Planets in Signs
Dates and Times of when Planets Enter and Leave the Signs of the Zodiac

This Planets in Signs program simulates the motion of the planets within the signs of the zodiac. It shows when a planet enters a sign, when it leaves, and how long it remains in that sign. It offers the option to display zodiac positions in either the geocentric or the heliocentric zodiac.

It might appear at first that since the planets move in regular orbits around the Sun, and the zodiac is divided into twelve equal parts of 30° each, there is nothing much to calculate. In fact the speed of each planet varies, since the further a planet is from the Sun the slower it moves in its orbit. Furthermore, with a geocentric zodiac planets can (appear to) reverse their motion in the heavens, and thus stay in one sign for a lengthy period, or even regress into the sign it was last in.

When a date and time are specified there are two modes of operation:

1. Calculate times of entry and exit for all planets (or a subset of planets).
2. Calculate times of entry and exit of one planet across a series of signs.

Here is a typical example of the first mode of operation (after selecting Heliocentric, checking the boxes for Mercury, Jupiter, Saturn and Pluto, then clicking on Calculate):

Here is a typical example of the second mode of operation (after selecting Moon from the drop-down menu and 4 for the number of sign entries, then clicking on the sign entries button):

Now follows the full explanation:

The ecliptic is the plane in which the planets of our solar system move around the Sun. This is the same plane in which the planets appear to move around the Earth. A physically accurate diagram of the solar system (planets, Sun and Moon) has the Sun at its center with (when viewed from "above"), the planets revolving in an anti-clockwise direction around the Sun (while the Moon revolves in an anti-clockwise direction around the Earth). In contrast, a traditional astrological diagram has the Earth at its center and the planets, Sun and Moon all revolving around the Earth (also in an anti-clockwise direction).

The plane of the orbit of each planet is slightly different from the others, and the ecliptic is the "average" plane. The planets actually move in orbits which pass above and below the plane of the ecliptic, but not by much, except in the case of Pluto, and astrologer's horoscopes are shown as if the planets all moved in the same plane.

The plane of the ecliptic (as viewed from the Earth or the Sun) is usually divided into twelve parts, each subtending an angle of 30°. These parts (called signs) are named Aries, Taurus and so on, up to Pisces, and this constitutes the 360° of the zodiac. The start of the zodiac is called zero degrees Aries, and in Western astrology this is defined as the direction of a line from the Earth to the Sun (or vice-versa) at the moment of the vernal (a.k.a. spring) equinox.

In geocentric astrology the planets are viewed as revolving around the Earth (as said above, in an anti-clockwise direction), whereas in heliocentric astrology they are viewed as revolving around the Sun. In both cases the planets move from one sign to an adjacent sign. In geocentric astrology a planet can appear to move backwards, passing from a sign back into the sign that it has just previously left. Such movement is said to be "retrograde". In heliocentric astrology planets never move backward. Retrograde motion in geocentric astrology is only apparent, due to the fact that the planets are viewed as moving around the Earth, whereas they actually move around the Sun.

The planets move around the Sun at different speeds. The closer a planet is to the Sun, the faster it moves. Thus the inner planets (Mercury, Venus, etc.) spend much less time in a sign than the outer planets (Jupiter, Saturn, etc.). And (with a geocentric zodiac) a planet may remain in a sign for an unusually long time due to moving backwards and forwards within that sign before passing to the next.