DESIGNATE POSITION ANGLE

Syntax

Fixed

   DESIGNATE @int(1001:10099)
      CENTER @body
      REFERENCE (1:1){ NORTH POSITION ANGLE
                     | SOLAR POSITION ANGLE }
      RELATIVE @body
      (2:2){ ANGLE  @number
           | RADIUS @number }
      (0:3){ EPOCH @calendar
           | FROM  @calendar
           | TO    @calendar }

Moving

   DESIGNATE @int(1001:10099)
      CENTER @body
      REFERENCE (1:1){ NORTH POSITION ANGLE
                    | SOLAR POSITION ANGLE }
      RELATIVE @body
      (2:2){ ANGLE  (1:2)@number
           | RADIUS (1:2)@number }
      EPOCH @calendar
      (0:2){ FROM  @calendar
           | TO    @calendar }

Description

When creating a designated object in this system, you must specify an observer (relative), an `angle' (longitude) and a `radius' (colatitude) within a spherical coordinate. The interpretation of the angle depends on which of the following reference meridians is selected: the one that contains celestial north or the one that contains the Sun.

Definition

Pick a body. The apparent position of this body on the celestial sphere (as seen from the observer) is the pole of a left-handed spherical coordinate system. The `radius' of a designated object is simply the colatitude of the object in this system (the angular `distance' from the pole).

The `angle' of a designated object is simply the longitude of the object, measured counter-clockwise from the reference meridian of the system.

In the NORTH POSITION ANGLE system, the reference meridian passes through celestial north:

              ^  reference meridian (through celestial north)
              |
            / |
       angle  |
            \ x <-- center
             /
            /
           / radius
          o

In the SOLAR POSITION ANGLE system, the reference meridian passes through the ap

              ^  reference meridian (through apparent sun)
              |
            / |
       angle  |
            \ x <-- center
             /
            /
           / radius
          o

Examples

Example 1

The gas tail of a comet moves away from the comet in (approximately) the anti-solar direction. The DESIGNATE SOLAR POSITION ANGLE command is particularly well suited for observing comet tails as illustrated in the following example, which places a designated object at a point half a degree into the tail of a comet.

   DEFINE GAS_TAIL 1002;
 
   DESIGNATE GAS_TAIL
        CENTER                  A_COMET
        REFERENCE  SOLAR POSITION ANGLE
        RELATIVE            EARTH
        ANGLE               180 DEGREES
        RADIUS              0.5 DEGREES;

Example 2

In the following example, the DESIGNATE NORTH POSITION ANGLE command is used to place a designated object at the anti-solar point. (Bodies have fewer Sun-related viewing constraints when they are near the anti-solar point.)

   DEFINE ANTISUN 1003;
 
   DESIGNATE ANTISUN
        CENTER                       SUN
        REFERENCE   NORTH POSITION ANGLE
        RELATIVE    EARTH
        ANGLE                          0
        RADIUS               180 DEGREES;