Brihat-Samhita is a encyclopedic text by author Varahamihira .The book discusses some subjects including astrology, planetary movements, eclipses, etc.
This topic is discussed in Brihat-Samhita Adhaya two , which is called "Samvatsara Sutra Adhaya" (सांवत्सर सूत्र अधाय) - The Astrologer. This is how the book describing them..
Here are some technical terms related to motion of planets , their distances etc. which a astrologer should be well familier with.
चतुर्णां च मानानां सौरसावननाक्षत्रचान्द्रणामधिमास-
कारणाभिद्न्य: ||Shloka 5||
He should be also familier with four kind of measurments of time
viz. saura (solar) : Savan , (a duration (day) intervening between the
first rising of any perticular planet or star and its corosponding
next rising ; Nakashtra (steller calculation in terms of lunar months
,Tithis etc. with the occurance of Adhimasa or inter-calary lunations
and intercalary days.
सुर्यदिनां च ग्रहाणां
शीघ्रमन्दयाम्योत्तरनीचोच्चगतिकार-णाभिद्न्या: ||Shloka 9 ||
He should also be conversant with the causes that lead the planets
from the sun onwards to proceeding different kind of motions - rapid
,slow ,southerly ,northerly ,towards apojee , periee etc.
He should in the case of every planet be able to calculate its
distance (in yojana) from the Earth and length of its orbit.
Here its said that a true astrologer is who understands the spirit of text and should not oppose to that, a true astrologer is one who is well in is mathematical as well as other concepts related to grahas and nakshatras. AS discussed in other verses.
कनकस्येवाधिकतरममलीकृतस्य शास्त्र्यस्य वक्ता तन्त्रज्ञ्नो भवति
He is a true Astrologer who is able to understand the objections and
points of differences and meet them in clear and convincing language
thus proving the truth of the science in its pristine purity.
And not only the text discusses the technical capabilities a astrologer should posses but like How should he live in society , how to react among people and how he emotionally should be etc.
तत्र गुणा: | शुचिर्दक्ष : प्रगल्भो वाग्मी प्रतिभावान् देशकालवित्
सात्विको न पर्षद्भिरु: सहाध्यायीभिरनभिभवनिय: कुशलो व्यसनि
शान्तिकपौष्टिकाभिचारस्नानविद्यभिज्ञो विबुधा - र्चन व्रतोपवासनिरत :
स्वतन्त्रच्श्रर्योत्पादितप्रभाव: पृष्टाभिधाय्यन्यत्र दैवात्ययात् |
ग्रहगणितसंहिताहोराग्रन्थार्थवेत्तेति ||Shloka 3 ||
The following are the good qualities of an astrologer. He must be
clean and active , bold and eloquent , have readiness of wit , fully
conversant with the details of time and place and sincere in
disposition. He should not be timid in an assembly , should not be
overpowered by his fellow students , must be well trained , should
understand heart of others ,should be free from vices , must be
conversant with art of propagation , with hygiene , magic and
ablutions ,should be engage in worship of devas ,should be observer of
fast and penance,should be endowed with great power resulting from the
wonderful achievements of his scientific knowledge , should be capable
of answering questions put by others ,and voluntarily suggesting
palliatives and remedies regarding things other than visitations of
God. He should also be thoroughly conversant with works dealing with
calculations of the positions etc. of planets ,samhita , Horoscopy and their sevral details.
It's even said that the person who studies the joytish shastra with it's mathematical part very well and through heart , his predictions will never prove ineffective.
जगति प्रसारितमिवालिखितमिव मतौ निषिक्तमिव ह्र्दये |
सभगणं नादेशा निष्फ़लातस्य ||Shloka 5||
He has got a clear grasp of the astrological science inclusive of the
mathematical portion as if it were spread before his eyes in the world
and engraved in his intellect and embedded in his heart the
predictions made by him will never prove ineffective.