Astronomy Concepts

This page introduces the foundational astronomical concepts used throughout the Surya Siddhanta. It begins with how the sky itself is observed and structured — the geocentric model, the ecliptic, the nakṣatras, and the grahas that travel along the ecliptic. It then introduces the orbital reference points used to correct a graha's idealized circular motion toward its true position.

In the Surya Siddhanta, the Earth is fixed in both position and rotation, and everything revolves around it. The text views the sky from the vantage of the northern hemisphere. This places the ecliptic, nakṣatras, zodiac, and planets to the south. This means east is to the left and west is to the right. These directions are reversed in the southern hemisphere.

Motion of Stars and Planets

The ecliptic is the eastward path the Sun traces against the backdrop of the stars. The planets all travel within a band centered on the ecliptic, and this band is what the zodiac and nakṣatras are measured along.

The ecliptic is tilted around 24 degrees from the celestial equator. The two points where the celestial equator and ecliptic cross are called the equinoxes. This tilt is what causes the seasons. When the Sun is north of the celestial equator, the northern hemisphere has summer while the southern hemisphere has winter — and the reverse when the Sun crosses south.

As the Sun travels along the ecliptic, its declination — its height relative to the celestial equator — rises and falls throughout the year. When the Sun's declination reaches its highest point, this is called the summer solstice. When it reaches its lowest point, this is the winter solstice.

The ecliptic gets its name from eclipses — which occur when the Sun and Moon are aligned not just in longitude, but also near the point where the Moon's path crosses the ecliptic.

The nakṣatras are 27 star groupings, each spanning 13°20′ (SS 2.64). Like the rest of the celestial sphere, they circle the sky once a day. The Surya Siddhanta states that at the end of Kṛta Yuga, all planets' mean positions were at 0° Aries (SS 1.57) — the end of Revatī, the last nakṣatra. We take this point, the start of Aśvinī, as the first nakṣatra.

Each nakṣatra's span comes from dividing the full circle, expressed in arcminutes, equally among the 27 nakṣatras.

#	Nakṣatra	Degree Range
1	Aśvinī	0°00′ – 13°20′
2	Bharaṇī	13°20′ – 26°40′
3	Kṛttikā	26°40′ – 40°00′
4	Rohiṇī	40°00′ – 53°20′
5	Mṛgaśīrṣa	53°20′ – 66°40′
6	Ārdrā	66°40′ – 80°00′
7	Punarvasu	80°00′ – 93°20′
8	Puṣya	93°20′ – 106°40′
9	Āśleṣā	106°40′ – 120°00′
10	Maghā	120°00′ – 133°20′
11	Pūrva Phalgunī	133°20′ – 146°40′
12	Uttara Phalgunī	146°40′ – 160°00′
13	Hasta	160°00′ – 173°20′
14	Citrā	173°20′ – 186°40′
15	Svātī	186°40′ – 200°00′
16	Viśākhā	200°00′ – 213°20′
17	Anurādhā	213°20′ – 226°40′
18	Jyeṣṭhā	226°40′ – 240°00′
19	Mūla	240°00′ – 253°20′
20	Pūrva Āṣāḍhā	253°20′ – 266°40′
21	Uttara Āṣāḍhā	266°40′ – 280°00′
	Abhijit*	~276°40′ – 283°20′
22	Śravaṇa	280°00′ – 293°20′
23	Dhaniṣṭhā	293°20′ – 306°40′
24	Śatabhiṣā	306°40′ – 320°00′
25	Pūrva Bhādrapadā	320°00′ – 333°20′
26	Uttara Bhādrapadā	333°20′ – 346°40′
27	Revatī	346°40′ – 360°00′

*Abhijit is a common 28th nakṣatra that sits between Uttara Āṣāḍhā and Śravaṇa.

The word graha comes from the Sanskrit root grah — to seize or grasp. They are the celestial bodies that move along the ecliptic: Sūrya (Sun), Candra (Moon), Maṅgala (Mars), Budha (Mercury), Bṛhaspati (Jupiter), Śukra (Venus), Śani (Saturn), and the lunar nodes Rāhu and Ketu. The Sun plays a central role even in this geocentric model, and its position is used to determine the true positions of the grahas.

In modern cosmology, the Sun is a star and the Moon is a natural satellite — neither qualifies as a planet. In the Vedic paradigm, both are grahas. Uranus and Neptune are not mentioned in this work. They are not visible to the naked eye. Bodies that cannot be seen exert no influence in the Vedic framework.

Rāhu and Ketu

The Surya Siddhanta tracks four kinds of reference points for a graha: its bhagaṇa (revolution rate), its mandocca (apogee), its śīghroca (node of conjunction), and its pāta (node of intersection). Not every graha has all four. The Sun has no śīghroca, since it is itself the reference point the śīghra correction is built around. It has no pāta either, since it defines the ecliptic itself, leaving no separate path for it to cross. The Moon also has no śīghroca, since its motion is purely geocentric rather than an artifact of Earth's own motion around the Sun. The five remaining grahas — Mercury, Venus, Mars, Jupiter, and Saturn — carry all four. Together, these reference points are what allow a graha's true position, in longitude and, where pāta applies, in latitude, to be calculated. Each is examined in its own section below.

Bhagaṇa

Mandocca - Node of Apsis

Śīghroca - Node of Conjunction

Epicycle

Pāta - Node of Intersection