Why Sidereal?

The short answer

Gumezishn Astrology uses the sidereal zodiac, signs measured against the actual, visible constellations, rather than the tropical zodiac often used in Western astrology today, which is measured against the seasons instead.

Gumezishn Astrology uses the sidereal zodiac, with planetary positions measured against the stellar background rather than against the seasonal equinoxes and solstices.

The historical evidence is more complex than a simple tropical–sidereal distinction, however. Sasanian astrology inherited substantial Hellenistic horoscopic material, while also preserving an unusually strong stellar astronomy and incorporating Indian astronomical traditions such as the lunar mansions and lunar nodes. The Greater Bundahishn itself contains both seasonal (tropical) and stellar ways of describing the zodiac. I therefore treat sidereal astrology as a historically informed reconstruction rather than claiming that the surviving sources settle the question beyond doubt. It is also the framework that best fits my own practice, which gives primary importance to fixed stars, Gozihr (the lunar nodes), and stellar lunar mansions.

The longer answer

1. The sources place the zodiac within a stellar astronomy: The Pahlavi sources fix the starting point of the zodiac at 0° Aries and describe the twelve axtarān (signs) in relation to the stars and other constellations. But 0° Aries by itself does not establish whether the zodiac was referenced tropically or sidereally. The Greater Bundahishn also connects the zodiac with months, seasons, equinoxes and solstices, while its twelve-sign horoscopic framework shows substantial Hellenistic inheritance. The historical reference point therefore remains a genuine reconstruction question. My choice of a sidereal framework reflects the much broader stellar character of the system rather than assuming that every element of Sasanian horoscopy originated independently of Hellenistic astrology.

2. Named stars are the tradition's own primary architecture, not an overlay on it: Zoroastrian astrology is not a system of abstract signs with planets moving through them, it is built, from its own primary texts, around specific, individually named stars in named combat with specific planets, where the signs are agents as well as the planets essentially. The Bundahishn names Polaris (Mēx ī mayān ī asmān), Ursa Major (Haftōring), Vega (Wanand), Sirius (Tishtar), and Fomalhaut (Sadwēs) as the four generals of the cardinal directions, under Polaris, each in a documented, named opposition to a specific planet, a structure so central it mirrors the actual command organization of the late Sasanian army (Bundahishn 5.4, 5a.3–4; Shkand-Gumānīg Wizār 4.31–36; Raffaelli 2001: 100–1). Additionally, Sirius (Tishtar) carries an entire Avestan hymn of his own, so too, Wanand/Vanant. You can find both here with a new translation and the Avestan to compare (Tir Yasht (Hymn to Sirius), page 148; Vanant Yasht (Hymn to Vanant), page 304).

A tropical chart can of course calculate astronomical contacts between planets and fixed stars. The distinction matters here because sidereal coordinates keep the zodiacal positions of the planets and the stellar background within the same reference frame. For a practice in which specific named stars are active participants in the cosmic contest, rather than supplementary points alongside the zodiac, I find sidereal a more consistent framework as precession continues to increase the divergence between the two reference frames. Fixed-star work is therefore not an optional overlay in Gumezishn Astrology; it is close to the centre of the method.

There are, however, complications that prevent one from treating the historical question as settled. The Greater Bundahishn associates Tishtar/Sirius with Cancer and has Sirius appear there, whereas Sirius falls in Gemini rather than Cancer in a modern sidereal zodiac. Likewise, Wanand, traditionally identified with Vega, is associated with Capricorn in the Bundahishn (it's tropical position), whereas Vega falls in Sagittarius in a modern sidereal zodiac. These correspondences are significant and deserve to be taken seriously rather than explained away. By contrast Fomalhaut only entered Pisces in the last few centuries and was tropically and sidereally in Aquarius in the Sasanian period but placed in Pisces in the Bundahishn.

Consequently, these stars may reflect differences between the historical Sasanian stellar reference point and modern sidereal ayanamsas, the syncretic transmission of Hellenistic and Indian astronomical material, or the fact that the Bundahishn preserves several overlapping ways of organising the heavens among other considerations.

3. It shares real, documented roots with Vedic astrology: Sasanian Persia's astral sciences were further developed through direct contact with Vedic astronomy and astrology, especially the nakshatra system of lunar mansions, and Sasanian royal astronomical tables leaned on Indian models as often as Greek ones (Pingree & Brunner, "Astrology and Astronomy in Iran," Encyclopaedia Iranica). Vedic astrology's sidereal zodiac and this practice's sidereal zodiac come from adjacent, historically connected traditions.

4. The Greater Bundahishn does preserve a strong seasonal dimension but needs to be set in its context or precession: The text repeatedly connects the zodiac with the months, seasons, equinoxes and solstices. This is important evidence for a seasonal conception of the zodiac and may reflect its Hellenistic inheritance. It is one reason I do not claim that the historical tropical-versus-sidereal question is conclusively settled.

At the same time, the Sasanian period was much closer to the astronomical conditions under which the zodiacal constellations received their names than the modern world is. Precession was already measurable, but its accumulated displacement was considerably smaller. Seasonal and stellar descriptions could therefore coexist much more closely than they do today.

For Gumezishn Astrology's preferred method, I've chosen the stellar reference frame because the fixed stars themselves are treated as active realities within the cosmic order. The tropical zodiac remains useful for understanding the seasonal layer preserved in the sources, but it is not the coordinate system I use for the practice.

5. The sources treat the stars themselves as cosmologically significant: A zodiac defined by the sun's position relative to the seasons doesn't need to know how many stars sit behind any given sign, the arcs would mean exactly the same thing whether the sky was crowded or empty. The Bundahishn does the opposite: it describes the twelve constellations "as a specimen of a warlike army, which is destined for battle," numbers the supporting stars at 6,480,000, and gives command of that host by direction to four named stars: Tishtar (Sirius) in the east, Sadwēs (Fomalhaut) and Wanand (Vega) in the west and south, Haftōring (Ursa Major) in the north (Bd. 2.5–7). This level of specific, numbered, directional detail suggests that the actual population of visible stars is doing real work in the system.

Indeed, in one Pahlavi text, the Drayishn-i Ahriman o Divan, Ahriman instructs his own demons directly to not look at Wanand (Vega) nor Haftōring (Ursa Major), because when the demons do look, their strength disappears. The two stars are described as specific points of real defensive power, strong enough that a hostile force is warned away from their gaze by name. Opting for sidereal allows me to retain the stellar reference frame when assessing where those stars are situated and how planets relate to them.

What this means practically & astrologically

The tropical and sidereal zodiacs answer different astronomical questions. The tropical zodiac remains anchored to the equinoxes and solstices, while the sidereal zodiac retains a reference to the stellar background. Because of precession, the two reference frames gradually diverge.

That divergence matters particularly for a modern practice centred on fixed stars. A tropical zodiac will continue to mark the same seasonal points even as those points move progressively through the stellar background over the approximately 26,000-year precessional cycle. A sidereal framework instead keeps the zodiac referenced to the stars themselves.

Bibliography

Primary sources

Bundahišn (Bd.), cited by chapter/section.
Škand-Gumānīg Wizār (ŠGW), cited by chapter/section.
Drayishn ī Ahriman ō Dēwān ("Ahriman's Utterance to the Divs"), Pahlavi text.
Kanga, Maneck Furdoonji (trans.). Khordeh Avesta: Transliterated and Translated into English, from the Gujarati original of Kavasji Edalji Kanga. Bombay Parsi Punchayet, 1993 (corrected ed. 2014).

For these translations, please see avesta.org

Secondary scholarship

Panaino, Antonio. "Zodiac." Encyclopaedia Iranica, 2009.
Pingree, David, and Christopher J. Brunner. "Astrology and Astronomy in Iran." Encyclopaedia Iranica, 1987.
Raffaelli, Enrico G. L'oroscopo del mondo: Il tema di nascita del mondo e del primo uomo secondo l'astrologia zoroastriana. Milan, 2001.