Indian astrology, the emphasis is on space, and the movement of the Sun, Moon and planets in the sky through each of the zodiac signs. In Chinese astrology, by contrast, the emphasis is on time, with the zodiac operating on cycles of years, months, and hours of the day.

Axial precession

In astronomy, axial precession is a gravity-induced, slow, and continuous change in the orientation of an astronomical body's rotational axis. In particular, it can refer to the gradual shift in the orientation of Earth's axis of rotation in a cycle of approximately 25,772 years. This is similar to the precession of a spinning-top, with the axis tracing out a pair of cones joined at their apices. The term "precession" typically refers only to this largest part of the motion; other changes in the alignment of Earth's axis—nutation and polar motion—are much smaller in magnitude.

Earth's precession was historically called the precession of the equinoxes, because the equinoxes moved westward along the ecliptic relative to the fixed stars, opposite to the yearly motion of the Sun along the ecliptic. This term is still used in non-technical discussions, that is, when detailed mathematics are absent. Historically,

the discovery of the precession of the equinoxes is usually attributed in the west to the Hellenistic-era (second-century BCE) astronomer Hipparchus, although there are claims of its earlier discovery, such as in the Indian text, Vedanga Jyotisha, dating from 700 BC.

With improvements in the ability to calculate the gravitational force between planets during the first half of the nineteenth century, it was recognized that the ecliptic itself moved slightly, which was named planetary precession, as early as 1863, while the dominant component was named lunisolar precession. Their combination was named general precession, instead of precession of the equinoxes.

Lunisolar precession is caused by the gravitational forces of the Moon and Sun on Earth's equatorial bulge, causing Earth's axis to move with respect to inertial space. Planetary precession (an advance) is due to the small angle between the gravitational force of the other planets on Earth and its orbital plane (the ecliptic), causing the plane of the ecliptic to shift slightly relative to inertial space. Lunisolar precession is about 500 times greater than planetary precession. In addition to the Moon and Sun, the other planets also cause a small movement of Earth's axis in inertial space, making the contrast in the terms lunisolar versus planetary misleading, so in 2006 the International Astronomical Union recommended that the dominant component be renamed the precession of the equator, and the minor component be renamed precession of the ecliptic, but their combination is still named general precession. Many references to the old terms exist in publications predating the change.

Axial tilt

In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, or, equivalently, the angle between its equatorial plane and orbital plane. It differs from orbital inclination.

At an obliquity of 0 degrees, the two axes point in the same direction; i.e., the rotational axis is perpendicular to the orbital plane. Earth's obliquity oscillates between 22.1 and 24.5 degrees on a 41,000-year cycle; Earth's mean obliquity is currently 23°26′12.4″ (or 23.43678°) and decreasing.

Over the course of an orbital period, the obliquity usually does not change considerably, and the orientation of the axis remains the same relative to the background of stars. This causes one pole to be directed more toward the Sun on one side of the orbit, and the other pole on the other side—the cause of the seasons on Earth.

Celestial coordinate system

In astronomy, a celestial coordinate system (or celestial reference system) is a system for specifying positions of celestial objects: satellites, planets, stars, galaxies, and so on. Coordinate systems can specify an object's position in three-dimensional space or plot merely its direction on a celestial sphere, if the object's distance is unknown or trivial.

The coordinate systems are implemented in either spherical or rectangular coordinates. Spherical coordinates, projected on the celestial sphere, are analogous to the geographic coordinate system used on the surface of Earth. These differ in their choice of fundamental plane, which divides the celestial sphere into two equal hemispheres along a great circle. Rectangular coordinates, in appropriate units, are simply the cartesian equivalent of the spherical coordinates, with the same fundamental (x, y) plane and primary (x-axis) direction. Each coordinate system is named after its choice of fundamental plane.

Celestial sphere

In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, which may be centered on Earth or the observer. If centered on the observer, half of the sphere would resemble a hemispherical screen over the observing location.

The celestial sphere is a practical tool for spherical astronomy, allowing astronomers to specify the apparent positions of objects in the sky if their distances are unknown or irrelevant. In the equatorial coordinate system, the celestial equator divides the celestial sphere into two halves: the northern and southern celestial hemispheres.

Conjunction (astronomy)

In astronomy, a conjunction occurs when two astronomical objects or spacecraft have either the same right ascension or the same ecliptic longitude, usually as observed from Earth.

The astronomical symbol for conjunction is ☌ (in Unicode U+260C) and handwritten .

The conjunction symbol is not used in modern astronomy. It continues to be used in astrology.When two objects always appear close to the ecliptic—such as two planets, the Moon and a planet, or the Sun and a planet—this fact implies an apparent close approach between the objects as seen on the sky. A related word, appulse, is the minimum apparent separation on the sky of two astronomical objects.Conjunctions involve either two objects in the Solar System or one object in the Solar System and a more distant object, such as a star. A conjunction is an apparent phenomenon caused by the observer's perspective: the two objects involved are not actually close to one another in space. Conjunctions between two bright objects close to the ecliptic, such as two bright planets, can be seen with the naked eye.

Ecliptic coordinate system

The ecliptic coordinate system is a celestial coordinate system commonly used for representing the apparent positions and orbits of Solar System objects. Because most planets (except Mercury) and many small Solar System bodies have orbits with slight inclinations to the ecliptic, using it as the fundamental plane is convenient. The system's origin can be the center of either the Sun or Earth, its primary direction is towards the vernal (northward) equinox, and it has a right-hand convention. It may be implemented in spherical or rectangular coordinates.

Equinox

An equinox is commonly regarded as the instant of time when the plane (extended indefinitely in all directions) of Earth's equator passes through the center of the Sun. This occurs twice each year: around 20 March and 23 September. In other words, it is the moment at which the center of the visible Sun is directly above the Equator.

The word is derived from the Latin aequinoctium, from aequus (equal) and nox (genitive noctis) (night). On the day of an equinox, daytime and nighttime are of approximately equal duration all over the planet. They are not exactly equal, however, due to the angular size of the Sun, atmospheric refraction, and the rapidly changing duration of the length of day that occurs at most latitudes around the equinoxes. Long before conceiving this equality primitive cultures noted the day when the Sun rises due East and sets due West and indeed this happens on the day closest to the astronomically defined event.

In the northern hemisphere, the equinox in March is called the Vernal or Spring Equinox; the September equinox is called the Autumnal or Fall Equinox. The dates are slightly variable, dependent as they are on the leap year cycle.Because the Moon (and to a lesser extent the planets) cause the motion of the Earth to vary from a perfect ellipse, the equinox is now officially defined by the Sun's more regular ecliptic longitude rather than by its declination. The instants of the equinoxes are currently defined to be when the longitude of the Sun is 0° and 180°.

Equinox (celestial coordinates)

In astronomy, equinox is either of two places on the celestial sphere at which the ecliptic intersects the celestial equator. Although there are two intersections of the ecliptic with the celestial equator, by convention the equinox associated with the sun's ascending node is used as the origin of celestial coordinate systems and referred to simply as the equinox. In contrast to the common usage of spring and fall, or vernal and autumnal, equinoxes, the celestial coordinate system equinox is a direction in space rather than a moment in time.

The equinox moves because of perturbing forces, therefore in order to define a coordinate system it is necessary to specify the date for which the equinox is chosen. This date should not be confused with the epoch. Astronomical objects show real movements such as orbital and proper motions, and the epoch defines the date for which the position of an object applies. Therefore, a complete specification of the coordinates for an astronomical objects requires both the date of the equinox and of the epoch.The currently used standard equinox and epoch is J2000.0, which is January 1, 2000 at 12:00 TT. The prefix "J" indicates that it is a Julian epoch. The previous standard equinox and epoch was B1950.0, with the prefix "B" indicating it was a Besselian epoch. Before 1984 Besselian equinoxes and epochs were used. Since that time Julian equinoxes and epochs have been used.

Full moon

The full moon is the lunar phase when the Moon appears fully illuminated from Earth's perspective. This occurs when Earth is located between the Sun and the Moon (more exactly, when the ecliptic longitudes of the Sun and Moon differ by 180°). This means that the lunar hemisphere facing Earth – the near side – is completely sunlit and appears as a circular disk, while the far side is dark. The full moon occurs once roughly every month.

When the Moon moves into Earth's shadow, a lunar eclipse occurs, during which all or part of the Moon's face may appear reddish due to the Rayleigh scattering of blue wavelengths and the refraction of sunlight through Earth's atmosphere. Lunar eclipses happen only during full moon and around points on its orbit where the satellite may pass through the planet's shadow. A lunar eclipse does not occur every month because the Moon's orbit is inclined 5.14° with respect to the ecliptic plane of Earth; thus, the Moon usually passes north or south of Earth's shadow, which is mostly restricted to this plane of reference. Lunar eclipses happen only when the full moon occurs around either node of its orbit (ascending or descending). Therefore, a lunar eclipse occurs approximately every 6 months and often 2 weeks before or after a solar eclipse, which occurs during new moon around the opposite node.

The interval period between a new or full moon and the next same phase, a synodic month, averages about 29.53 days. Therefore, in those lunar calendars in which each month begins on the day of the new moon, the full moon falls on either the 14th or 15th day of the lunar month. Because a calendar month consists of a whole number of days, a lunar month may be either 29 or 30 days long.

Longitude of the ascending node

The longitude of the ascending node (☊ or Ω) is one of the orbital elements used to specify the orbit of an object in space. It is the angle from a reference direction, called the origin of longitude, to the direction of the ascending node, measured in a reference plane. The ascending node is the point where the orbit of the object passes through the plane of reference, as seen in the adjacent image. Commonly used reference planes and origins of longitude include:

For a geocentric orbit, Earth's equatorial plane as the reference plane, and the First Point of Aries as the origin of longitude. In this case, the longitude is also called the right ascension of the ascending node, or RAAN. The angle is measured eastwards (or, as seen from the north, counterclockwise) from the First Point of Aries to the node.

For a heliocentric orbit, the ecliptic as the reference plane, and the First Point of Aries as the origin of longitude. The angle is measured counterclockwise (as seen from north of the ecliptic) from the First Point of Aries to the node.

For an orbit outside the Solar System, the plane tangent to the celestial sphere at the point of interest (called the plane of the sky) as the reference plane, and north, i.e. the perpendicular projection of the direction from the observer to the North Celestial Pole onto the plane of the sky, as the origin of longitude. The angle is measured eastwards (or, as seen by the observer, counterclockwise) from north to the node., pp. 40, 72, 137; , chap. 17.In the case of a binary star known only from visual observations, it is not possible to tell which node is ascending and which is descending. In this case the orbital parameter which is recorded is the longitude of the node, Ω, which is the longitude of whichever node has a longitude between 0 and 180 degrees., chap. 17;, p. 72.

Lunar node

A lunar node is either of the two orbital nodes of the Moon, that is, the two points at which the orbit of the Moon intersects the ecliptic. The ascending (or north) node is where the Moon moves into the northern ecliptic hemisphere, while the descending (or south) node is where the Moon enters the southern ecliptic hemisphere.

A lunar eclipse can occur only when the full Moon is near (within 11° 38' ecliptic longitude) either lunar node, while a solar eclipse can occur only when the new Moon is near (within 17° 25') either lunar node.

Because the orbital plane of the Moon precesses in space, the lunar nodes also precess around the ecliptic, completing one revolution (called a draconic or nodal period) in 18.612958 years (6,798.383 days). (This is not the same length as a saros.) The same cycle measured against an inertial frame of reference, such as International Celestial Reference System (ICRS), a coordinate system relative to the fixed stars), is 18.599525 years.

Both solar eclipses of July 2000 (on the 1st and 31st days) occurred around the time when the Moon was at its ascending node. Ascending-node eclipses recur after one draconic year on average, which is about 0.94901 Gregorian year, as do descending-node eclipses.

Non-inclined orbit

A non-inclined orbit is an orbit coplanar with a plane of reference. The orbital inclination is 0° for prograde orbits, and π (180°) for retrograde ones. If the plane of reference is a massive spheroid body's equatorial plane, these orbits are called equatorial; if the plane of reference is the ecliptic plane, they are called ecliptic.

As these orbits lack nodes, the ascending node is usually taken to lie in the reference direction (usually the vernal equinox), and thus the longitude of the ascending node is taken to be zero. Also, the argument of periapsis is undefined.

Geostationary orbit is a geosynchronous example of an equatorial orbit.

Orbit of the Moon

Not to be confused with Lunar orbit (the orbit of an object around the Moon).The Moon orbits Earth in the prograde direction and completes one revolution relative to the stars in about 27.32 days (a sidereal month) and one revolution relative to the Sun in about 29.53 days (a synodic month). Earth and the Moon orbit about their barycenter (common center of mass), which lies about 4,600 km (2,900 mi) from Earth's center (about 3/4 of the radius of Earth). On average, the distance to the Moon is about 385,000 km (239,000 mi) from Earth's center, which corresponds to about 60 Earth radii.

With a mean orbital velocity of 1.022 km/s (0.635 miles/s), the Moon covers a distance approximately its diameter, or about half a degree on the celestial sphere, each hour. The Moon differs from most satellites of other planets in that its orbit is close to the ecliptic plane instead of that of its primary (in this case, Earth's) equatorial plane. The Moon's orbital plane is inclined by about 5.1° with respect to the ecliptic plane, whereas the Moon's equatorial plane is tilted by only 1.5°.

Scorpio (astrology)

Scorpio is the eighth astrological sign in the Zodiac, originating from the constellation of Scorpius. It spans 210°–240° ecliptic longitude. Under the tropical zodiac (most commonly used in Western astrology), the Sun transits this area on average from October 23 to November 22. Under the sidereal zodiac (most commonly used in Hindu astrology), the Sun is in Scorpio from approximately November 16 to December 15. Depending on which zodiac system one uses, an individual born under the influence of Scorpio may be called a Scorpio or a Scorpion.

Sidereal and tropical astrology

Sidereal and tropical are astrological terms used to describe two different definitions of a year. They are also used as terms for two systems of ecliptic coordinates used in astrology. Both divide the ecliptic into a number of "signs" named after constellations, but while the sidereal system defines the signs based on the fixed stars, the tropical system defines it based on the position of vernal equinox in the northern hemisphere (i.e., the intersection of the ecliptic with the celestial equator). Because of the precession of the equinoxes, the two systems do not remain fixed relative to each other but drift apart by about 1.4 arc degrees per century. The tropical system was adopted during the Hellenistic period and remains prevalent in Western astrology. A sidereal system is used in Hindu astrology, and in some 20th century systems of Western astrology.

While classical tropical astrology is based on the orientation of the Earth relative to the Sun and planets of the solar system, sidereal astrology deals with the position of the Earth relative to both of these as well as the stars of the celestial sphere. The actual positions of certain fixed stars as well as their constellations is an additional consideration in the horoscope.

Solstice

A solstice is an event occurring when the Sun appears to reach its most northerly or southerly excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around June 21 and December 21. The seasons of the year are determined by reference to both the solstices and the equinoxes.

The term solstice can also be used in a broader sense, as the day when this occurs. The day of a solstice in either hemisphere has either the most sunlight of the year (summer solstice) or the least sunlight of the year (winter solstice) for any place other than the Equator. Alternative terms, with no ambiguity as to which hemisphere is the context, are "June solstice" and "December solstice", referring to the months in which they take place every year. The word solstice is derived from the Latin sol ("sun") and sistere ("to stand still"), because at the solstices, the Sun's declination appears to "stand still"; that is, the seasonal movement of the Sun's daily path (as seen from Earth) stops at a northern or southern limit before reversing direction.

Year

A year is the orbital period of the Earth moving in its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by change in weather, the hours of daylight, and, consequently, vegetation and soil fertility. The current year is 2019.

In temperate and subpolar regions around the planet, four seasons are generally recognized: spring, summer, autumn, and winter. In tropical and subtropical regions, several geographical sectors do not present defined seasons; but in the seasonal tropics, the annual wet and dry seasons are recognized and tracked.

A calendar year is an approximation of the number of days of the Earth's orbital period as counted in a given calendar. The Gregorian calendar, or modern calendar, presents its calendar year to be either a common year of 365 days or a leap year of 366 days, as do the Julian calendars; see below. For the Gregorian calendar, the average length of the calendar year (the mean year) across the complete leap cycle of 400 years is 365.2425 days. The ISO standard ISO 80000-3, Annex C, supports the symbol a (for Latin annus) to represent a year of either 365 or 366 days. In English, the abbreviations y and yr are commonly used.

In astronomy, the Julian year is a unit of time; it is defined as 365.25 days of exactly 86,400 seconds (SI base unit), totalling exactly 31,557,600 seconds in the Julian astronomical year.The word year is also used for periods loosely associated with, but not identical to, the calendar or astronomical year, such as the seasonal year, the fiscal year, the academic year, etc. Similarly, year can mean the orbital period of any planet; for example, a Martian year and a Venusian year are examples of the time a planet takes to transit one complete orbit. The term can also be used in reference to any long period or cycle, such as the Great Year.

Zodiac

The zodiac is an area of the sky that extends approximately 8° north or south (as measured in celestial latitude) of the ecliptic, the apparent path of the Sun across the celestial sphere over the course of the year. The paths of the Moon and visible planets are also within the belt of the zodiac.In Western astrology, and formerly astronomy, the zodiac is divided into twelve signs, each occupying 30° of celestial longitude and roughly corresponding to the constellations Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces.The twelve astrological signs form a celestial coordinate system, or more specifically an ecliptic coordinate system, which takes the ecliptic as the origin of latitude and the Sun's position at vernal equinox as the origin of longitude.

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