Historical, Babylonian and Galactic Ayanamshas: A Research Guide

Hipparchos, Babylonian · Huber, Galactic Centre at 0° Sagittarius and J2000 appear in the same Hermes ayanamsha menu. These names define origins in different ways. Reconstructing a historical calculation and testing a galactic geometry require different selection criteria.

First identify the basis of the option: a historical reconstruction, a stellar reference, galactic geometry, or the coordinate reference of a particular epoch. Then compare it using the same input data.

Following the sixteen entries in our Lahiri, Raman, KP and Fagan–Bradley guide, this guide covers the remaining twelve historically grouped, six Babylonian, ten galactically grouped, three epoch-based and one custom option. Together, the two guides cover the Hermes inventory of 47 built-in references plus one custom entry. The groups organise a catalogue; their members do not all have the same origin.

What does a historical name tell you?

“Valens” in a menu does not establish that every calculation in an ancient text has been reproduced exactly. A modern researcher may have derived an origin from old observations or charts. Another option may reinterpret an older work’s reference epoch through modern calculations.

Ask two questions separately: How does the software calculate this reference? What information is actually present in the historical source? The tables below primarily explain Swiss Ephemeris technical definitions and the Hermes menu inventory. The relevant original tablets, Sanskrit texts and critical editions have not each been read for this article.

The twelve entries in the historical group

Do not merge Surya Siddhanta and its Mean Sun variant because their names are similar. They use different definitions of the starting point in the Swiss implementation. The corresponding distinction in the Aryabhata family matters for the same reason.

Polar longitude in SS Revati/Citra concerns the geometry used to project the star onto the ecliptic. True Revati/True Citra, covered in the previous guide, are separate definitions anchoring the star through ordinary ecliptic longitude. Sharing a star’s name does not establish the same projection or epoch. These distinctions are discussed in Swiss Ephemeris §2.8.3–2.8.5.

Six Babylonian reconstructions

The precision of ancient records, the selection of observations, and the method used to estimate an origin affect the result. “Babylonian ayanamsha” is insufficient as the name of one numerical definition.

Eta Piscium is not zeta Piscium, the star used as Revati. Do not read ETPSC as another name for Revati.

Swiss Ephemeris §2.8.2 describes these reconstructions and a correction to Huber’s value following an earlier transcription error. That is a concrete reason to record the library version. Before comparing old and new outputs labelled Huber, establish that their definitions and versions match.

Using these options does not turn a reconstruction into a direct re-observation of antiquity. Read the result alongside the researcher’s data and assumptions, rather than ranking historical certainty from menu labels alone.

Galactic centre, galactic equator and the Mula star

The galactic centre identifies a direction on the sky. The galactic equator is a great circle; its intersection with the ecliptic gives two opposite directions. Assigning a target longitude to the centre and targeting one of those intersections are different constructions.

True Mula in this group directly references a star, lambda Scorpii. Its inclusion in the galactic group does not make that star the galactic centre.

Wilhelm’s Galactic Centre Middle Mula choice and Galactic Equator Middle Mula do not target the same object. Similarly, anchoring the centre and anchoring the galactic intersection at 0° Sagittarius need not give the same result.

“IAU1958” identifies a galactic-coordinate definition; it does not ask you to enter 1958 as the chart date. The history of that coordinate definition and its relationship to modern reference systems are discussed in the abstract of Liu, Zhu and Zhang’s Reconsidering the Galactic Coordinate System. This astronomical coordinate discussion does not validate an astrological interpretation of a galactic ayanamsha.

The software definitions in the table follow Swiss Ephemeris §2.8.7–2.8.9. Giving the centre, star and intersection separate columns in your notes helps prevent similar names from obscuring different references.

Why do J2000, J1900 and B1950 form another group?

An epoch is a reference date used for a definition or coordinate. A chart’s calculation date, its coordinate reference, and its calendar system are separate pieces of information.

J denotes a Julian epoch and B a Besselian epoch. Selecting J2000 does not change a birth date to the year 2000 or select the Julian calendar.

J2000.0 is precisely 1 January 2000 at 12:00 TT, or JD2451545.0 TT. TT and UTC are different time scales; the U.S. Naval Observatory’s TT explanation also gives the UTC equivalent. Do not equate it with 1 January 2000 at 12:00 UTC.

For a precise coordinate comparison using these choices, checking a longitude offset alone may be insufficient. The epoch of the ecliptic and the transformation applied also matter. The Swiss Ephemeris programming manual, §12.1, discusses appropriate coordinate transformations for these special references. A menu label alone does not explain every technical detail of every output column.

Which epoch does the custom degree belong to?

Custom, enter degrees · user is the single custom-entry option alongside the 47 built-in records.

In the Hermes calculation, the custom degree is supplied as the initial ayanamsha at J2000.0. Calculations for other dates proceed from that definition through the library’s model. Entering a number does not mean subtracting one unchanging angle on every date.

For example, 23.5° specifies an initial value of 23°30′ at J2000. Copying a calculated ayanamsha of 23.5° from another date into this field would assign that other date’s value to J2000. Establish the correct initial value and definition before attempting to reproduce a built-in method.

Check decimal degrees too: 24°30′ = 24.5°, whereas 24.30° = 24°18′. These inputs differ by 12′.

A custom number does not automatically reproduce an entire algorithm that holds a star at its target on every date or uses a particular galactic projection. A close result on one date does not establish equivalent definitions for all dates.

The Swiss Ephemeris custom-reference documentation explains why both the reference epoch and initial ayanamsha are required.

Choose one research question in Hermes

Use the synthetic sample 20 March 2000, 12:00:00 UTC, 41° north, 29° east; Gregorian calendar. Verify the appropriate time-zone conversion if the form requires local time.

Fix Sidereal, Placidus, True Node, Normal cusp setting, Normal transformation and Normal coordinate view. Change only the ayanamsha in each trial.

This is a study plan; actual Hermes results for all 32 choices have not been generated here. If you add other dates, compare choices within each date first. Do not carry a difference found on a modern date centuries backwards as a constant correction.

Keep the full name/code, reference object, definition epoch, calculation date and time scale, version, output and source in your archive. When reconstructing an old date, specify the calendar and year-numbering convention too.

See the Hermes Chart Settings guide for saved preferences and new-form defaults. A useful research record answers both “Which ayanamsha did I select?” and “Which reference am I reconstructing under this name?”