The Forgotten Meridian of Ujjain

 The Forgotten Meridian of Ujjain

How Ancient India Built Its Own Prime Meridian Centuries Before Greenwich

 

Location Parallax

Utpala calls up her friend Rohini from Gauhati. ‘We are moving into our new house next month. There will be Grah Pravesh Pooja on Saptami at…’ Rohini interrupted her and remarked, Saptami is not on that day and time, it is…

Utpala corrected her, here in Gauhati it is correct, in Ahmedabad it will be later...

I can see the moon low in the sky; Revati pointed at the Moon in her video call to Swati from Leh. No, it is high up in the sky here in Kanyakumari, Swati replied

Our ancient astronomers did visualise this anomaly and found how to overcome it.

Celestial orientation makes a location feel alive—defined not by abstract numbers but by its relationship to the zodiac. It tells you what the sky does there, which is far more intuitive for cultures that lived by the rhythms of stars and seasons.

Vedic Period

Vedic and Vedang periods saw the evolution of observational astronomy. It established time divisions, calendrical functions, seasons, equinoxes and Solstices, and luni-solar cycles.

The nakshatras provided a celestial reference system, but they could not relate to the place on the earth in relation to the zodiac.

Puranic texts then covered, to a great extent, a conceptual geography. It defined directions and spatial relationships with the zodiac and nakshatra system. (example Mahabharat text). A foundation of cosmography was laid down. Places and events could be referred to this Cosmogram at a later stage.

Siddhantic Period

 In this period,  Indian astronomy developed a mathematically rigorous system for locating places on the terrestrial sphere and astronomical calculations for their geographical separation. Prominent astronomers from the  4^th century CE onwards, right up to the  15^th century CE, developed and updated concepts and formulae to further improve the astronomical data

 A requirement was felt for

• Determining local planetary positions
• Calculation of eclipses, zenith distances, and other celestial events
• Adopting astronomical tables for different regions.
• Accurate determination of time and place, which was required for religious observations and calendric/ panchang creation.

 Coordinate Astronomy /Elements of the Hybrid System

Indian astronomy possessed both geographical and celestial angular concepts, but they served different purposes.

Traditional Indian astronomy did not necessarily express every locality in precisely this modern notation, but it possessed the conceptual components necessary to do so:

• latitude / अक्षांश akṣāṃśa
• Longitude/ लम्बांश/ रेखांश
• longitudinal difference / देशान्तर  
• astronomical reference meridian
• angular celestial positions
• corrections associated with locality.

The main components of the system comprised अक्षांश akṣāṃśa (Latitude), लम्बांश रेखांश / (Longitude), देशान्तर (Difference of Longitude), equivalent to, but not exactly similar to today’s coordinate system and अयनांश.

अयनांश belongs to a different celestial reference frame, it is the accumulated displacement of the equinox. It is annotated as the angular difference between the Sidereal and Tropical Zodiac. It is used as a celestial reference frame and not for geographical coordinates.

The zero point of celestial longitude is fiducial, though it did create a definitional problem because the अयनांश shifts the celestial reference to zero.

अक्षांश(akṣāṃśa) ≃ Latitude

It is analogous to latitude. Aksa अक्ष in Indian astronomical tradition is axis, and अक्षांश becomes the angular quantity associated with terrestrial /equatorial geometry.

It was calculated by measuring the sun's zenith distance at noon, especially on equinoxes, using shadows and trigonometric tables. The shadow cast by the Gnomon/Shanku is used to know the अक्षांश of the place. The method and mathematical formula are given in the Surya Siddhant

लम्बांश रेखांश / ≃ Longitude

It is analogous to Longitude, its reference meridian and geometrical context are peculiar to the Indian astronomical system.

In Indian astronomical terminology, अंश means a degree, and लम्बांश concerns angular longitude on the celestial sphere.

शङ्कुच्छायाहते त्रिज्ये विषुवत्कर्णभाजिते ।
लम्बाक्षज्ये तयोश्चापे लम्बाक्षौ दक्षिणौ सदा ॥   — Sūryasiddhānta 3.14                                                This Shlok explains a spherical trigonometrical method to determine a location's अक्षांश akṣāṃśa and लम्बांश based on the shadow cast by the Gnomon/Shanku.

Side-by-side illustration comparing a gnomon's noon shadow length in summer versus winter solstice, used to calculate akṣāṃśa (latitude) in Surya Siddhanta astronomy

 The shadow is known as Palabha

A similar process was also used to calculate the exact position of the planets at the observer’s meridian.    लम्बांश/ रेखांश was also calculated by comparing the local time of a known celestial event with its time at a reference meridian.

देशान्तर

Literally, it means difference in places. However, in the Indian astronomical context, it is an astronomical/time correction due to the angular separation of two meridians. Analogous to difference in longitude.

Surya Siddhant verses 1.59 to 1.61 establish the देशान्तर correction required for planetary motion

षष्ट्या विभज्य लब्धैस्तु योजनैः प्रागथापरैः ।

स्वदेशःपरिधौज्ञेयःकुर्याद्देशान्तरंहितैः॥— Sūryasiddhānta 1.65

"The position of one's local place must be understood on the Earth's circumference; using these [values/coordinates], one should calculate the longitudinal difference (देशान्तर) between two places." देशान्तर value is the astronomical/time correction required to compute for a given longitude

In fact, a देशान्तर shifts the observer's terrestrial reference position.

लङ्कासमयाम्योत्तररेखायां भास्करोदये मध्याः ।
देशान्तरोनयुक्ता रेखायाः प्रागपरदेशेषु ॥ ३५ ॥ Bramhagupt I

It conveys that the mean planetary positions calculated for the reference meridian must be modified by the देशान्तर for places east or west of that meridian. The value was mentioned in Yojanas.

Thus, the astronomer could determine a place's position through astronomical angular measurements, relate it to a conventional reference meridian, and calculate the resulting correction needed when applying astronomical data from that reference location to another place.

Reference Meridian

A common reference meridian was required, more so for standardisation and computing astronomical calculations.

उदयः यस्य लङ्कायां स अस्तमयः सवितुरेव सिद्धपुरे ।^*
मध्याह्नं यमकोट्यां रोमकविषयेऽर्धरात्रं स्यात् ॥       Āryabhaṭīya, Golapād 

The verse mentions that the Sunrise at one place will be sunset / at zenith or at other place, at the same time. Aryabhat in this verse explains the global concept of time zones.

This establishes four reference locations around the terrestrial sphere, Laṅkā, Siddhapura, Yamakoti, and Romaka

"The Surya Siddhanta refers to a line joining Avanti (Ujjayini) and Rohitaka and Lanka, in its discussion of the reference meridian (Chapter I, verse 62, commentary). Avanti is identified with modern Ujjain, while the location of Rohitaka is uncertain, but it can be safely assumed to be North of Ujjain. Later astronomers generally used Ujjayini as their computational reference. Modern scholarship confirms that the canonical mean motions were normally referred to the central meridian of Ujjain.

Map of South Asia showing the Ujjain Meridian as a red vertical line running through Ujjain and Lanka, with the equator marked, illustrating the ancient Indian prime meridian

Almost all panchang,17^th century onwards, mentioned all celestial events in reference to this prime meridian passing through Ujjain. They also mentioned Palbha for the place to calculate अक्षांश 

Why Ujjain?

1. Mathematically, any place can be used for reference/prime meridian purposes and defined as a  0⁰ meridian. Selection of Ujjain was not arbitrary either. There are a lot of theories purporting the claim for Ujjain to be used for the prime meridian. However, the most plausible explanations as to why Ujjain was considered could be (there is no direct evidence of these theories)
2. It was a prominent city of the Malwa region, and Ujjayini/Avanti was one of the major political, commercial and cultural centres of ancient India.
3. Its latitude was astronomically convenient because the Sun is at maximum zenith during its northward travel. The shadow /Palabha of the solstice day is used as a marker for further calculations. Tropic of Cancer was not a defined line in that period.
4. The city got associated with astronomical studies and observations, and  later astronomers/ scholars used this location for their calculations
5. The texts adopted this meridian and formed a convention for further use as zero-degree meridian.
6. Ujjain was a sacred city, associated with various traditions and religious rituals. Its selection as a zero meridian gave astronomical calculations a religious, cultural and geographical legitimacy.
7. The meridian of Ujjain became a prime meridian for both astronomical calculations and geographical purposes. All zodiac calculations in panchang were based on this meridian. देशान्तर values were applied keeping this as a reference.

The mathematical idea of locating a place by its position north or south and east or west did not appear suddenly with the modern terms अक्षांश akṣāṃśa (Latitude), लम्बांश (Longitude). Its development can be traced from the directional and calendrical astronomy of the Vedic and Vedang traditions, through the spherical cosmography of the Purāṇas, to Āryabhaṭa's explicit terrestrial geometry. Surya Siddhānta formalised अक्षांश akṣāṃśa (Latitude), लम्बांश (Longitude), through Jya geometry (trigonometry), and Brahmagupta developed देशान्तर into a rigorous mathematical operation.

One can say a Hybrid system of astronomical and geographical coordinates got evolved.

Location Parallax solved

Rohini insisted Saptami hadn't yet arrived in Ahmedabad while Utpala's priest in Gauhati was ready to begin. 

Revati saw the Moon riding high over Kanyakumari while Swati still watched it hugging the horizon in Leh. 

Both were living out precisely the problem Brahmagupta and the Surya Siddhanta had already solved a millennium and a half earlier.

 Every place on the sphere sees its own sky in its own time. The astronomical and geographical coordinate system  was the astronomers' answer to that anomaly: a correction that let a single reference meridian, running quietly through Ujjain, translate one place's sky into another's.

The video call may be new. The problem it revealed is not.

 

Some Prime Meridian Facts

The Surya Siddhanta coordinate system is notable for its originality. There are similarities with the Greek and Babylonian systems. Hindu astronomers determined their own constants and methods, adapting some foreign ideas to create a uniquely  Indian framework

Arabic astronomers and all Southeast Asian countries used Ujjain’s meridian as the prime meridian for their calculations. Arabic astronomers referred to it as Arin

The historical analysis notes that Ujjain, Rohitaka and Kurukṣetra and Lanka (not Sri Lanka) were traditionally treated as lying on the same meridian, although their actual modern longitudes differ by nearly one degree

IIT Roorkee Study Finds Sacred Shiva Temple Alignment, along 79⁰ E meridian, were based on Natural Resource Hotspots

John Harrison calculated longitude in 1761 CE; Indian astronomers could calculate it, nearly 1000 years earlier.

Prime meridians considered by various countries before Greenwich

• Ptolemy-Fortunate Isles (Ancient and Medieval Europe)
• France- Paris
• Russia Pulkovo, near St. Petersburg
• Spain Madrid; early Spanish and European maps used the Ferro Meridian passing through El Hierro in the Canary Islands.
• USA – Washington DC

 The International Meridian Conference, held in Washington, DC, voted to make Greenwich the universal 0° longitude in 1884

Methods, instruments and mathematical formulaes to calculate अक्षांश (akṣāṃśa), लम्बांश, रेखांश will be covered in the next blog

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References

Bhartiya Jyotish Shastra Part 1 and 2 by SB Dixit 1886

Surya Siddhanta, English translation by Burgess, Ebenezer (1935). 

The Meridians of Reference of Indian cannons by Raymond Mercier,2016

Study Report by IIT Roorkee https://www.pib.gov.in/PressReleasePage.aspx?PRID=2170961&reg=48&lang=2

Mentions of Dhruva in Indian Astronomical Treatises – a Possible Pole Star. By Vinay Iyer, Dr. Ramakrishna Pejathaya. Int J Sanskrit Res 2024;10(3):72-80. DOI: 10.22271/23947519.2024.v10.i3b.2370