Ecliptic coordinate system explained

Ecliptic coordinate system should not be confused with Elliptic coordinate system.

In astronomy, the ecliptic coordinate system is a celestial coordinate system commonly used for representing the apparent positions, orbits, and pole orientations of Solar System objects. Because most planets (except Mercury) and many small Solar System bodies have orbits with only 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 March equinox, and it has a right-hand convention. It may be implemented in spherical or rectangular coordinates.^[1]

Primary direction

See also: Axial precession and Astronomical nutation.

The celestial equator and the ecliptic are slowly moving due to perturbing forces on the Earth, therefore the orientation of the primary direction, their intersection at the March equinox, is not quite fixed. A slow motion of Earth's axis, precession, causes a slow, continuous turning of the coordinate system westward about the poles of the ecliptic, completing one circuit in about 26,000 years. Superimposed on this is a smaller motion of the ecliptic, and a small oscillation of the Earth's axis, nutation.^[2] ^[3]

In order to reference a coordinate system which can be considered as fixed in space, these motions require specification of the equinox of a particular date, known as an epoch, when giving a position in ecliptic coordinates. The three most commonly used are:

Mean equinox of a standard epoch: (usually the J2000.0 epoch, but may include B1950.0, B1900.0, etc.) is a fixed standard direction, allowing positions established at various dates to be compared directly.

Spherical coordinates

! colspan="3" style="background-color:#89CFF0;"
Longitude	Rectangular
Spherical	Rectangular
Geocentric
Heliocentric		,, ^[5]

Ecliptic longitude

Ecliptic longitude or celestial longitude (symbols: heliocentric, geocentric) measures the angular distance of an object along the ecliptic from the primary direction. Like right ascension in the equatorial coordinate system, the primary direction (0° ecliptic longitude) points from the Earth towards the Sun at the March equinox. Because it is a right-handed system, ecliptic longitude is measured positive eastwards in the fundamental plane (the ecliptic) from 0° to 360°. Because of axial precession, the ecliptic longitude of most "fixed stars" (referred to the equinox of date) increases by about 50.3 arcseconds per year, or 83.8 arcminutes per century, the speed of general precession.^[6] ^[7] However, for stars near the ecliptic poles, the rate of change of ecliptic longitude is dominated by the slight movement of the ecliptic (that is, of the plane of the Earth's orbit), so the rate of change may be anything from minus infinity to plus infinity depending on the exact position of the star.

Ecliptic latitude

Ecliptic latitude or celestial latitude (symbols: heliocentric, geocentric), measures the angular distance of an object from the ecliptic towards the north (positive) or south (negative) ecliptic pole. For example, the north ecliptic pole has a celestial latitude of +90°. Ecliptic latitude for "fixed stars" is not affected by precession.

Distance

Distance is also necessary for a complete spherical position (symbols: heliocentric, geocentric). 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.

Historical use

From antiquity through the 18th century, ecliptic longitude was commonly measured using twelve zodiacal signs, each of 30° longitude, a practice that continues in modern astrology. The signs approximately corresponded to the constellations crossed by the ecliptic. Longitudes were specified in signs, degrees, minutes, and seconds. For example, a longitude of is 19.933° east of the start of the sign Leo. Since Leo begins 120° from the March equinox, the longitude in modern form is .^[8]

Rectangular coordinates

A rectangular variant of ecliptic coordinates is often used in orbital calculations and simulations. It has its origin at the center of the Sun (or at the barycenter of the Solar System), its fundamental plane on the ecliptic plane, and the -axis toward the March equinox. The coordinates have a right-handed convention, that is, if one extends their right thumb upward, it simulates the -axis, their extended index finger the -axis, and the curl of the other fingers points generally in the direction of the -axis.^[9]

These rectangular coordinates are related to the corresponding spherical coordinates by

\begin{align} x&=r\cosb\cosl\\ y&=r\cosb\sinl\\ z&=r\sinb\end{align}

Conversion between celestial coordinate systems

See main article: Celestial coordinate system.

Converting Cartesian vectors

Conversion from ecliptic coordinates to equatorial coordinates

$\begin x_\text \\ y_\text \\ z_\text \\ \end = \begin 1 & 0 & 0 \\ 0 & \cos \varepsilon & -\sin \varepsilon \\ 0 & \sin \varepsilon & \cos \varepsilon \\ \end \begin x_\text \\ y_\text \\ z_\text \\ \end$ ^[10]

Conversion from equatorial coordinates to ecliptic coordinates

$\begin x_\text \\ y_\text \\ z_\text \\ \end = \begin 1 & 0 & 0 \\ 0 & \cos \varepsilon & \sin \varepsilon \\ 0 & -\sin \varepsilon & \cos \varepsilon \\ \end \begin x_\text \\ y_\text \\ z_\text \\ \end$ where is the obliquity of the ecliptic.

External links

The Ecliptic: the Sun's Annual Path on the Celestial Sphere Durham University Department of Physics
Equatorial ↔ Ecliptic coordinate converter
MEASURING THE SKY A Quick Guide to the Celestial Sphere James B. Kaler, University of Illinois

Notes and References

Book: Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac . Nautical Almanac Office, U.S. Naval Observatory . H.M. Nautical Almanac Office, Royal Greenwich Observatory . H.M. Stationery Office, London (reprint 1974) . 1961 . 24–27.
Explanatory Supplement (1961), pp. 20, 28
Book: Explanatory Supplement to the Astronomical Almanac . U.S. Naval Observatory . Nautical Almanac Office . P. Kenneth Seidelmann . University Science Books, Mill Valley, CA (reprint 2005) . 1992 . 1-891389-45-9 . 11–13.
Book: Meeus , Jean . Astronomical Algorithms . Willmann-Bell, Inc., Richmond, VA . 1991 . 137 . 0-943396-35-2 .
Occasional use;,, are usually reserved for equatorial coordinates.
N. Capitaine. P.T. Wallace. J. Chapront. Expressions for IAU 2000 precession quantities. Astronomy & Astrophysics. 2003. 412. 2. 581. 10.1051/0004-6361:20031539. https://web.archive.org/web/20120325210757/http://syrte.obspm.fr/iau2006/aa03_412_P03.pdf . 2012-03-25 . live. 2003A&A...412..567C. free.
J.H. Lieske et al. (1977), "Expressions for the Precession Quantities Based upon the IAU (1976) System of Astronomical Constants". Astronomy & Astrophysics 58, pp. 1-16
Book: Leadbetter, Charles . A Compleat System of Astronomy . 1742 . J. Wilcox, London . 94.

numerous examples of this notation appear throughout the book.
Explanatory Supplement (1961), pp. 20, 27
Explanatory Supplement (1992), pp. 555-558