Vedic and Western astrology: what is different, and why your sign changes

The zodiac: sidereal and tropical

Both divide the ecliptic into twelve signs of 30°. Western astrology fixes 0° Aries at the point where the Sun stands at the March equinox; because the equinox slides slowly backward against the stars (precession, about one degree in 72 years), that point has moved about 24° since the two zodiacs coincided, around 285 CE. Vedic astrology keeps the signs tied to the stars, and the difference — the ayanāṃśa — is subtracted from the tropical longitudes. horoAstro computes the planets from NASA JPL's DE440 tables and subtracts the Lahiri ayanāṃśa (the value fixed by India's Calendar Reform Committee in 1955); a reader may choose Raman's instead.

• A Western Aries born 21 March–13 April is usually a Vedic Pisces; born 14–19 April, a Vedic Aries. The same shift of about three weeks runs through the year.
• The nakṣatras — the 27 lunar mansions of 13°20′ each — exist only in the sidereal zodiac; they name the Moon's place at birth (the birth star) and start the daśā clock.

The Moon before the Sun

Western astrology's 'sign' is the Sun's; a Vedic reading names three things before it — the lagna (the sign rising in the east at the moment of birth), the Moon's sign (rāśi), and the Moon's nakṣatra. The daily and yearly transits (Gochar) are counted from the Moon, not the Sun or the lagna (Phaladīpikā 26.1).

Houses, planets and aspects

• Houses: Parāśara's houses are whole signs counted from the lagna; the sign rising is the whole 1st house. Western astrology mostly divides the sky by quadrant systems (Placidus and others), so a planet can fall in a different house in the two charts even after the sign shift.
• Planets: nine grahas — the Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn, and the two nodes Rāhu and Ketu. Uranus, Neptune and Pluto have no verse in the texts and no place in the reading.
• Aspects: each planet aspects the 7th sign from itself in full; Mars also the 4th and 8th, Jupiter the 5th and 9th, Saturn the 3rd and 10th (Bṛhat Parāśara Horā Śāstra 26.4). Western aspects are angles between planets (trine, square, sextile) regardless of sign.
• Strength: ṣaḍbala, the six strengths, and the aṣṭakavarga points, with no counterpart in Western practice.

Timing: daśās, not progressions

Western astrology times a life by transits, progressions and returns. Vedic astrology has a clock of its own: the Vimśottarī daśā, in which the nine planets rule in turn for 120 years — Ketu 7, Venus 20, Sun 6, Moon 10, Mars 7, Rāhu 18, Jupiter 16, Saturn 19, Mercury 17 — each period divided among the nine again. What a planet promises in the birth chart comes in its own period (Bṛhat Parāśara Horā Śāstra 33.96). horoAstro's Timing page places every result of your reading in the period that gives it, at your own age.

Which is right?

Neither can claim to be measured true: astrology is a matter of belief, and horoAstro does not claim scientific accuracy for either system. What it claims is narrower and checkable — that every line of its reading is a rule of a named classical text, read as the text's own rules of judgement decide, with the verse shown. If you want the Western chart for comparison, any Western site will draw it from the same birth data; the sign shift above tells you what to expect.