Ecliptical coordinate system

The ecliptical coordinate system is one of the coordinate systems used in spherical astronomy as well as astrology . In contrast to the equatorial coordinate system , which as a reference, the plane of the equator of the earth used which is reference plane of the ecliptic plane described (which is approximately the orbital plane of the Earth in its orbit around the sun ).

definition

1. The orbital angular momentum vector of the middle earth defines the ecliptic pole on the celestial sphere . The normal plane to is called the middle ecliptic . ${\displaystyle \mathbf {C} }$ ${\displaystyle P}$
   ${\displaystyle \mathbf {C} }$ ${\displaystyle E}$
2. ${\displaystyle \mathbf {W} }$be a direction in . defines the point on the celestial sphere , the equinox of the coordinates . ${\displaystyle E}$
   ${\displaystyle \mathbf {W} }$ ${\displaystyle {\mathcal {W}}}$

The position of an object is given by two coordinates :

• The ecliptical length indicates the angle opposite . is measured positively in the ecliptic plane in the direction of the earth's orbit and is 0 ° in . The following applies: ${\displaystyle \lambda }$ ${\displaystyle {\mathcal {W}}}$ ${\displaystyle \lambda }$ ${\displaystyle {\mathcal {W}}}$

${\displaystyle -180^{\circ }<\lambda \leq +180^{\circ }}$

• The ecliptical latitude indicates the angle opposite - the height above the ecliptic - and is 0 ° in and 90 ° in . The following applies: ${\displaystyle \beta }$ ${\displaystyle E}$ ${\displaystyle E}$ ${\displaystyle P}$

${\displaystyle -90^{\circ }\leq \beta \leq +90^{\circ }}$

Since the ecliptical coordinate system refers neither to the current true vernal equinox nor to the current mean vernal equinox , a time reference must also be specified, which is called the epoch . The mean spring point at exactly this point in time is the equinox of the coordinates . The specified coordinates refer to this point. The point in time has to be given because the mean spring equinox shifts due to the precession against the starry sky, and the true spring equinox fluctuates around the mean spring equilibrium due to further disturbances .

The coordinate equinox can in principle be freely chosen (this is stated in definition 2.). Either a standard equinox (e.g. J2000.0) for catalogs or the equinox of the date of the measurement for direct sky observations is selected .

The system also refers to the middle ecliptic , not the current earth-sun constellation: Therefore, the ecliptic in 1. is formally defined via the orbital angular momentum vector, which is more stable in front of the sky than the earth's orbit, which is subject to orbital disturbances by the other planets .

The direction of rotation of the coordinate system, i.e. positive values ​​of the variables, follow the usual conventions of physics ( three-finger rule ), with the celestial north pole as the primary point.

See also

• Equatorial coordinate system : The reference plane is the plane of the equator .
• The conversion between the different astronomical coordinate systems is described in the main article Astronomical coordinate systems .

Web links

• Ecliptic coordinates at museogalileo.it (English)