The word rāśi refers to a quantity of something, and is a measure of this quantity implying something tangible. It means a heap, mass or pile of anything. For example, dhana (wealth) rāśi can mean the quantum of wealth. It also means a group or multitude of beings like animals or men. It is a quantity indicated by a number. For example Vṛṣabha (bull) rāśi can mean a herd of bulls or cattle thereby referring to such a group of animals.
In jyotiṣa, it specifically refers to a sign of the zodiac which represents all of men, material and money that is measured by the quantum of degrees of longitude. As such the ecliptic is 360° and is composed of twelve rāśi each measuring 30° longitude. Now, when we divide 360° by 30°, we get 12 rāśi. Thus, if each rāśi is a measure of exactly 30°, then there can only be 12 such signs mathematically and any modern revision is not going to fit into this ancient fundamental jyotiṣa paradigm.
How did we get 30° for each sign? The Sun and Moon were observed to conjoin in the sky every 30 days. And it was observed that the average motion of the Sun is about 1° per day. Therefore, in 30 days the Sun would have moved ahead by 30°. This measure by which the Sun moved ahead between two conjunctions with the Moon is called a rāśi. It is a measure or quantity of degrees of solar motion between subsequent conjunctions with the Moon. Therefore these rāśi of 30° are called ‘Sun signs’ as they measure out the solar motion. The day the Sun enters a sign is called Saṅkrānti.
Most of these signs look like the creatures they represent. When a sign represents one creature, it is blessed not to have a flaw whereas when a sign represents two or more, it is said to have built in flaws. Seven of these twelve signs have flaws as taught in PJC Year-1.
Sūrya the Sun god, has manifested in twelve forms and these are the dvādasa (twelve) Āditya. The name Āditya is from their mother Aditi who gave birth to these Āditya who govern the twelve signs. Of these the greatest is Viṣṇu Āditya for He grants mokṣa and forgiveness from sins, good sleep and rejuvenation etc. The twelve āditya are Dhātṛ (Savitṛ), Aryaman, Mitra, Varuṇa, Indra, Vivasvat, Pūṣan, Parjanya, Aṅśumān, Bhaga, Tvaṣṭṛ and Viṣṇu in the order of the twelve signs.