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{{short description|Sanskrit text on Indian astronomy}}
 
{{italic title}}
[[File:1st verse of the 1st chapter of the Surya Siddhanta Hindu astronomy, 1847 Sanskrit manuscript edition.jpg|thumb|Verse 1.1 (prayer to [[Brahman]])]]
The '''''Surya Siddhanta''''' ({{IAST3|Sūrya Siddhānta}}; {{Literal translation|Sun Treatise}}) is a [[Sanskrit]] treatise in [[Indian astronomy]] dated to 4th to 5th century,<ref name=britsuryasiddhan>Menso Folkerts, Craig G. Fraser, Jeremy John Gray, John L. Berggren, Wilbur R. Knorr (2017), [https://www.britannica.com/topic/mathematics Mathematics], Encyclopaedia Britannica, Quote: "(...) its Hindu inventors as discoverers of things more ingenious than those of the Greeks. Earlier, in the late 4th or early 5th century, the anonymous Hindu author of an astronomical handbook, the ''Surya Siddhanta'', had tabulated the sine function (...)"</ref><ref name="Bowman2005p596"/> in fourteen chapters.<ref name=burgess/><ref name="Markanday551–561"/><ref name="Plofker2009p71-72">Plofker, [https://books.google.com/books?id=DHvThPNp9yMC&pg=PA71 pp. 71–72].</ref> The ''Surya Siddhanta'' describes rules to calculate the motions of various planets and the moon relative to various [[constellation]]s, diameters of various planets, and calculates the [[orbit]]s of various [[astronomical bodies]].<ref name="Thompson2007p76" /><ref name="montgomery104" /> The text is known from a {{Nowrap|15th-century CE}} [[palm-leaf manuscript]], and several newer [[manuscripts]].<ref name=":1">{{Cite book|last=Thompson|first=Richard L.|url=https://books.google.com/books?id=3TZmDSr-1msC|title=The Cosmology of the Bhāgavata Purāṇa: Mysteries of the Sacred Universe|publisher=Motilal Banarsidass|year=2007|isbn=978-81-208-1919-1|pages=15–18|language=en}}</ref> It was composed or revised probably c. 800 CE from an earlier text also called the ''Surya Siddhanta''.<ref name="Plofker2009p71-72" /> The ''Surya Siddhanta'' text is composed of verses made up of two lines, each broken into two halves, or ''pãds'', of eight syllables each.<ref name=burgess/>

As per [[al-Biruni]], the 11th-century Persian scholar and polymath, a text named the ''Surya Siddhanta'' was written by '''Lāṭadeva''', a student of [[Aryabhata|Aryabhatta I]].<ref name=":1" /><ref>{{Cite book |last=Hockey |first=Thomas |url=http://link.springer.com/10.1007/978-1-4419-9917-7 |title=Biographical Encyclopedia of Astronomers |date=2014 |publisher=Springer New York |isbn=978-1-4419-9916-0 |editor-last=Hockey |editor-first=Thomas |location=New York, NY |pages=1283 |language=en |chapter=Latadeva |doi=10.1007/978-1-4419-9917-7 |bibcode=2014bea..book.....H |s2cid=242158697 |editor-last2=Trimble |editor-first2=Virginia |editor-last3=Williams |editor-first3=Thomas R. |editor-last4=Bracher |editor-first4=Katherine |editor-last5=Jarrell |editor-first5=Richard A. |editor-last6=Marché |editor-first6=Jordan D. |editor-last7=Palmeri |editor-first7=JoAnn |editor-last8=Green |editor-first8=Daniel W. E.}}</ref> The second verse of the first chapter of the ''Surya Siddhanta'' attributes the words to an emissary of the [[solar deity]] of [[Hindu mythology]], [[Surya]], as recounted to an ''[[asura]]'' called [[Mayasura|Maya]] at the end of [[Satya Yuga]], the first golden age from Hindu texts, around two million years ago.<ref name=":1" />{{sfn|Gangooly|1989|p=ix (Introduction)|ps=: Calculated date of 2163102 B.C. for "the end of the Golden Age (Krta yuga)" mentioned in Surya Siddhanta 1.57.}}

The text asserts, according to Markanday and Srivatsava, that the Earth is of a spherical shape.<ref name="Markanday551–561">{{cite book | last1=Markanday | first1=Sucharit | last2=Srivastava | first2=P. S. | title=Oceanography: The Past | chapter=Physical Oceanography in India: An Historical Sketch | publisher=Springer New York | year=1980 | isbn=978-1-4613-8092-4 | doi=10.1007/978-1-4613-8090-0_50 | pages=551–561}}, Quote: "According to Surya Siddhanta the earth is a sphere."</ref> It treats Earth as stationary globe around which Sun orbits, and makes no mention of Uranus, Neptune and Pluto.<ref>{{cite book|author=Richard L. Thompson|title=Vedic Cosmography and Astronomy|url=https://books.google.com/books?id=9oi64rTVwNMC|year=2004|publisher=Motilal Banarsidass |isbn=978-81-208-1954-2|page=[https://books.google.com/books?id=9oi64rTVwNMC&pg=PA10 10]}}</ref> It calculates the Earth's diameter to be 8,000 miles (modern: 7,928 miles),<ref name="Thompson2007p76" /> the diameter of the [[Moon]] as 2,400 miles (actual ~2,160)<ref name="Thompson2007p76" /> and the [[Lunar distance (astronomy)|distance between the Moon and the Earth]] to be 258,000 miles<ref name="Thompson2007p76">{{cite book|author=Richard L. Thompson|title=The Cosmology of the Bhagavata Purana |url=https://books.google.com/books?id=3TZmDSr-1msC&pg=PA76 |year=2007|publisher=Motilal Banarsidass |isbn=978-81-208-1919-1|pages=16, 76–77, 285–294}}</ref> (now known to vary: {{convert|221500-252700|miles|km|abbr=off}}.<ref name="millimeter challenge">{{cite journal|last1=Murphy|first1=T W|date=1 July 2013|title=Lunar laser ranging: the millimeter challenge|url=http://physics.ucsd.edu/~tmurphy/papers/rop-llr.pdf|journal=Reports on Progress in Physics|volume=76|issue=7|page=2|arxiv=1309.6294|bibcode=2013RPPh...76g6901M|doi=10.1088/0034-4885/76/7/076901|pmid=23764926|s2cid=15744316}}</ref> The text is known for some of the earliest known discussions of fractions and [[trigonometric functions]].<ref name="britsuryasiddhan"/><ref name="Bowman2005p596">{{cite book|author=John Bowman|url=https://books.google.com/books?id=cYoHOqC7Yx4C&pg=PA596|title=Columbia Chronologies of Asian History and Culture|publisher=Columbia University Press|year=2000|isbn=978-0-231-50004-3|page=596}}, Quote: "c. 350-400: The Surya Siddhanta, an Indian work on astronomy, now uses sexagesimal fractions. It includes references to trigonometric functions. The work is revised during succeeding centuries, taking its final form in the tenth century."</ref><ref>{{cite book|author=Brian Evans|title=The Development of Mathematics Throughout the Centuries: A Brief History in a Cultural Context|url=https://books.google.com/books?id=3CPwAgAAQBAJ&pg=PT60 |year= 2014|publisher= Wiley|isbn= 978-1-118-85397-9|page=60}}</ref>

The ''Surya Siddhanta'' is one of several astronomy-related Hindu texts. It represents a functional system that made reasonably accurate predictions.<ref name="pingree2292">David Pingree (1963), Astronomy and Astrology in India and Iran, Isis, Volume 54, Part 2, No. 176, pages 229-235 with footnotes</ref><ref name="duke5632">{{cite journal|last=Duke|first=Dennis|year=2005|title=The Equant in India: The Mathematical Basis of Ancient Indian Planetary Models|journal=Archive for History of Exact Sciences|publisher=Springer Nature|volume=59|issue=6|pages=563–576|doi=10.1007/s00407-005-0096-y|bibcode=2005AHES...59..563D|s2cid=120416134}}</ref><ref>{{cite journal | last=Pingree | first=David | title=On the Greek Origin of the Indian Planetary Model Employing a Double Epicycle | journal=Journal for the History of Astronomy | publisher=SAGE Publications | volume=2 | issue=2 | year=1971 | doi=10.1177/002182867100200202 | pages=80–85| bibcode=1971JHA.....2...80P | s2cid=118053453 }}</ref> The text was influential on the [[Solar Year|solar year]] computations of the luni-solar [[Hindu calendar]].<ref>{{cite book|author=Roshen Dalal|title=Hinduism: An Alphabetical Guide|url=https://books.google.com/books?id=DH0vmD8ghdMC |year=2010|publisher=Penguin Books |isbn=978-0-14-341421-6|page=[https://books.google.com/books?id=DH0vmD8ghdMC&pg=PA89 89]}}, Quote: "The solar calendar is based on the Surya Siddhanta, a text of around 400 CE."</ref> The text was translated into [[Arabic]] and was influential in medieval [[Islamic geography]].<ref>{{Citation|last=Canavas|first=Constantin|title=Geography and Cartography|date=2014|url=https://orca.cardiff.ac.uk/id/eprint/154649/1/Optics%20and%20Optical%20Theories.pdf|encyclopedia=The Oxford Encyclopedia of Philosophy, Science, and Technology in Islam|publisher=Oxford University Press|language=en|doi=10.1093/acref:oiso/9780199812578.001.0001|isbn=978-0-19-981257-8|access-date=2020-07-19}}</ref> The Surya Siddhanta has the largest number of commentators among all the astronomical texts written in India. It includes information about the mean orbital parameters of the planets, such as the number of mean revolutions per ''[[Yuga Cycle|Mahayuga]],'' the longitudinal changes of the orbits, and also includes supporting evidence and calculation methods.<ref name=burgess/>

==Textual history==
{{further|Jyotisha}}
In a work called the ''[[Pancha-siddhantika|Pañca-siddhāntikā]]'' composed in the sixth century by [[Varāhamihira]], five astronomical treatises are named and summarised: ''[[Paulisa Siddhanta|Paulīśa-siddhānta]]'', ''[[Romaka Siddhanta|Romaka-siddhānta]]'', ''[[Vasishtha Siddhanta|Vasiṣṭha-siddhānta]]'', ''Sūrya-siddhānta'', and ''Paitāmaha-siddhānta''.{{rp|50}} Most scholars place the surviving version of the text variously from the 4th century to the 5th century CE,<ref name="Bowman2005p5962">{{cite book|author=John Bowman|url=https://books.google.com/books?id=cYoHOqC7Yx4C&pg=PA596|title=Columbia Chronologies of Asian History and Culture|publisher=Columbia University Press|year=2005|isbn=978-0-231-50004-3|page=596}}, Quote: "c. 350-400: The Surya Siddhanta, an Indian work on astronomy, now uses sexagesimal fractions. It includes references to trigonometric functions. The work is revised during succeeding centuries, taking its final form in the tenth century."</ref><ref name="BoyerMerzbach2011p1882">{{cite book|author1=Carl B. Boyer|url=https://books.google.com/books?id=bR9HAAAAQBAJ|title=A History of Mathematics|author2=Uta C. Merzbach|publisher=John Wiley & Sons|year=2011|isbn=978-0-470-63056-3|page=188|author2-link=Uta Merzbach}}</ref> although it is dated to about the 6th-century BCE by Markandaya and Srivastava.<ref name="Markanday551–5612">{{cite book|last1=Markanday|first1=Sucharit|title=Oceanography: The Past|last2=Srivastava|first2=P. S.|publisher=Springer New York|year=1980|isbn=978-1-4613-8092-4|pages=551–561|chapter=Physical Oceanography in India: An Historical Sketch|doi=10.1007/978-1-4613-8090-0_50}}, Quote: "According to Surya Siddhanta the earth is a sphere."</ref>

According to John Bowman, the version of the text existed between 350 and 400 CE wherein it referenced fractions and trigonometric functions, but the text was a living document and revised through about the 10th century.<ref name="Bowman2005p5962" /> One of the evidence for the ''Surya Siddhanta'' being a living text is the work of medieval Indian scholar [[Utpala (astronomer)|Utpala]], who cites and then quotes ten verses from a version of ''Surya Siddhanta'', but these ten verses are not found in any surviving manuscripts of the text.<ref>Romesh Chunder Dutt, ''[https://books.google.com/books?id=CS8QAAAAYAAJ&pg=PA208 A History of Civilization in Ancient India, Based on Sanscrit Literature]'', vol. 3, {{ISBN|0-543-92939-6}} p. 208.</ref> According to [[Kim Plofker]], large portions of the more ancient ''Sūrya-siddhānta'' was incorporated into the ''Panca siddhantika'' text, and a new version of the ''Surya Siddhanta'' was likely revised and probably composed around 800 CE.<ref name="Plofker2009p71-72"/> Some scholars refer to ''Panca siddhantika'' as the old ''Surya Siddhanta'' and date it to 505 CE.<ref name="Selin2008p10352">{{cite book|author=George Abraham|url=https://books.google.com/books?id=kt9DIY1g9HYC|title=Encyclopaedia of the History of Science, Technology, and Medicine in Non-Western Cultures|publisher=Springer Science|year=2008|isbn=978-1-4020-4559-2|editor=Helaine Selin|pages=1035–1037, 1806, 1937–1938}}</ref>

Based on a study of the longitude variation data from the text, Indian scientist Anil Narayanan (2010) concludes that the text has been updated several times in the past, with the last update around 580 CE. Narayan obtained a match for the [[nakshatra]] latitudinal data in the period 7300-7800 BCE based on a computer simulation.<ref>Narayanan, Anil, {{citation | title=Dating the Surya Siddhanta using Computational Simulation of Proper Motions and Ecliptic Variations | url=https://insa.nic.in/writereaddata/UpLoadedFiles/IJHS/Vol45_4_1_ANarayan.pdf | publisher=Indian Journal of History of Science}}.</ref>

=== Vedic influence ===
The ''Surya Siddhanta'' is a text on astronomy and time keeping, an idea that appears much earlier as the field of [[Jyotisha]] ([[Vedanga]]) of the Vedic period. The field of Jyotisha deals with ascertaining time, particularly forecasting auspicious dates and times for Vedic rituals.<ref name="jameslochtefeldsca3262">James Lochtefeld (2002), "Jyotisha" in The Illustrated Encyclopedia of Hinduism, Vol. 1: A–M, Rosen Publishing, {{ISBN|0-8239-2287-1}}, pages 326–327</ref> [[Vedic sacrifice]]s state that the ancient Vedic texts describe four measures of time – ''savana'', solar, lunar and sidereal, as well as twenty seven constellations using ''Taras'' (stars).<ref>{{cite book|author=Friedrich Max Müller|url=https://books.google.com/books?id=h8cFAAAAQAAJ|title=On Ancient Hindu Astronomy and Chronology|publisher=Oxford University Press|year=1862|pages=37–60 with footnotes|bibcode = 1862ahac.book.....M|author-link=Max Müller}}</ref> According to mathematician and classicist [[David Pingree]], in the Hindu text ''[[Atharvaveda]]'' (~1000 BCE or older) the idea already appears of twenty eight constellations and movement of astronomical bodies.<ref name="pingree2292" />

According to Pingree, the influence may have flowed the other way initially, then flowed into India after the arrival of [[Darius the Great|Darius]] and the [[Achaemenid conquest of the Indus Valley]] about 500 BCE. The mathematics and devices for time keeping mentioned in these ancient Sanskrit texts, proposes Pingree, such as the water clock may also have thereafter arrived in India from Mesopotamia. However, Yukio Ôhashi considers this proposal as incorrect,{{Sfn|Yukio Ôhashi|1999|pp=719–721}} suggesting instead that the Vedic timekeeping efforts, for forecasting appropriate time for rituals, must have begun much earlier and the influence may have flowed from India to Mesopotamia.{{Sfn|Yukio Ôhashi|1993|pp=185–251}} Ôhashi states that it is incorrect to assume that the number of civil days in a year equal 365 in both Indian (Hindu) and Egyptian–Persian year.{{Sfn|Yukio Ôhashi|1999|pp=719–720}} Further, adds Ôhashi, the Mesopotamian formula is different than Indian formula for calculating time, each can only work for their respective latitude, and either would make major errors in predicting time and calendar in the other region.<ref>{{cite book|author=Yukio Ôhashi|url=https://books.google.com/books?id=8X0iCQAAQBAJ|title=History of Oriental Astronomy|publisher=Springer Science|year=2013|isbn=978-94-015-9862-0|editor=S.M. Ansari|pages=75–82}}</ref>

Kim Plofker states that while a flow of timekeeping ideas from either side is plausible, each may have instead developed independently, because the loan-words typically seen when ideas migrate are missing on both sides as far as words for various time intervals and techniques.{{Sfn|Plofker|2009|pp=[https://books.google.com/books?id=DHvThPNp9yMC&pg=PA41 41–42]}}<ref>{{cite journal|last=Sarma|first=Nataraja|year=2000|title=Diffusion of astronomy in the ancient world|journal=Endeavour|publisher=Elsevier|volume=24|issue=4|pages=157–164|doi=10.1016/s0160-9327(00)01327-2|pmid=11196987}}</ref>

=== Greek influence ===
It is hypothesized that contacts between the ancient Indian scholarly tradition and [[Hellenistic Greece]] via the [[Indo-Greek Kingdom]] after the [[Indian campaign of Alexander the Great]], specifically regarding the work of [[Hipparchus]] (2nd-century BCE), explain some similarities between ''Surya Siddhanta'' and [[Ancient Greek astronomy#Hellenistic astronomy|Greek astronomy]] in the [[Hellenistic period]]. For example, ''Surya Siddhanta'' provides table of [[sine]]s function which parallel the Hipparchian table of [[Chord (geometry)|chords]], though the Indian calculations are more accurate and detailed.<ref name="p. 1862">"There are many evident indications of a direct contact of Hindu astronomy with Hellenistic tradition, e.g. the use of epicycles or the use of tables of chords which were transformed by the Hindus into tables of sines. The same mixture of elliptic arcs and declination circles is found with Hipparchus and in the early Siddhantas (note: [...] In the Surya Siddhanta, the zodiacal signs are used in similar fashion to denote arcs on any great circle." Otto Neugebauer, ''The Exact Sciences in Antiquity'', vol. 9 of Acta historica scientiarum naturalium et medicinalium, Courier Dover Publications, 1969,  [https://books.google.com/books?id=JVhTtVA2zr8C&pg=PA186 p. 186].</ref> According to Alan Cromer, the Greek influence most likely arrived in India by about 100 BCE.<ref name="p. 111">"The table must be of Greek origin, though written in the Indian number system and in Indian units. It was probably calculated around 100 B.C. by an Indian mathematicisn familiar with the work of Hipparchus." Alan Cromer, ''Uncommon Sense : The Heretical Nature of Science'',  Oxford University Press, 1993,  [https://books.google.com/books?id=8cT2C87tb-sC&pg=PA111 p. 111].</ref> The Indians adopted the Hipparchus system, according to Cromer, and it remained that simpler system rather than those made by [[Ptolemy]] in the 2nd century.<ref>"The epicyclic model in the ''Siddnahta Surya'' is much simpler than Ptolemy's and supports the hypothesis that the Indians learned the original system of Hipparchus when they had contact with the West." Alan Cromer, ''Uncommon Sense : The Heretical Nature of Science'',  Oxford University Press, 1993,  [https://books.google.com/books?id=8cT2C87tb-sC&pg=PA111 p. 111].</ref>

The influence of Greek ideas on early medieval era Indian astronomical theories, particularly zodiac symbols ([[astrology]]), is broadly accepted by the Western scholars.<ref name="p. 1862"/> According to Pingree, the 2nd-century CE cave inscriptions of [[Nasik]] mention sun, moon and five planets in the same order as found in [[Babylon]], but "there is no hint, however, that the Indian had learned a method of computing planetary positions in this period".<ref name="pingree234">David Pingree (1963), Astronomy and Astrology in India and Iran, Isis, Volume 54, Part 2, No. 176, pages 233-238 with footnotes</ref> In the 2nd-century CE, a scholar named Yavanesvara translated a Greek astrological text, and another unknown individual translated a second Greek text into Sanskrit. Thereafter started the diffusion of Greek and Babylonian ideas on astronomy and astrology into India.<ref name="pingree234" /> The other evidence of European influential on the Indian thought is ''Romaka Siddhanta'', a title of one of the Siddhanta texts contemporary to ''Surya Siddhanta'', a name that betrays its origin and probably was derived from a translation of a European text by Indian scholars in [[Ujjain]], then the capital of an influential central Indian large kingdom.<ref name="pingree234" />

According to mathematician and historian of measurement John Roche, the astronomical and mathematical methods developed by Greeks related arcs to chords of spherical trigonometry.<ref name="Roche1998p482">{{cite book|author=John J. Roche|url=https://books.google.com/books?id=eiQOqS-Q6EkC&pg=PA48|title=The Mathematics of Measurement: A Critical History|publisher=Springer Science|year=1998|isbn=978-0-387-91581-4|page=48}}</ref> The Indian mathematical astronomers, in their texts such as the ''Surya Siddhanta,'' developed other linear measures of angles, made their calculations differently, "introduced the versine, which is the difference between the radius and cosine, and discovered various trigonometrical identities".<ref name="Roche1998p482" /> For instance "where the Greeks had adopted 60 relative units for the radius, and 360 for circumference", the Indians chose 3,438 units and 60x360 for the circumference thereby calculating the "ratio of circumference to diameter [pi, π] of about 3.1414".<ref name="Roche1998p482" />
The ''Surya Siddhanta'' was one of the two books in Sanskrit that were translated into Arabic in the later half of the eighth century during the reign of Abbasid caliph [[Al-Mansur]].
 {{Citation needed|date=April 2022}}

== Importance in history of science ==
{| class="wikitable" align="center" style=" background: transparent; "
|+Astronomical calculations: Estimated time per sidereal revolution{{r|burgess|pages=26–27}}</ref>
| width="60px" |Planet
| width="200px" style="background: #ffad66;" |''Surya Siddhanta''
| width="200px" |[[Ptolemy]]
| width="200px" |20th-century
|- style="text-align: center;"
|Mangala (Mars)
|686 days, 23 hours, 56 mins, 23.5 secs
|686 days, 23 hours, 31 mins, 56.1 secs
|686 days, 23 hours, 30 mins, 41.4 secs
|- style="text-align: center;"
|Budha (Mercury)
|87 days, 23 hours, 16 mins, 22.3 secs
|87 days, 23 hours, 16 mins, 42.9 secs
|87 days, 23 hours, 15 mins, 43.9 secs
|- style="text-align: center;"
|Bṛhaspati (Jupiter)
|4,332 days, 7 hours, 41 mins, 44.4 secs
|4,332 days, 18 hours, 9 mins, 10.5 secs
|4,332 days, 14 hours, 2 mins, 8.6 secs
|- style="text-align: center;"
|Shukra (Venus)
|224 days, 16 hours, 45 mins, 56.2 secs
|224 days, 16 hours, 51 mins, 56.8 secs
|224 days, 16 hours, 49 mins, 8.0 secs
|- style="text-align: center;"
|Shani (Saturn)
|10,765 days, 18 hours, 33 mins, 13.6 secs
|10,758 days, 17 hours, 48 mins, 14.9 secs
|10,759 days, 5 hours, 16 mins, 32.2 secs
|}

The tradition of Hellenistic astronomy ended in the West after [[Late Antiquity]]. According to Cromer, the ''Surya Siddhanta'' and other Indian texts reflect the primitive state of Greek science, nevertheless played an important part in the [[history of science]], through its translation in Arabic and stimulating the Arabic sciences.<ref>{{Cite news|date=2020-01-24|title=Surya Siddhanta the basis of space studies, says Governor|language=en-IN|work=The Hindu|url=https://www.thehindu.com/news/national/kerala/surya-siddhanta-the-basis-of-space-studies-says-governor/article30644090.ece|access-date=2021-09-02|issn=0971-751X}}</ref><ref>Alan Cromer (1993), ''[https://books.google.com/books?id=8cT2C87tb-sC Uncommon Sense : The Heretical Nature of Science]'',  Oxford University Press, pp. 111-112.</ref> According to a study by Dennis Duke that compares Greek models with Indian models based on the oldest Indian manuscripts such as the ''Surya Siddhanta'' with fully described models, the Greek influence on Indian astronomy is strongly likely to be pre-[[Ptolemy|Ptolemaic]].<ref name="duke5632"/>

The ''Surya Siddhanta'' was one of the two books in Sanskrit translated into Arabic in the later half of the eighth century during the reign of Abbasid caliph [[Al-Mansur]]. According to Muzaffar Iqbal, this translation and that of Aryabhatta was of considerable influence on geographic, astronomy and related Islamic scholarship.<ref name="Muzaffar Iqbal 2007 36–38">{{cite book|author=Muzaffar Iqbal|url=https://books.google.com/books?id=3KVAmUrZ_bcC&pg=PA36|title=Science and Islam|publisher=Greenwood Publishing|year=2007|isbn=978-0-313-33576-1|pages=36–38}}</ref>

==Contents==
[[File:Surya Siddhantha Mean Motion of Planets.ogv|thumb|The mean (circular) motion of planets according to the ''Surya Siddhantha''.]]
[[File:Surya Siddhantha True Position of Mercury.ogv|thumb|The variation of the true position of Mercury around its mean position according to the ''Surya Siddhantha''.]]
The contents of the ''Surya Siddhanta'' is written in [[classical Indian poetry]] tradition, where complex ideas are expressed lyrically with a rhyming meter in the form of a terse ''[[shloka]]''.<ref name="Gittleman1975p104"/> This method of expressing and sharing knowledge made it easier to remember, recall, transmit and preserve knowledge. However, this method also meant secondary rules of interpretation, because numbers don't have rhyming synonyms. The creative approach adopted in the ''Surya Siddhanta'' was to use [[Symbolic language (literature)|symbolic language]] with double meanings. For example, instead of one, the text uses a word that means moon because there is one moon. To the skilled reader, the word moon means the number one.<ref name="Gittleman1975p104">{{cite book|author=Arthur Gittleman|title=History of mathematics|url=https://books.google.com/books?id=gtbuAAAAMAAJ| year=1975| publisher=Merrill| isbn=978-0-675-08784-1|pages=104–105}}</ref> The entire table of trigonometric functions, sine tables, steps to calculate complex orbits, predict eclipses and keep time are thus provided by the text in a poetic form. This cryptic approach offers greater flexibility for poetic construction.<ref name="Gittleman1975p104"/><ref name="Mercier2004p53">{{cite book|author=Raymond Mercier|title=Studies on the Transmission of Medieval Mathematical Astronomy |url=https://books.google.com/books?id=UW7vAAAAMAAJ |year=2004|publisher=Ashgate |isbn=978-0-86078-949-9|page=53}}</ref>

The ''Surya Siddhanta'' thus consists of cryptic rules in Sanskrit verse. It is a compendium of astronomy that is easier to remember, transmit and use as reference or aid for the experienced, but does not aim to offer commentary, explanation or proof.<ref name="BoyerMerzbach2011p1882"/> The text has 14 chapters and 500 shlokas. It is one of the eighteen astronomical [[siddhanta]] (treatises), but thirteen of the eighteen are believed to be lost to history. The ''Surya Siddhanta'' text has survived since the ancient times, has been the best known and the most referred astronomical text in the Indian tradition.<ref name=montgomery104>{{cite book|author1=Scott L. Montgomery|author2=Alok Kumar|title=A History of Science in World Cultures: Voices of Knowledge|url=https://books.google.com/books?id=kkLeCQAAQBAJ&pg=PA104|year=2015|publisher=Routledge|isbn=978-1-317-43906-6|pages=104–105}}</ref>

The fourteen chapters of the ''Surya Siddhanta'' are as follows, per the much cited Burgess translation:<ref name="Markanday551–561"/><ref>{{cite book|author=Enrique A. González-Velasco|title=Journey through Mathematics: Creative Episodes in Its History|url=https://books.google.com/books?id=0sTd4qJgOmsC&pg=PA28 |year=2011|publisher=Springer Science|isbn=978-0-387-92154-9|pages=27–28 footnote 24}}</ref>
# Of the Mean Motions of the [[Planet]]s<ref name=burgess>{{cite book |last1=Burgess |first1=Ebenezer |editor1-last=Gangooly |editor1-first=Phanindralal |title=Surya Siddhanta Translation |date=1935 |publisher=University of Calcutta |page=1 |url=https://archive.org/details/SuryaSiddhantaTranslation/surya_siddhanta_english?view=theater |access-date=14 March 2024}}</ref>
#On the True Places of the Planets{{r|burgess|p=53}}
#Of Direction, Place and Time{{r|burgess|p=108}}
#Of Eclipses, and Especially of Lunar Eclipses{{r|burgess|p=143}}
#Of Parallax in a Solar Eclipse{{r|burgess|p=161}}
#The Projection of Eclipses{{r|burgess|p=178}}
#Of Planetary Conjunctions{{r|burgess|p=187}}
#Of the Asterisms{{r|burgess|p=202}}
#Of Heliacal (Sun) Risings and Settings{{r|burgess|p=255}}
#The Moon's Risings and Settings, Her Cusps{{r|burgess|p=262}}
#On Certain Malignant Aspects of the Sun and Moon{{r|burgess|p=273}}
#Cosmogony, Geography, and Dimensions of the Creation{{r|burgess|p=281}}
#Of the Armillary Sphere and other Instruments{{r|burgess|p=298}}
#Of the Different Modes of Reckoning Time{{r|burgess|p=310}}
The methods for computing time using the shadow cast by a [[gnomon]] are discussed in both Chapters 3 and 13.

=== Description of Time ===
The author of ''Surya Siddhanta'' defines time as of two types: the first which is continuous and endless, destroys all animate and inanimate objects and second is time which can be known. This latter type is further defined as having two types: the first is ''Murta'' (Measureable) and ''Amurta'' (immeasureable because it is too small or too big). The time ''Amurta'' is a time that begins with an infinitesimal portion of time (''[[Truti]]'') and ''Murta'' is a time that begins with 4-second time pulses called ''Prana'' as described in the table below. The further description of ''Amurta'' time is found in [[Puranas]] where as ''Surya Siddhanta'' sticks with measurable time.<ref name=":0">{{Cite book|last=Deva Shastri|first=Pandit Bapu|title=Translation of the Surya Siddhanta|pages=2–3}}</ref>
{| class="wikitable"
|+Time described in ''Surya Siddhanta''<ref name=":0" />
!Type
!''Surya Siddhanta'' Units
!Description
!Value in modern units of time
|-
|''Amurta''
|Truti
|1/33750 seconds
|29.6296 micro seconds
|-
|''Murta''
|Prana
| -
|4 seconds
|-
|''Murta''
|Pala
|6 Pranas
|24 seconds
|-
|''Murta''
|Ghatika
|60 Palas
|24 minutes
|-
|''Murta''
|Nakshatra Ahotra
|60 Ghatikas
|One [[Sidereal day]]
|}
The text measures a ''savana'' day from sunrise to sunrise. Thirty of these ''savana'' days make a ''savana'' month. A solar (''saura'') month starts with the entrance of the sun into a [[Zodiac Sign|zodiac sign]], thus twelve months make a year.<ref name=":0" />

The text further states there are nine modes of measuring time. "Of four modes, namely solar, lunar, sidereal, and civil time, practical use is made among men; by that of Jupiter is to be determined the year of the cycle of sixty years; of the rest, no use is ever made".<ref>{{cite book |last1=Burgess |first1=Ebenezer |title=TRANSLATION OF THE SURYA-SIDDHANTA |date=1860 |publisher=UNIVERSITY OF CALCUTTA |page=310 |edition=1935}}</ref>

===North pole star and South pole star===
''Surya Siddhanta'' asserts that there are two pole stars, one each at north and south [[celestial pole]]. ''Surya Siddhanta'' chapter 12 verse 43 description is as following:

<nowiki>
मेरोरुभयतो मध्ये ध्रुवतारे नभ:स्थिते।
निरक्षदेशसंस्थानामुभये क्षितिजाश्रिये॥१२:४३॥
</nowiki>

This translates as "On both sides of the Meru (i.e. the north and south poles of the earth) the two polar stars are situated in the heaven at their zenith. These two stars are in the horizon of the cities situated on the equinoctial regions".<ref name="Pundit Sastri">{{cite book |last1=Deva Sastri |first1=Pundit Bapu |title=The Translation of Surya Siddhanta |date=1861 |publisher=Baptist Mission Press |location=Calcutta |pages=80–81 |url=https://www.wilbourhall.org/pdfs/suryaEnglish.pdf}}</ref>

=== The Sine table ===
{{further|Āryabhaṭa's sine table}}
The ''Surya Siddhanta'' provides methods of calculating the sine values in chapter 2. It divides the quadrant of a circle with radius 3438 into 24 equal segments or sines as described in the table. In modern-day terms, each of these 24 segments has angle of 3.75°.<ref>{{Cite book|last=Deva Shastri|first=Pundit Bapu|title=Translation of the Surya Siddhanta|year=1861|pages=15–16}}</ref>
{| class="wikitable"
|+''Table of Sines'' {{r|burgess|pages=115}}
!No.
!Sine
!1st order 
differences
!2nd order 
differences
!No.
!Sine
!1st order 
differences
!2nd order 
differences
|-
|'''0'''
|0
|<nowiki>-</nowiki>
|<nowiki>-</nowiki>
|'''13'''
|2585
|154
|10
|-
|'''1'''
|225
|225
|1
|'''14'''
|2728
|143
|11
|-
|'''2'''
|449
|224
|2
|'''15'''
|2859
|131
|12
|-
|'''3'''
|671
|222
|3
|'''16'''
|2978
|119
|12
|-
|'''4'''
|890
|219
|4
|'''17'''
|3084
|106
|13
|-
|'''5'''
|1105
|215
|5
|'''18'''
|3177
|93
|13
|-
|'''6'''
|1315
|210
|5
|'''19'''
|3256
|79
|14
|-
|'''7'''
|1520
|205
|6
|'''20'''
|3321
|65
|14
|-
|'''8'''
|1719
|199
|8
|'''21'''
|3372
|51
|14
|-
|'''9'''
|1910
|191
|8
|'''22'''
|3409
|37
|14
|-
|'''10'''
|2093
|183
|9
|'''23'''
|3431
|22
|15
|-
|'''11'''
|2267
|174
|10
|'''24'''
|3438
|7
|15
|-
|'''12'''
|2431
|164
|10
|
|
|
|
|}
The 1st order difference is the value by which each successive sine increases from the previous and similarly the 2nd order difference is the increment in the 1st order difference values. ''Burgess'' says, it is remarkable to see that the 2nd order differences increase as the sines and each, in fact, is about 1/225th part of the corresponding sine.<ref name=burgess />

=== Calculation of tilt of Earth's axis (Obliquity) ===
The tilt of the ecliptic varies between 22.1° to 24.5° and is currently 23.5°.<ref>{{Cite web|date=2000-03-24|title=Milutin Milankovitch|url=https://earthobservatory.nasa.gov/features/Milankovitch/milankovitch_2.php#:~:text=Today,%20the%20Earth%27s%20axis%20is,know%20them%20can%20become%20exaggerated.|access-date=2020-08-15|website=earthobservatory.nasa.gov|language=en}}</ref> Following the sine tables and methods of calculating the sines, ''Surya Siddhanta'' also attempts to calculate the Earth's tilt of contemporary times as described in chapter 2 and verse 28, the obliquity of the [[Earth's Axis|Earth's axis]], the verse says "The sine of greatest declination is 1397; by this multiply any sine, and divide by radius; the arc corresponding to the result is said to be the declination".{{r|burgess|p=65}} The greatest declination is the inclination of the plane of the ecliptic. With radius of 3438 and sine of 1397, the corresponding angle is 23.975° or 23° 58' 30.65" which is approximated to be 24°.{{r|burgess|p=118}}

===Planets and their characteristics===
{{Quote box
 |quote  = '''Question: How Can the Earth Be a Sphere?'''
<poem>
Thus everywhere on the terrestrial globe (bhūgola),
people suppose their own place higher,
yet this globe (gola) is in space where there is no above nor below.
</poem>
 |source = —''Surya Siddhanta, XII.53''<br>Translator: Scott L. Montgomery, Alok Kumar<ref name=montgomery104/>{{r|burgess|page=289|at=verse 53}}
 |bgcolor=#FFE0BB
 |align  = right
}}
The text treats earth as a stationary globe around which sun, moon and five planets orbit. It makes no mention of Uranus, Neptune and Pluto.<ref name=thompson10>{{cite book|author=Richard L. Thompson|title=Vedic Cosmography and Astronomy|url=https://books.google.com/books?id=9oi64rTVwNMC&pg=PA10 |year=2004| publisher=Motilal Banarsidass |isbn= 978-81-208-1954-2|pages=10–11}}</ref> It presents mathematical formulae to calculate the orbits, diameters, predict their future locations and cautions that the minor corrections are necessary over time to the formulae for the various astronomical bodies.<ref name=burgess />

The text describes some of its formulae with the use of very large numbers for "''[[Yuga Cycle|divya-yuga]]''", stating that at the end of this ''[[yuga]]'', Earth and all astronomical bodies return to the same starting point and the cycle of existence repeats again.<ref name="thompson14">{{cite book|author=Richard L. Thompson|url=https://books.google.com/books?id=9oi64rTVwNMC&pg=PA10|title=Vedic Cosmography and Astronomy|publisher=Motilal Banarsidass|year=2004|isbn=978-81-208-1954-2|pages=12–14 with Table 3}}</ref> These very large numbers based on ''divya-yuga'', when divided and converted into decimal numbers for each planet, give reasonably accurate [[sidereal period]]s when compared to modern era western calculations.<ref name=thompson14/> 
{| class="wikitable"
|+[[Orbital period|Sidereal Periods]]<ref name="thompson14" />
!
!''Surya Siddhanta''
!Modern values
|-
|Moon
|27.322 days
|27.32166 days
|-
|Mercury
|87.97 days
|87.969 days
|-
|Mars
|687 days
|686.98 days
|-
|Venus
|224.7 days
|224.701 days
|-
|Jupiter
|4,332.3 days
|4,332.587 days
|-
|Saturn
|10,765.77 days
|10,759.202 days
|}

===Calendar===
{{See also|Astronomical basis of the Hindu calendar}}
The solar part of the luni-solar [[Hindu calendar]] is based on the ''Surya Siddhanta''.<ref name="Dalal2010p145">{{cite book|author=Roshen Dalal|title=The Religions of India: A Concise Guide to Nine Major Faiths|url=https://books.google.com/books?id=pNmfdAKFpkQC&pg=PA145 |year=2010|publisher=Penguin Books |isbn=978-0-14-341517-6|page=145}}</ref> The various old and new versions of ''Surya Siddhanta'' manuscripts yield the same solar calendar.<ref>{{cite book|author1=Robert Sewell|author2=Śaṅkara Bālakr̥shṇa Dīkshita|title=The Indian Calendar |year=1896|url= https://books.google.com/books?id=Q8MiAQAAMAAJ&pg=PA53| publisher=S. Sonnenschein & Company | pages=53–54}}</ref> According to J. Gordon Melton, both the Hindu and Buddhist calendars that are in use in South and Southeast Asia are rooted in this text, but the regional calendars adapted and modified them over time.<ref>{{cite book|author=J. Gordon Melton|title=Religious Celebrations: An Encyclopedia of Holidays, Festivals, Solemn Observances, and Spiritual Commemorations|url=https://books.google.com/books?id=KDU30Ae4S4cC&pg=PA161|year=2011|publisher=ABC-CLIO|isbn=978-1-59884-205-0|pages=161–162}}</ref><ref>{{cite book|author= Yukio Ôhashi| editor= Helaine Selin|title=Encyclopaedia of the History of Science, Technology, and Medicine in Non-Western Cultures |url=https://books.google.com/books?id=kt9DIY1g9HYC&pg=PA354 |year=2008|publisher=Springer Science |isbn=978-1-4020-4559-2|pages=354–356}}</ref>

The ''Surya Siddhanta'' calculates the solar year to be 365 days 6 hours 12 minutes and 36.56 seconds.<ref>{{cite book|author=Lionel D. Barnett|title=Antiquities of India |url=https://books.google.com/books?id=LnoREHdzxt8C&pg=PA193|year= 1999|publisher=Atlantic |isbn=978-81-7156-442-2|page=193}}</ref><ref>{{cite book|author1=V. Lakshmikantham|author2=S. Leela|author3=J. Vasundhara Devi|title=The Origin and History of Mathematics|url= https://books.google.com/books?id=KaofAQAAIAAJ|year= 2005|publisher= Cambridge Scientific Publishers|isbn =978-1-904868-47-7|pages=41–42}}</ref> On average, according to the text, the lunar month equals 27 days 7 hours 39 minutes 12.63 seconds. It states that the lunar month varies over time, and this needs to be factored in for accurate time keeping.<ref>{{cite book|author1=Robert Sewell|author2=Śaṅkara Bālakr̥shṇa Dīkshita|title=The Indian Calendar|url=https://books.google.com/books?id=CQ8iAVwA-yEC |publisher=Motilal Banarsidass | year= 1995 |pages=21 with footnote, cxii–cxv|isbn=9788120812079}}</ref>

According to Whitney, the Surya Siddhanta calculations were tolerably accurate and achieved predictive usefulness. In Chapter 1 of ''Surya Siddhanta'', "the Hindu year is too long by nearly three minutes and a half; but the moon's revolution is right within a second; those of Mercury, Venus and Mars within a few minutes; that of Jupiter within six or seven hours; that of Saturn within six days and a half".<ref>{{cite book|author=William Dwight Whitney|title=Oriental and Linguistic Studies|url=https://books.google.com/books?id=SfAcCY0xvqQC|year=1874|publisher=Scribner, Armstrong|page=368}}</ref>

The ''Surya Siddhanta'' was one of the two books in Sanskrit translated into [[Arabic]] during the reign of [[Abbasid Caliphate|'Abbasid]] caliph [[al-Mansur]] ({{Reign|754|775|era=CE}}). According to [[Muzaffar Iqbal]], this translation and that of [[Aryabhata]] was of considerable influence on geographic, astronomy and related Islamic scholarship.<ref name="Muzaffar Iqbal 2007 36–38"/>

==Editions==
*[https://archive.org/details/Bibliotheca_Indica_Series ''The Súrya-Siddhánta, an antient system of Hindu astronomy''] ed. FitzEdward Hall and Bápú Deva Śástrin (1859).
*[https://books.google.com/books?id=jpE7AAAAcAAJ ''Translation of the Sûrya-Siddhânta: A text-book of Hindu astronomy, with notes and an appendix'' by Ebenezer Burgess] Originally published: ''Journal of the American Oriental Society'' '''6''' (1860) 141–498. Commentary by Burgess is much larger than his translation.
*[https://books.google.com/books?id=W0Uo_-_iizwC ''Surya-Siddhanta: A Text Book of Hindu Astronomy'' translated by Ebenezer Burgess, ed. Phanindralal Gangooly] (1989/1997) with a 45-page commentary by P. C. Sengupta (1935).
* [http://www.wilbourhall.org/pdfs/suryaEnglish.pdf Translation of the ''Surya Siddhanta'' by Bapu Deva Sastri] (1861) {{ISBN|3-7648-1334-2}}, {{ISBN|978-3-7648-1334-5}}. Only a few notes. Translation of ''Surya Siddhanta'' occupies first 100 pages; rest is a translation of the ''Siddhanta Siromani'' by [[Lancelot Wilkinson]].

== Commentaries ==

The historical popularity of ''Surya Siddhanta'' is attested by the existence of at least 26 commentaries, plus another 8 anonymous commentaries.<ref>{{cite book |author=Amiya K. Chakravarty |title=The Sūryasiddhānta: The Astronomical Principles of the Text |year=2001 |publisher=Asiatic Society |page=viii |isbn=9788172361129 |url=https://books.google.com/books?id=atNfAAAAMAAJ }}</ref> Some of the Sanskrit-language commentaries include the following; nearly all the commentators have re-arranged and modified the text:<ref name="DP_1981">{{cite book |author=David Pingree |title=Jyotiḥśāstra: Astral and Mathematical Literature |series=A History of Indian Literature |year=1981 |publisher=Otto Harrassowitz |isbn=3-447-02165-9 |pages=23–24 |url=https://books.google.com/books?id=L0UIAQAAIAAJ }}</ref>

* ''Surya-siddhanta-tika'' (1178) by Mallikarjuna Suri
* ''Surya-siddhanta-bhashya'' (1185) by Chandeshvara, a [[Maithil Brahmin|Maithila Brahmana]]
* ''Vasanarnava'' (c. 1375–1400) by ''Maharajadhiraja'' Madana-pala of Taka family
* ''Surya-siddhanta-vivarana'' (1432) by [[Parameshvara Nambudiri|Parameshvara]] of Kerala
* ''Kalpa-valli'' (1472) by Yallaya of Andhra-desha
* ''Subodhini'' (1472) by Ramakrishna Aradhya
* ''Surya-siddhanta-vivarana'' (1572) by Bhudhara of Kampilya
* ''Kamadogdhri'' (1599) by Tamma Yajvan of Paragipuri
* ''Gudhartha-prakashaka'' (1603) by Ranganatha of [[Varanasi|Kashi]]
* ''Saura-bhashya'' (1611) by Nrsimha of Kashi
* ''Gahanartha-prakasha'' ([[IAST]]: Gūḍhārthaprakāśaka, 1628) by Vishvanatha of Kashi
* ''Saura-vasana'' (after 1658) by [[Kamalakara]] of Kashi
* ''Kiranavali'' (1719) by Dadabhai, a [[Chitpavan Brahmin|Chittpavana Brahmana]]
* ''Surya-siddhanta-tika'' (date unknown) by Kama-bhatta of southern India
* ''Ganakopakarini'' (date unknown) by Chola Vipashchit of southern India
* ''Gurukataksha'' (date unknown) by Bhuti-vishnu of southern India

Mallikarjuna Suri had written a [[Telugu language]] commentary on the text before composing the Sanskrit-language ''Surya-siddhanta-tika'' in 1178.<ref name="DP_1981"/> Kalpakurti Allanarya-suri wrote another Telugu language commentary on the text, known from a manuscript copied in 1869.<ref>{{cite book |editor=David Pingree |editor-link=David Pingree |title=Census of the Exact Sciences in Sanskrit Series A |volume=1 |publisher=American Philosophical Society |year=1970 |page=47 |url=https://archive.org/details/PingreeCESS/Pingree_CESS_A1_1970/ }}</ref>

==See also==
*[[Hindu units of measurement]]
*[[Indian science and technology]]

==References==
{{reflist}}

===Bibliography===
*{{cite book|last=Gangooly|first=Phanindralal|title=Sûrya-Siddhânta: A Text-book of Hindu Astronomy|publisher=Motilal Banarsidass Publ.|year=1989|isbn=9788120806122}}
*{{cite journal | last=Pingree | first=David |author-link=David Pingree| title=The Mesopotamian Origin of Early Indian Mathematical Astronomy | journal=Journal for the History of Astronomy | publisher=SAGE | volume=4 | issue=1 | year=1973 | pages=1–12 | doi=10.1177/002182867300400102 | bibcode=1973JHA.....4....1P | s2cid=125228353 }}
*{{cite book|last=Plofker|first=Kim|author-link=Kim Plofker|title=Mathematics in India|publisher=Princeton University Press|year=2009|isbn=978-0-691-12067-6|title-link= Mathematics in India (book)}}
*{{cite book|last=Pingree |first= David | title= Jyotihśāstra : Astral and Mathematical Literature| publisher= Otto Harrassowitz| year= 1981| isbn= 978-3447021654 }}
* [[K. V. Sarma]] (1997), "Suryasiddhanta", [[Encyclopaedia of the History of Science, Technology, and Medicine in Non-Western Cultures]] edited by [[Helaine Selin]], Springer, {{ISBN|978-0-7923-4066-9 }}
*{{cite book|author=Yukio Ôhashi|chapter=The Legends of Vasiṣṭha – A Note on the Vedāṅga Astronomy|editor=Johannes Andersen|title=Highlights of Astronomy, Volume 11B|url=https://books.google.com/books?id=gQYscrT0fgQC|year=1999|publisher=Springer Science|isbn=978-0-7923-5556-4}}
*{{cite journal|author=Yukio Ôhashi| title=Development of Astronomical Observations in Vedic and post-Vedic India| year=1993|journal=Indian Journal of History of Science|volume=28|number=3}}
*{{cite book| author=Maurice Winternitz| author-link=Moriz Winternitz| title=History of Indian Literature, Volume 1| year=1963| publisher=Motilal Banarsidass| isbn=978-81-208-0056-4 }}

==Further reading==
* Victor J. Katz. ''A History of Mathematics: An Introduction'', 1998.

==External links==
{{wikisourcelang|sa|सूर्यसिद्धान्त|Surya Siddhanta (सूर्यसिद्धान्त): Sanskrit text}}
*[https://www.youtube.com/@ahargana Ahargana - The Astronomy of the Hindu Calendar] Explains the various calendric elements of the Hindu calendar by means of astronomical simulations created using [[Stellarium (software)|Stellarium]]. The definitions of the various calendric elements are obtained from Surya Siddhantha.
*[https://www.geogebra.org/m/DRhaaKPp ''Surya Siddhantha'' Planetary Model] A geometric model that illustrates the Surya Siddhantha model of the orbital movement of the planets. In this model, the asterism are not stationary but exhibit high-speed movement which is faster than the planets. As a result, the planets seem to "fall behind" thus creating orbital movement.
*[http://vedicreserve.mum.edu/jyotish/surya_siddhanta.pdf ''Surya Siddhanta''] Sanskrit text in Devanagari
*[https://books.google.com/books?id=NJpeAAAAcAAJ Remarks on the Astronomy of the Brahmins], John Playfair
*[https://www.cc.kyoto-su.ac.jp/~yanom/pancanga/ Online Surya Siddhanta panchanga ]
([http://sandhi.hss.iitb.ac.in/Sandhi/Mathematics%20and%20Astronomy%20articles/Playfair%20J/Playfair%20-%20On%20the%20Trignometry%20of%20%20Brahmins%20(1798).pdf Archive])

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