There have been many astrologers in our Indian history. Here are given some names of them.

Gunaakar's disciple was Kaalakaachaarya (599-527 BC - 72 yrs)
Kaalakaachaarya (599-527 BC = 72 yrs) disciple was Kumaaril Bhatt
(557-493 BC = 64 yrs)

Gunaakar
Gunaakar, the author of "Horaa Makarand", was a Jain sage, and Kaalakaachaarya was his student. Kaalakaachaarya is the one who caused the spread of Ramal System of astrology. Their period is believed to be the 1st century BC. If so, then how can Gunaakar imitate Varaah Mihir? Because Varaah Mihir is supposed to be in Vikramaaditya's period - 6th century AD.Either Mihir's period was before 1st century BC or Gunakar and Kaalakaachaaryaa might have lived after 6th century.

The following is the ending section of "Horaa Makarand" which speaks about his tradition, village, father, and Horaa Makarand. This helps us to locate that period of Gunaakar is 11th century AD. This is the translation of the last 4 Shlok of Horaa Makarand ----
1. --"In the town of Kharjura, which is the only Tilakak (forehead ornament) of Avantee, Shree Naaraayan was born in the Gotra of Kaundinya, in a house which is a playground of virtues. That excellent Braahman was very fond of learning and he took the Shraut Vidhi (Yagya and all) in Kali Yug to the same height where it was in Krit Yug."

2. --"From him, who is like an ocean containing gems of pure virtues, like a Moon was born the Ganak Shree Shreepati. The rays of whose (knowledge) are drunk day and night by men with intelligent minds, who are like Chakor, using their ears as beaks."

3. --"After Shree Brahmgupt, Aaryabhatt, Ulpal, Varaah and Lalla, ascended to heaven, then (he) seeing (the goddess of) speech being without any support (of texts), composed a text, which is like an abode for (the goddess of) speech (or composed a text named Vaanee Vilaas - Vasa)."

4. --"Seeing the three worlds brightly light by the moon-rays of its fame, the astounded men devoted to (learning), who are like female Chakor birds devoted to moonlight, drink its nectar. By his son, Gunaakar, a Ganak, who is like a Moon, has been composed this Jaatak named "Horaa Makarand", for the happiness of Jyotirvid (knower of astrology)."

--"Whatever has not been mentioned here out of lack of knowledge or whatever has been said wrongly out of less intelligence, the learned men themselves strive to make it devoid of defects."
(The second half of the Shlok is erroneous and is unintelligible)

Kaalakaachaarya
He is well known among the Guru of Jain school of astrology. He was the son of King Vayar Singh who was the King of Dhaaraa Nagaree. His mother's name was Surasundaree and sister's name was Saraswatee. He was the student of Jain Muni Gunaakar. Kaalakaachaarya was also known as Kaalakaa Sooree', meaning the knowledgeable person with the name Kaalakaa. Kaalakaachaarya is the first Guru of Jain school of astrology who mixed this ancient knowledge with the systems of Yavan, and thus originated the system of astrology known as Ramal Vidyaa or Ramal Jyotish. The Ramal system of astrology originated from the mix of Phaarasee (Paarasee) and Indian system of astrology. (So the word Yavan means Phaarasee or Persian in this context).

This also gives some indications about the period of Kaalakaachaarya. Kaalakaachaarya has written a Sanhitaa text as well. [The text known as Ramal Vidyaa written by Bhojsaagar Gani, mentions the name of Kaalakaachaarya as the originator of this system] His period was before, Varaah Mihir. Mihir mentions "Kaalakaachaaryaa Sanhitaa in Brihat Sanhitaa.
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Jinavijaya Mahaa Kaavya is the biography of Jain Muni Kaalakaachaarya of Ujjain who was the Guru of Kumaaril Bhatt. So, it gives dates of Kaalakaachaarya, Kumaaril Bhatt and Aadi Shankaraachaarya - allin Jaian Yudhishthir Shak of 2634 BC (possibly Sanyaas date of Paarshwanawth, the King of Kaashi - 8 generations after Yudhishthir)

Kaalakaachaarya (599-527 BC) revived Jain Aagam, that is why he was called Veer, an image of Mahaaveer, the 24thTeerthankar
. His death time is rightly observed as Veer Nirvaan Samvat, but under British influence, it is mistook as Nirvaan of Mahaaveer.

Kumaaril Bhatt (557-493 BC) learnt Jain Aagam and possibly Ved from Kaalakaachaarya and used that to re-establish Vaidik system. Much after the death of Kaalakaachaarya, it was projected as treachery with Guru, though Kaalakaachaarya never thought that learning Ved means attack on Jainism knowledge is always one for real scholar. At the age of 63, Kumaaril Bhatt burnt himself in straw-fire at Prayaag, when Shankaracharya at the age of 15 met him after he had completed his Bhaashya on Brahm Sootra - already read and appreciated by Kumaaril Bhatt.

As per this date, the date of Shankaraachaarya is 509-476 BC. All the 4 Peeth of Shankaraachaarya were set up in 483 BC as per their inscriptions and order of king Sudhanvaa (his Patta about authority of 4 Peeth).

Varaah Mihir and Brahmgupt - both were contemporaries of Vikramaaditya of Ujjain (82 BC-19 AD).

Jishnugupt was also of the same age as Varaah Mihir, He was the son of Avantivarman, the Nepal king from 103 BC to 33 BC during whose reign Vikramaaditya started Vikram Samvat in 57 BC at Pashupati Naath. Brahmgupt was always called the son of Jishnugupt. Times of Varaah Mihir and Brahmgupt cannot be calculated in Shaalivaahan Shak of 78 AD started by the grandson of Vikramaaditya which was after the death of these 2 astronomers. Varaah Mihir has indicated the Shak used by him as 612 BC in his Brihat Sanhtaa. Samvat is Luni-solar calendar for festivals and fastings etc. Shak is used for calculation of planetary position from an epoch for which Solar months are convenient. So, Varaah Mihir was using Chaahamaan Shak of 612 BC for calculation even after Vikramaaditya started his Shak in 57 BC. Later on, Shaalivaahan Shak of 78 AD was used by astronomy texts for calculation.

All inscriptions of Avantivarman use Chahaman Shak before Vikram Samvat and his later inscriptions use Vikram Samvat.
All dates for Varaah Mihir in Panch Saiddhaantikaa, Brihat Sanhitaa tally only with epoch of 612 BC - not with Cyrus Shak of 550 BC. Cyrus of Persia was not ruling from that period, he neither captured any part of India, nor started any Shak. In fact Al Biruni (Chronology of Ancient Nations) has stated very clearly that no Shak king had ever started any Shak (calendar) and in Persia also, there was no running calendar. Years were counted from regional periods of each king. Ptolemy (Al-Magest) has also followed the same system for Assyrian kings.

Naturally, Aaryabhatt has been quoted by Varaah Mihir at least twice, so Aaryabhatt has to be much before Varaah Mihir. Many astronomers in between had been named by him. Time of Aaryabhata was changed from 360 Kali to 3600 Kali by Sudhakara Dvivedi to please Thebaut so that he could be appointed him the Principal of Queen's Sanskrit College, Kashi (now Sampurnananda Sanskrit University). Naturally, Aaryabhata had no idea of Nanda Shak used by Kharavel and other inscriptions, Shoodrak Shak of 756 BC, Chaahamaan Shak of 612 BC, Shree Harsh Shak of 456 BC, Vikram Samvat of 57 BC, or Shaalivaahan Shak of 78 AD. Aaryabhatteeya and Mahaa Siddhaant have clearly stated several times that Aaryabhatt wrote his book slightly after Kali to preserve Aarya Mat (of Swaayambhuv or Pitaamaha, Aarya = Pitaamaha in most Indian languages). The other stream of Paraashar (Vivaswaan = Soorya Siddhaant) was already described in Vishnu Puraan.

Ujjayinee And Kaalakaachaarya
Source: http://www.jainworld.com/book/historyofjainism/ch7a.asp

Ujjayinee
The next two centuries appear to be quite dark so far as any information about the Jains is concerned. There are no contemporary epigraphs or literary records. The later Jain historians say that Jainism had spread to Ujjayinee at the time of Ashok's grandson Samprati Maurya
(224-215 BC). Hemchandra (12th century) wrote that Suhastin, the head of the Jain Church at that time was living in Ujjayinee when ruled from his capital there, and Samprati was a patron of the Jains. This might have been actually so but there is no epigraphic or other independent proof of Samprati's affinity to Jainism.

Ujjayinee, however, was the scene of an important event which is said to have occurred in the 1st century BC This legend is connected with the
Shak conquest of Ujjayinee and the origin of the Vikram era. The legend mentions about Gardabhilla, a king of Ujjayinee. He had abducted the sister of Kaalakaachaarya, a celebrated Jain teacher (Kaalakaa was a king's son and had later become Jain. His sister whose name was Saravatee was herself a Jain nun). Kaalakaachaaryaa approached one of the Scythian Kings, the Shahis, in Shak Sthaan for help. But that king was afraid of attacking Gardabhilla, a powerful ruler enjoying the protection of the goddess Rasabhi, who by the spell of her voice made it impossible for an enemy to approach within 24 kilometers of the King. On his part, Kaalakaa also had magic powers and could produce wealth at will. He persuaded the Shak King to raise an army and march against Ujjayinee. When he encamped at a distance of 24 kilometers from Ujjayinee, the goddess began to raise her voice for the protection of Gardbhilla, but the Shak army stopped her mouth with their arrows, and she became unable to utter a sound. The Gardabhilla was easily made captive and Kaalakaa's sister was recovered. When he was later forgiven and released, Gardabhilla retired to a forest where he was devoured by a tiger.

Some years afterwards, the son of Gardabhilla, according to some accounts the glorious Vikramaaditya, came up from Pratishthaan with an army, expelled the invaders from Ujjayinee, and ruled there for many years in great splendor and established the era that goes by his name (58-57 BC).

Though the exact historical foundation for this legend cannot easily be ascertained, its setting fits the 1st century BC very well, as it was clearly a period of Shak inroads into India and of the attempts of Indian rulers, particularly the Satavaahan to resists them. The Hindu Puraan which describe the Saatvaahan as Aandhra, count Gardabhilla among the feudatories (Bhritya) of the Aandhra. Thus the Jain story is partly corroborated. There might thus be some historical truth in this legend of Kaalakaachaarya.

It is possible that the legend existed in some form since the 1st century but its first recorded form is found in Kaalakaachaarya Kathaanak,
a work by Mahesara Suri who probably existed at the time of Hemachandra Suri (12th century). Thus the legend or history of Kaalakaachaaryaa was put down in writing about 1200 years after the alleged event.

Reproduced by permission from Mr. Rahul Gupta from his post in the Facebook group ‘Astrologers from ICAS’