PLANETARY HELIOCENTRIC LONGITUDES
1999

    The heliocentric longitude of a planet is the angle between the vernal equinox and the planet, as seem from the Sun. It is measured in the ecliptic plane, in the direction of the orbital motion of the planet (counterclockwise as viewed from the north side of the ecliptic plane).  Knowing   the heliocentric longitudes and the distances of the planets from the Sun, one can construct a diagram or model showing the relative orientations of the Sun and planets on any date.