Sastra means Science, systematized knowledge of Nature. Not only physical sciences, Sastra includes spiritual sciences as well, as Indian Philosophy deals with both  Matter and Spirit, Purusha & Prakriti.

Siddhanta Sastra means the Science of Astronomy & Hora Sastra means the Science of Astrology.

 

P N Oak was right when he said that the English word Hour is a malpronunciation of the Sanskrit word Hora. Hora Sastra means the Science of Hour, Time.

When I met the astro scholar, Avanoor Asan, he was              ecstatic when he described the Master, Varaha.  Said the Master was an Avatar, who came to redeem this infinite Ocean of Knowledge which is the  Science of Time, this Hora Sastra , Hora Sastra Maharnava pratharane.

Aditya Dasathanayastava apta Bodha Kapishtalo Savithru Labda vara prasada  
Avantiko Muni manany avalokya Samyak  
Horaam Varaha Mihiram ruchiram chakara
 
Savithru Labda vara prasada refers to Revealations received by the Master in higher states of Consciousness, symbolically depicted as boons from the  Sun !  

An astronomer, mathematician and astrologer par excellence, Varaha strode the scene like an intellectual colossus, contributing to all the three branches of the triune Astro Physics, the Thriskanda Jyotis Sastra - Siddanta, Samhita & Hora.

See how he describes The Significations of the Twelve Houses  in just two lines

Kalya  Swa     Vikrama Griha Prathibha Kshatani
Chittotha Randhra Guru  Mana Bhava Vyayani.
The Quintessence of BHS are these two  verses

Planetary states are    seven,
Exalted is Sampoorna,
Devoid of strength positional is Riktha
In inimical and debilitation houses,
Is Anishta Phalada called !

Fallen from Exaltation is Avarohee
Madhya in houses of  fiends,
Arohinee when rising,
And Adhama in houses inimical !

Samyak balina  swa tunga bhage
Sampoorna balavarjitasya Riktha
Neechamsa gathasya satrubhage
Jneya  Anishtaphala  Dasa   prasoothu
 
Bhrishtasya tungath Avarohi samjna
Madhya bhaveth sa Suhrud Ucchabhamse
Arohinee nimna parichyutasya
Neecharibhageth Adhama bhaveth Sa !


BHS is Varaha’s magnum opus. wherein he spells out the principles of the Science of Time, in 384  verses of golden Sanskrit Poetry.

BHS has a great competitor, BPHS, Brihat Parasara Hora Sastra, consisting of 4000 verses.