Jyotiṣam, Jyautiṣam, and Jyotiśśāstram (astrology) are synonymous. This Vedāṅga is considered the eyes of Vedapuruṣa. The term jyotiḥ, here, refers to a Graha (planet) or Nakṣatra (star). The system that deals with the movement of planets and stars and related matters is called Jyotiṣam. The auspicious/suitable time for the performance of Vedic rites is fixed by applying different calculations (Gaṇitaśāstram); this is the only Vedāṅga that deals with a lot of mathematics. This is also the only Vedāṅga, which is required even by common people in day-to-day life for selecting the auspicious times.
Jyotiṣam, unlike the other Vedāṅgas, is known around the world, especially for determining auspicious times and making predictions.
Origin of Jyotiṣam
The origin of Jyotiṣam can be traced to the Vedas. The Śikṣavallī of Taittirīyopaniṣat (Kṛṣṇayajurveda) explains that there is an Upaniṣat called Mahāsaṃhitā (the great text); it is divided into five Adhikaraṇas (chapters) that deal with Jñānam (cognition). Adhijyautiṣam is the second, and it deals with Jyotiṣam:
अथाधिज्यौतिषम्। अग्निः पूर्वरूपम्। आदित्य उत्तररूपम्। आपस्सन्धिः। वैद्युतस्सन्धानम्। इत्यधिज्यौतिषम् ॥३, शिक्षावल्ली, तैत्तिरीयोपनिषत् ॥
athādhijyautiṣam. agniḥpūrvarūpam. ādityauttararūpam. āpassandhiḥ. vaidyutassandhānam. ityadhijyautiṣam॥3,śikṣāvallī, taittirīyopaniṣat॥
atha = after the first Adhikaraṇam: adhilokam, adhijyautiṣam = the second one called “adhijyautiṣam” is being explained.agniḥ = agni (fire) is, pūrvarūpam = the first letter. ādityaḥ = āditya (Sun) is, uttararūpam = the last letter. āpaḥ = water is, sandhiḥ = in the middle, vaidyutaḥ = electricity is, sandhānam = the connecting factor. iti = this, adhijyautiṣam = the chapter called adhijyautiṣam explains.
The term “saṃhitā” is used (in Pāṇini also) in the sense of morphophonemics, that is, when two letters are uttered in quick succession, there will be phonemic change. Here the mantra says: consider the first letter as the form of Agni, and the second one as that of Āditya. Water is in between; electricity is the uniting factor. Thus the Adhikaraṇam called Adhijyautiṣam is explained. One should worship (upāsanā) in the same fashion.
The system is divided into three Skandhas (parts): Muhūrta-skandha (dealing with auspicious and inauspicious times), Jātaka-skandha or Horāskandha or Phalitajyotiṣam (deals with predictions), and Siddhānta-skandha (dealing with preparation of Pañcāṇgam or almanac).