The daily worship of the sun is called Sandhyā. The word ‘sandhyā’ literally means ‘twilight’ but also indicates the prayer performed during the morning and evening twilight. This act of adoration to the sun is generally styled ‘sandhyopāsana’ or ‘sandhyāvandana’ or simply ‘sandhyā.’ The word ‘sandhyā’ can also mean ‘the junction between night and day
Daṇḍa – The Staff 
The ācārya gave a daṇḍa (staff, stick) to the student, who accepted it by reciting the verse: “My daṇḍa fell down to the ground and I pick it up, for the sake of long life, to adhere to the path of brahman, and to begin student-hood.” The daṇḍa represented control over the mind, speech, and body.
After tying the mekhalā, the boy was invested with the yajñopavīta (sacred thread). While the yajñopavīta as a ‘sacred thread’ was largely unknown in ancient times, it became the focus of the upanayana saṃskāra in later years. In later times, the young vaṭu was given the yajñopavīta and made to recite the well-known mantra, “The yajñopavīt
The Right Age for Conducting Upanayana
There are several varying rules and prescriptions about the age of the boy who is to undergo the upanayana (the boy is referred to as the ‘vaṭu’). What is interesting to note is that the age was counted from conception. From the earliest times we see in ancient India the notion that the age of a child must be counted.
Ayurveda, being a medical science, does not consider cosmology as one of its central preoccupations. Vagbhata’s Ashtanga-Hrdayam, which contains the definitive summary of classical Ayurveda, makes not even a passing reference to cosmological theories.
Elaborate references to these theories, based mostly upon the Sankhya system, may however be found in the texts of Charaka and Sushruta. These theories owe their presence in these texts to two major reasons:
Most of our ancient thinkers were of the opinion that only the male members of the brāhmaṇa, kṣatriya, and vaiśya varṇas were eligible to study the Vedas. And since the upanayana saṃskāra was primarily meant as an entry to the study of the Vedas, it was not applicable to women of all varṇas and to śūdras.
It was in the Sūtra period that the upanayana saṃskāra seems to have been fully established. Most of the details of the ceremony are laid out in the gṛhya-sūtras. The Dharma-sūtras and Smṛtis have nothing new to say about the ritualistic aspects apart from what has already been said earlier; they primarily develop the social side of the saṃskāra. It was also perhaps during this period that the Upanayana became compulsory for men from the first three varṇas.