Astrology in the Realms
Astrology has been practiced by the peoples of Toril since the beginning, with each race (and even individual societies, including the subterranean dwarves) having their own constellations and accompanying myths.
For humans of Faerûn, the art of astrology extends back to the ancient empire of Raurin. Although not the first humans to look up into the night skies and see images (that honor going to their primitive cave-dwelling ancestors), the Imaskari were the first to identify the visible planets and set down a codified set of constellations. It is believed that the Imaskari gained much of their astronomical knowledge from the elves of the South, as well as through contact with the eastern cultures of Kara-Tur. The Imaskari's contemporaries to the north, the Netherese, knew of spelljamming and of magics that allowed one to travel the stars, and it is believed that this more scientific orientation to the heavens prevented the rise of frivolous myth making (calling it 'science').
Following the fall of the Imaskari, observations of the heavens were carried on by their descendants, the priests of the Mulhorand empire. It is from the Mulhorandi that much of the rest of Faerûn gained their knowledge of the constellations (although each culture often re-named and re-attributed these heavenly bodies to fit their own needs).
In modern times, astrology has become widespread throughout Faerûn, although the field of observational astronomy has dwindled until, today, it is mostly practiced only by mages and adherents of Selûne. However, increasing contact with the societies of Zakhara and Maztica (both highly-developed in the field of astronomy) is re-kindling Faerûn's interest in the night sky.
Traditionally, the astrologer's job has been to help determine the most propitious times and places for such things as treaty signings, royal weddings, new business ventures, and so on. (Horary and electional astrology.) Also traditionally, astrologers study how the cycles of the planets coincide with the rise and fall of nations, cities and groups (mundane astrology). In a society where the average individual is expected to follow the dictates of family and station, the astrology of the individual focuses on how many children a woman might expect to bear or rear, or when an individual might expect his or her hard work to bring him success. Personality and personal compatibility are not major concerns in a society where marriages are arranged to preserve property and to cement treaties.
Until Robinton Handreth's development of theories and techniques on how a more in depth personal astrology could be used by the ordinary individual for his or her personal development, the ordinary individual had only standard Sun/Selûne combinations that any self-respecting mage could (and can) look up in widely available almanacs and give a quick interpretation. (See the Forgotten Realms Birthday Finder for standard combinations/interpretations.) Handreth's studies changed this, observing that while societies and nations evolve and change, it was individuals that initiated these changes and these changes, for good or ill, most often involved the balance between the individual's need for self-determination and the demands of the larger society for conformity and obedience. This being the case, the individual obviously deserved deeper study than that offered by simple almanac interpretations.
The Planets of the Toril System
Of the eight planets that orbit Toril's sun, the inhabitants of Toril can only see four with the naked eye in the night skies. To most people, these four planets are all there is, although many mages, astronomers, and spelljammers know better. It is believed that the Netherese were the first to give names to these planets, and those names have since spread to the rest of Faerûn.
For humans and halflings, the four naked eye planets are associated with the four elements and with the four Elemental Gods (Akadi, Grumbar, Ishtishia, and Kossuth). Handreth extended the idea of these associations to include the other planets known to orbit Toril's sun.
(Selûne): Toril's moon governs navigation, prophecy, questers, and lycanthropes. By extension, Selûne also governs all things with a cyclic nature, such as women's issues, and emotions (whose extremes can lead to madness).
(Kossuth): reddish color, also associated with Garagos the Reaver (humans).
Anadia governs fire and destruction, and by association, war and violence.
(Akadi): bluish-white color.
Coliar governs air, flight, illusions and trickery. By extension, it also governs communication, thought and travel.
(Ishtishia): blue-green color.
Karpri governs storms, water, and destruction and by extension, the destruction of antiquated and unworkable forms and ideas.
(Grumbar): brownish color.
Chandos governs earth, metal, and time. By extension, it also governs discipline, limitation and fate.
(Sune): dull gray, but surrounded by a spectacular ring and three lesser bodies, visible only with magical aid.
Glyth governs beauty, love, and, passion, and by extension, relationships, aesthetics, and art.
(Chauntea): a tiny green sparkle, rarely seen and never seen without magical aid.
Garden governs protection, renewal, and all growing things. By extension, Garden also governs expansion, opportunity and prosperity.
(Mystra): a very large world very far away, a gleam of diamond white. Never seen without magical aid.
H'Catha governs knowledge and magic, and by extension, also governs the power of transformation, change, revolution and evolution.
A few notes on astrology in general:
Despite the confusion caused by terminology, any astrologer or mage knows that the planets do not 'govern' or command anything on Toril. Those mages familiar with spelljamming and Realmspace recognize that the planets, (excluding Selûne which does have observable physical effects on Toril's tides) are all too far away to cause any physically discernible changes on the surface of Toril. HOWEVER, the planets do reflect underlying cycles that are observed on Toril and it is this relationship that makes astrology work. Anadia does not 'cause' fire or war, but its cycle coincides with that of the observed cycles of fire and violence on Toril. Likewise, Selûne at its fullest does not cause lycanthropes to change, but the moon cycle coincides exactly with the natural cycle of the lycanthrope's condition.