What would Ptolemy have seen?

The zodiac Ptolemy defined, against the stars he saw: slide from 500 BCE to today.

Ptolemy's zodiac: signs and stars almost aligned, and anchored to the seasons by definition.

Ptolemy anchored the zodiac to the seasons: the vernal equinox is 0° Aries, by definition. Because Earth’s axis slowly precesses, that point creeps westward along the ecliptic, about one degree every 72 years. It carries the whole tropical zodiac with it, past stars that barely move. Slide through the centuries and watch the signs drift across the constellations. The planets are not moving into different places; only the labels change.

“It is reasonable to reckon the beginnings of the signs also from the equinoxes and solstices.”

Tetrabiblos I.22, trans. Robbins [18]

The vernal equinox among the stars

Tropical 0° Aries lies in the constellation (IAU boundaries, a 1930 convention).

Close up: 16° either side of the equinox, north up and east to the left, as in the sky. Top scale tropical, bottom scale sidereal (Lahiri).

Named stars nearest the vernal equinox in longitude
Nearest named stars	Δλ	β
ν Psc	0°14′ west	−4°48′
Alpherg (η Psc)	1°05′ east	+5°16′
Torcular (ο Psc)	1°57′ east	−1°45′

Two zodiacs, one sky

The is the gap between them: tropical minus sidereal longitude.

Ayanamsa for each sidereal definition
Sidereal: tropical longitude minus the ayanamsa, Lahirimodern standard	−1°53′
Sidereal: tropical longitude minus the ayanamsa, Fagan–Bradleymodern standard	−1°00′
Sidereal: tropical longitude minus the ayanamsa, Bab. (Huber)reconstruction	−1°00′
Sidereal: tropical longitude minus the ayanamsa, Bab. (Britton)reconstruction	−1°07′

In the Babylonian star-fixed zodiac the equinox fell at 1°00′ of sidereal Aries on Huber's reconstruction and 1°07′ on Britton's.

Bright stars in Ptolemy's frame

Tropical longitudes of five bright zodiacal stars
Spicaα Vir	28°08′Virgotropical
Regulusα Leo	4°13′Leotropical
Aldebaranα Tau	14°01′Taurustropical
Antaresα Sco	14°02′Scorpiotropical
Alcyone (Pleiades)η Tau	4°15′Taurustropical

Mean places of date: the stars' own motions and the long-term precession model.

Ptolemy's 36″ a year against the sky

Real rate for the date: 49.89″a year
Ptolemy (Almagest VII): 36″a year

Drift of his model since 137 CE

Since the Almagest epoch the sky has moved +0°10′; at Ptolemy's rate it would have moved +0°07′. His model lags the sky by 0°03′.

Before counting the catalogue's own offset of about 1° at its epoch. Real precession: the long-term model (Vondrák, Capitaine & Wallace 2011).

Every number on this page is computed by the app's astronomy engine for the year shown: mean places on the long-term precession model of Vondrák, Capitaine & Wallace (2011), with each star's own motion.

Tropical meets sidereal

When did the two zodiacs coincide?

The answer depends entirely on how the sidereal zodiac is defined. The two most used modern standards give 221 CE (Fagan–Bradley) and 285 CE (Lahiri). Other definitions spread the answer over roughly 220–570 CE.

Fagan–Bradley
222 CE
Anchor
Synetic vernal point; Spica at 29°06′05″ Vir (1950)
Stated
c. 221 CE (the widely quoted figure; Kollerstrom derives 221 ± 50 from Huber’s data)
Sources
[1][8]
Babylonian (Huber 1958)
222 CE
Anchor
−4°28′ ± 20′ at year −100
Stated
not stated
Sources
[1]
Babylonian (Britton 2010)
230 CE
Anchor
C = 3.20° ± 0.1° at year 0
Stated
not stated
Sources
[2][1]
Lahiri (ICRC)
285 CE
Anchor
23°15′00″ at 21 March 1956; Lahiri’s own zero at the mean equinox of 22 March 285
Stated
285 CE
Sources
[1]
True Chitra
285 CE
Anchor
Spica always at 180°
Stated
not stated
Sources
engine
Krishnamurti
292 CE
Anchor
Zero “in the year 291” (no exact date given)
Stated
291 CE
Sources
[1]
Raman
389 CE
Anchor
Raman’s own statement
Stated
389 CE
Sources
[1]
Āryabhaṭa / Sūrya Siddhānta (equinox-based)
not in the app's registry
Anchor
Kali year 3600
Stated
499 CE (522 CE in a later tradition)
Sources
[1]
Sassanian / Revatī (Mercier)
564 CE
Anchor
Zero point 10′ east of ζ Piscium
Stated
564 CE
Sources
[1]

The year the tropical and sidereal zodiacs coincided, by definition of the sidereal zodiac
Definition	Anchor	Stated zero year	Engine	Sources
Fagan–Bradley	Synetic vernal point; Spica at 29°06′05″ Vir (1950)	c. 221 CE (the widely quoted figure; Kollerstrom derives 221 ± 50 from Huber’s data)	222 CE	[1][8]
Babylonian (Huber 1958)	−4°28′ ± 20′ at year −100	not stated	222 CE	[1]
Babylonian (Britton 2010)	C = 3.20° ± 0.1° at year 0	not stated	230 CE	[2][1]
Lahiri (ICRC)	23°15′00″ at 21 March 1956; Lahiri’s own zero at the mean equinox of 22 March 285	285 CE	285 CE	[1]
True Chitra	Spica always at 180°	not stated	285 CE	engine
Krishnamurti	Zero “in the year 291” (no exact date given)	291 CE	292 CE	[1]
Raman	Raman’s own statement	389 CE	389 CE	[1]
Āryabhaṭa / Sūrya Siddhānta (equinox-based)	Kali year 3600	499 CE (522 CE in a later tradition)	not in the app's registry	[1]
Sassanian / Revatī (Mercier)	Zero point 10′ east of ζ Piscium	564 CE	564 CE	[1]

In Ptolemy’s own lifetime, the frames were only about 1–2° apart: at 150 CE the Fagan–Bradley value was −1°00′ and the Lahiri −1°53′.
By modern IAU boundaries, the vernal point had already left the constellation Aries for Pisces around 68 BCE [65][66].

One event, many dates

The “Age of Aquarius” depends on the convention

The equinox moves steadily; the year it “enters Aquarius” depends on where you draw the line.

When does the vernal point “enter Aquarius”?

Astrologers' published claimsCampion’s survey (1999) [66][67]1447–3597
Equal 30° signs: Fagan–Bradleythe ayanamsa reaches 30° (engine)2376 CE
Equal 30° signs: Lahirithe ayanamsa reaches 30° (engine)2439 CE
IAU boundary into Aquarius1930 constellation boundaries (engine)2597 CE

Now: 2026 CE

Using IAU constellation boundaries

The boundaries were drawn by Delporte along lines of right ascension and declination for the equinox of 1875, approved in 1928 and published in 1930 [68]. The vernal point crossed from Aries into Pisces about 68 BCE and will enter Aquarius about 2597 CE (Meeus [66]; Strous [65]). The engine finds the crossings in 68 BCE and 2597 CE. Some sources round to 2595 or “about 2600” [67].

Using equal 30° sidereal signs

The “age” changes when the ayanamsa reaches 30°. The engine puts that in 2376 CE for Fagan–Bradley and 2439 CE for Lahiri. At the present rate, each 30° takes 2,147 years; one source gives 2,147.5 [67].

Using astrologers' published claims

A survey by Nicholas Campion (1999) found proposed start dates ranging from 1447 to 3597 CE, with the twentieth century the most common choice [66][67].

Why they disagree

Constellations are unequal, and their borders are a 1930 convention. Equal signs need an anchor, which is an ayanamsa, and there are dozens. Some authors instead count from symbolic or historical events. The equinox moves steadily; the “age” boundary depends on the convention chosen.

Hipparchus, 2nd century BCE

How precession was found

His discovery came from comparing old and new measurements. Timocharis, observing early in the third century BCE, had placed Spica about 8° west of the autumnal equinox. Hipparchus found it about 6° west. Stars near the ecliptic had shifted about 2° relative to the equinoxes in some 150 years. He considered other explanations before settling on the essentially correct one: the whole sphere of stars turns slowly eastward relative to the equinoxes. As Ptolemy reports, he committed himself only to a minimum, “not less than” 1/100 of a degree a year. The true rate is about 1° in 72 years.

Spica against the autumnal equinox

Timocharis: lunar eclipse of 17 Mar 284 BCE (Julian), total: Spica at 172.14°, 7°52′ west of the autumnal equinox. Ptolemy quotes 172°.
Hipparchus: lunar eclipse of 21 Mar 135 BCE (Julian), total: Spica at 174.20°, 5°48′ west of the autumnal equinox. Ptolemy quotes 174°.

A shift of 2.06° in 149 years: 49.8″ a year by the engine. Hipparchus committed himself only to “not less than” 36″ a year.

Eclipses found by the engine's own eclipse search, as identified by Duke.

Measuring the drift

The rate, from Hipparchus to today

Arcseconds a year

Hipparchus (as quoted by Ptolemy)not less than 1/100 of one degree a year · a lower bound ≥ 36.0″
Ptolemy, Almagest VII1° in about 100 years (2°40′ in about 265 years) · adopted 36.0″
al-Battānī (c. 880)1° in 66 years · measured 54.5″
Maragha (for post-Ptolemaic data)1° in 70 years · adopted 51.4″
Latin Alfonsine Tables (mean, 16–1252 CE)about 1° in 72 years, the mean of a variable model · mean of a variable model 50.0″

Modern rate for today: 50.29″ a year

The Almagest epoch

1 Thoth 885 Nabonassar

= 20 July 137 CE (Julian) = JD 1771298

The catalogue longitudes are given “for the beginning of the reign of Antoninus”: 1 Thoth 885 of the era of Nabonassar = 20 July 137 CE (Julian) = JD 1771298 [26][19]. On this date the catalogue’s longitudes run about 1° lower than the true tropical positions [19][25][26].

Through the Julian calendar: JD 1771298
Through the Egyptian calendar of Nabonassar: JD 1771298

The engine reproduces the published Julian Day both ways.

Fact sheet

Every claim on this page

Each claim keeps its identifier from the research behind this page; values in mono are computed by the engine.

Show 19 claims

WWP-1Tetrabiblos I.22: “reckon the beginnings of the signs also from the equinoxes and solstices” [18].
WWP-2The rates in the precession-rate table [21][26][31][38][39][28]. Engine values: 49.89″ per year at 150 CE and 49.75″ at 500 BCE.
WWP-3The Almagest epoch is 1 Thoth 885 Nabonassar = 20 July 137 CE = JD 1771298 [26][19].
WWP-4The catalogue longitudes are about 1° too low for their epoch [19][25][26].
WWP-5The Fagan–Bradley zero is c. 221 CE (±50 years) [8], from the definition in [1]. Engine check: 222 CE.
WWP-6Huber’s value, −4°28′ ± 20′ at year −100, gives an engine zero year of 222 CE ± 24 years [1].
WWP-7Britton’s C = 3.20° ± 0.1° gives an engine zero year of 230 CE [2][1].
WWP-8The Lahiri zero is the mean vernal equinox of 22 March 285 CE (Lahiri, 1972) [1]. Engine check from the ICRC value: 285 CE.
WWP-9True Chitra: Spica stood at 180°00′17″ in the Lahiri frame in 285 [1]. Spica reaches tropical 180° in 285 CE.
WWP-10The Krishnamurti zero is 291 CE; Raman’s is 389 CE; the Āryabhaṭa/Sūrya Siddhānta equinox-based zero is 499 CE (522 CE in a later tradition); the Sassanian zero is 564 CE [1].
WWP-11ζ Psc reaches tropical 0° in 575 CE. The Swiss Ephemeris places the Sassanian zero point 10′ east of ζ Psc in 564 [1]. The engine check gives ζ Psc at 359.84° in 564, which is consistent.
WWP-12At 150 CE the frames were 1–2° apart: Fagan–Bradley −1°00′ and Lahiri −1°53′. Kollerstrom: “In his lifetime, the two wheels were only one degree apart” [8].
WWP-13The vernal point crossed from IAU Aries to Pisces in 68 BCE and will cross to Aquarius in 2597 (Meeus) [66][65]. Engine check: 68 BCE and 2597 CE. Another source gives 2595 [67].
WWP-14IAU boundaries: Delporte, along right ascension and declination for 1875, approved at Leiden in 1928 and published in 1930 [68].
WWP-15Equal-sign “Aquarius” years: Fagan–Bradley 2376 CE and Lahiri 2439 CE. One sign takes 2,147 years at the current rate; one source gives 2,147.5 [67].
WWP-16Campion (1999) found proposed start dates from 1447 to 3597 CE, with the twentieth century most common (29 claims) [66][67].
WWP-17The descriptions of the stars at each position of the slider are computed by the engine. “Pisces’ brightest star” for η Psc (V = 3.62) comes from BSC5 magnitudes.
WWP-18Robbins (1940): “with the precession of the equinoxes, the fictive sign Aries is now almost entirely in Pisces” [18].
WWP-19The true rate is about 1° per 72 years [25].

Sources

Where these statements come from

Researched and fact-checked for this app on 22 September 2026. Numbers in brackets refer to this list; the letter after each entry is how directly the source was read. Every value marked as computed comes from the app's astronomy engine, never from this list.

[1]Koch, D. & Treindl, A. Swiss Ephemeris documentation, Astrodienst: §2.8 “Ayanamshas” (the Babylonian, Hipparchan, Sūrya Siddhānta and Āryabhaṭa, Spica/Citra and Lahiri, Krishnamurti and other traditions) and §6.2 “Astrological house systems”. www.astro.com/swisseph/swisseph.htm. (technical documentation that quotes its primary definitions: ICRC, IAE, Lahiri 1972, Fagan and Firebrace 1971, Huber 1958, Britton 2010, Mercier 1976–77, Holden 1995).Level R: reference work, read
[2]Britton, J. P. (2010). “Studies in Babylonian lunar theory: part III. The introduction of the uniform zodiac.” Archive for History of Exact Sciences 64: 617–663. doi.org/10.1007/s00407-010-0064-z. (abstract read).Level S: scholarly source, read
[8]Kollerstrom, N. (1997). “The Star Zodiac of Antiquity.” Culture and Cosmos 1(2): 5–22. www.cultureandcosmos.org/pdfs/1-2/1_2_Kollerstrom_Star_Zodiac.pdf. (full text; the author argues one side of a debate, see §5).Level S: scholarly source, read
[18]Ptolemy, Tetrabiblos, ed. and trans. F. E. Robbins (Loeb Classical Library 435, 1940), Book I, chs. 11 and 22 with Robbins’s notes 62, 121 and 122; LacusCurtius transcription. penelope.uchicago.edu/thayer/e/roman/texts/ptolemy/tetrabiblos/1b*.html.Level P: primary text or a scholarly translation, read
[19]Duke, D. W. (2002). “Dating the Almagest Star Catalogue Using Proper Motions: A Reconsideration.” Journal for the History of Astronomy 33: 45–55. Author’s copy: people.sc.fsu.edu/~dduke/pmotion.pdf. (it quotes G. J. Toomer, Ptolemy’s Almagest (1984), pp. 339–340).Level S: scholarly source, read
[20]Duke, D. W. (c. 2003). “Ancient Declinations and Precession.” Manuscript (footer dated 6/13/2003), people.sc.fsu.edu/~dduke/decs.pdf. (not formally published, as far as could be determined).Level S: scholarly source, read
[21]Schironi, F. (2024). “Hipparchus’ Star Catalogues.” arXiv:2410.19228. (includes her own translation of Almagest VII.2).Level S: scholarly source, read
[23]Jones, A. R. “Hipparchus.” Encyclopaedia Britannica online. www.britannica.com/biography/Hipparchus-Greek-astronomer.Level R: reference work, read
[25]Linton, C. M. (2004). From Eudoxus to Einstein: A History of Mathematical Astronomy (Cambridge UP), ch. 3 “The Ptolemaic universe”, section “The motion of the Sun and precession”, with notes 9, 10 and 20.Level S: scholarly source, read
[26]Verbunt, F. & van Gent, R. H. (2012). “The star catalogues of Ptolemaios and Ulugh Beg.” Astronomy & Astrophysics 544, A31. arXiv:1206.0628.Level S: scholarly source, read
[28]Kaplan, G. H. (2005). The IAU Resolutions on Astronomical Reference Systems, Time Scales, and Earth Rotation Models. USNO Circular 179, Table 5.1. arXiv:astro-ph/0602086. (the IAU 2006 value 5028.796195″ per century comes from Capitaine et al. 2003).Level R: reference work, read
[29]Vondrák, J., Capitaine, N. & Wallace, P. (2011). “New precession expressions, valid for long time intervals.” Astronomy & Astrophysics 534, A22, as implemented in ERFA eraLtpequ, eraLtpecl and eraLtp (BSD-3). Used for the computed checks.Level S/C: scholarly source, read / computed check
[31]MacTutor, “Al-Battani”. mathshistory.st-andrews.ac.uk/Biographies/Al-Battani/.Level R: reference work, read
[38]Nothaft, C. P. E. (2017). “Criticism of trepidation models and advocacy of uniform precession in medieval Latin astronomy.” Archive for History of Exact Sciences 71: 211–244. doi.org/10.1007/s00407-016-0184-1. Author’s copy on ORA (Oxford). (the page numbers of the published version were not seen).Level S: scholarly source, read
[39]Mozaffari, S. M. (2016). “A medieval bright star table: the non-Ptolemaic star table in the Īlkhānī Zīj.” Journal for the History of Astronomy 47: 294–316. (abstract read, via VizieR catalogue J/other/JHA/47.294).Level S: scholarly source, read
[65]Strous, L. “Constellations”, Astronomy Answers (Utrecht), Table 2 and §7 “The Age of Aquarius”. www.aa.quae.nl/en/antwoorden/sterrenbeelden.html. (by an astronomer).Level W: general web source
[66]EarthSky, “When will the Age of Aquarius begin?” (reporting J. Meeus’s computations and N. Campion’s survey). earthsky.org/human-world/when-will-the-age-of-aquarius-begin/. (Meeus and Campion: not read directly; cited through another source).Level W: general web source
[67]Wikipedia, “Age of Aquarius”, citing N. Campion, The Book of World Horoscopes (1999), pp. 489–495. en.wikipedia.org/wiki/Age_of_Aquarius. (Campion 1999: not read directly; cited through another source).Level W: general web source
[68]Ridpath, I. “Constellation boundaries” and “Délimitation Scientifique des Constellations” (with a translation of Delporte’s 1930 introduction). www.ianridpath.com/boundaries.html and www.ianridpath.com/delporte.html. (Delporte 1930: primary text or a scholarly translation, read in translation).Level W: general web source

Computed with astronomy-engine 2.1.19 (MIT, Don Cross) and the long-term precession model of Vondrák, Capitaine & Wallace (2011), ported from ERFA (BSD-3-Clause). Stars: the Yale Bright Star Catalogue, 5th revised edition (Hoffleit & Warren 1991, via CDS) and SIMBAD (CDS, Strasbourg). Constellation figures: d3-celestial (Olaf Frohn, BSD-3-Clause). Constellation boundaries: IAU (Delporte 1930).

04Time Machine01

What would Ptolemy have seen?

The zodiac Ptolemy defined, against the stars he saw: slide from 500 BCE to today.

Turning the sky back to the Almagest…