Why polar coordinates are integral part of Surya Siddhanta?

Surya Siddhanta mentions both celestial longitudes as well as polar longitudes. While the true places of the planets are their celestial longitudes, the bhogas mentioned in chapter 8 about the prominent stars in various nakshatras signify their polar longitudes. The word "Dhruva" mentioned in that chapter confirms the same.

On the other hand, the word that is used throughout Surya Siddhanta for latitude is Vikshepa, which means “deflection”. Obviously as the deflection from the ecliptic could be referred to even from the ecliptic pole one may consider “Vikshepa” as the celestial latitude. But there is a catch. If we read the context wherever the “vikshepa” has been explained, it is in relation to the declination circle e.g. Verse 2.6, 2.58 etc. In fact, Burgess in his commentary has accepted this peculiarity on page 46 after verse 1.70. He mentions:

"The Hindus measure the latitude, as we shall have occasion to notice more particularly hereafter, upon a circle of declination, and not upon a secondary to the ecliptic."

We can easily notice it ourselves wherever the "vikshepa" is employed in the context. So, if I say only polar latitude appears in the Surya Siddhanta would not be a wrong statement.

Now the question is:

"Why polar coordinates?"

I would begin with observations. When we're observing, we observe the apparent longitude and latitude. Since apparent would differ from place to place we find it easy to specify the equatorial coordinates where terrestrial latitude is zero. In Surya Siddhanta, though we find the methods to calculate the declination of planets, I am yet to locate a verse where we can learn how RA is calculated. In place of that we are told to convert celestial longitudes to polar longitudes as one of the steps in finding the apparent longitudes in ch7 under "Drkkarma".

Thus, polar longitude serves the purpose of what RA is supposed to do. Similarly, it would be polar latitude on the declination circle. The verse 13.13 further confirms what I'm saying.

Even in the case of the longitudes of the stars as specified in chapter 8, apart from the word “Dhruva” that is self-speaking, the same is the reason why they are polar longitudes. One can verify it in verse 8.12.

Hence, polar coordinates seem to be the need of the hour then.

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