How a Siddhamsha D24 Chart Is Calculated

The Siddhāṁśa passage and two readings

Bṛhat Parāśara Horā Śāstra 6.22–23 names Leo for odd signs and Cancer for even signs, and describes a reversed sequence of presiding deities in even signs. The conventional D24 sign assignment counts forward from both starts: Leo, Virgo, Libra … in an odd D1 sign and Cancer, Leo, Virgo … in an even D1 sign. Each sequence runs twice across twenty-four portions. The choice to keep sign counting forward in even signs is a specific interpretation, not something silently inferred from the word “reverse” in the deity passage.

Rao's 2012 study of the Sanskrit passage proposes a reverse-even sign assignment: Cancer, Gemini, Taurus … in an even D1 sign. The study explicitly identifies the forward method as conventional and the backward method as its alternative. This calculator defaults to forward for easier comparison with common software and lets readers switch to the alternative. The presiding deity order is not used as a sign algorithm and is omitted from the result table. Other editions and varga settings may differ.

The odd D1 signs are Aries, Gemini, Leo, Libra, Sagittarius, and Aquarius. The even signs are Taurus, Cancer, Virgo, Scorpio, Capricorn, and Pisces. Parity is based on sign order starting with Aries as sign one; it is not determined by a planet's gender or dignity.

All twenty-four exact intervals

Each row starts at the included boundary and ends just before the next boundary. At exactly 1°15′ within any D1 sign, portion two begins. At exactly 30° the longitude has entered the next D1 sign, whose parity supplies a new Leo or Cancer start. Rows thirteen through twenty-four repeat the signs of rows one through twelve, while the derived degree within each part differs.

Calculate a D24 placement by hand

Normalize the sidereal D1 longitude to 0° through just under 360°. Let s be the zero-based D1 sign index (Aries = 0) and d its degree inside that sign. Set the zero-based portion p = floor(d ÷ 1.25), from 0 to 23. In an odd sign (s even), D24 sign index = (4 + p) modulo 12. In an even sign, the default index = (3 + p) modulo 12; the alternate index = (3 − p) modulo 12. The derived degree is 24 × (d − 1.25p).

At 10° Taurus, p = 8, the ninth portion. Taurus is even: the forward count from Cancer lands in Pisces, while the backward count lands in Scorpio. Both display 0° within that derived sign, because 10° is an exact portion start. At 10°30′ Taurus, the same signs display 12° Pisces or 12° Scorpio respectively.

At exactly 1°15′ Aries, the second portion begins in Virgo. One arcsecond earlier remains in the first portion in Leo. At exactly 30° Aries, the D1 sign changes to Taurus, so its first D24 portion begins at Cancer. Checking both sides of each boundary matters because this chart magnifies a narrow D1 interval.

Traditional use and calculation limits

Bṛhat Parāśara Horā Śāstra 7.5 associates Chaturvimshamsha with vidyā, or learning. That is a traditional interpretive domain, not evidence that a chart can quantify learning ability or determine an educational future. Study the D24 beside the D1 chart, with its mathematical and interpretive status clear.

A recorded birth time, place, and historical UTC offset are needed for D24 Lagna. A position within 0.05° of a 1°15′ boundary is flagged by the calculator. Birth-time uncertainty, different ayanamshas, ephemerides, lunar-node conventions, and the even-sign rule can all alter a comparison. The base D1 chart uses an approximate Lahiri ayanamsha and Astronomy Engine 2.1.19, not Swiss Ephemeris. The local date form covers 1800–2100 and does not reconstruct historical time zones.