Astrology (Jyotiṣa) is known as the eye of the Vedas.
The most crucial text in Hindu Astrology - The Brihat Parāśara Horā Śāstra (BPHS) declares it in the very beginning (Ch 1. Verse 2).
Offering his obeisance’s to all-knowing sage Parasara and with folded hands, Maitreya said: O venerable, Astrology, the supreme limb of the Vedas, has three divisions, viz. Hora, Ganita and Samhita. Among the said three divisions, Hora is still far excellent. I desire to know of its glorious aspects from you. Kindly relate them to me. BPHS I.1-3
भगबन् परमं पुण्यं गुह्यं वेदाङ्गमुत्तमम्।
त्रिस्कन्धं ज्यौतिषं होरा गणितं संहितेति च॥ २
Here obviously, the supreme limb is the eye.
Another classical Astrological text Praśna Mārga says it even more clearly.
The Vedas have six limbs, viz. Jyotisha, Kalpa, Nirukta, Siksha, Vyakarana and Chanda. For the Vedas, the six important limbs, viz. the feet, the face, the hands, the eyes, the nose, and the ears, are represented by Chanda, Vyakarana, Kalpa, Jyotisha, Siksha and Nirukta, respectively. As Astrology is the eye of the Vedas, it is given the pride of place. No person possessing all the organs intact but without eyesight can have individuality.
- Brihat Parasara Hora Sastra (Vol. 1) translated by R. Santhanam
- Prasna Marga (Vol. 1) translated by B. V. Raman
- Origin and Development of Jyotisha Shastra by Sundarjee published in the International Journal of Scientific Research (IJSR) Vol. 3 Issue 4. April 2014 https://www.worldwidejournals.com/international-journal-of-scientific-research-(IJSR)/recent_issues_pdf/2014/April/April_2014_1396444412_131fa_10.pdf