The more I understand the Surya Siddhantha, the impressive its geocentric model  looks, considering it is at least 1800 year old.

The Surya Siddhantha model is as follows:

• The Earth is at the center of Brahmánda, fixed and unmoving.
• The following bodies revolve around the Earth, East to West, in order of distance from the Earth:
• Moon (Soma)
• Mercury (Budha)
• Venus (Shukra)
• Sun (Ravi)
• Mars (Mangala / Angaraka)
• Jupiter (Guru / Bruhaspathi)
• Saturn (Shani)
• Beyond them all revolves the Nakshatra Mandala (Rashi Mandala, zodiac, asterisms, fixed stars, call it what you will).

So far so good, but how does this model simulate the axial rotation of the Earth? Let us take a closer look the what the Surya Siddhantha actually says.

“This Brahmánda is hollow; within it is the universe consisting of earth, sky etc. It has the form of a sphere like a receptacle (samputa) made of a pair of  caldrons. A circle within the Brahmánda is styled the circle of the ether (vyoman); within that is the revolution of the asterisms (bha); and likewise in order one below the other revolve Saturn, Jupiter, Mars, the Sun, Venus, Mercury, and the Moon;…”
Surya Siddhantha, Chapter 12, Verse 29 – 31

“The planets, moving westward with exceeding velocity, but constantly beaten by the asterisms, fall behind, at a rate precisely equal, proceeding each in its own path. Hence they have an eastward motion. From the number of their revolutions is derived their daily motion, which is different according to the size of  their orbits; in proportion to this daily motion they pass through the asterisms.”
- Surya Siddhantha, Chapter 1, Verse 25,26

It took me a long time to understand the above.

• Firstly, the Nakshatra mandala is not stationary but is revolving East to West (i.e. westward). Since  the Earth is fixed and unmoving at the center of Brahmanda, the asterisms have to move instead thus simulating the daily east to west movement of the Nakshatra Mandala around the Earth.
• Now consider the Sun. It too has to revolve around  the earth to simulate the axial rotation of the Earth. More specifically, to simulate the apparent daily westward  movement of the Sun that results in day and night (i.e. “civil” days).
• Moreover, it is made to revolve slower than the Nakshatra Mandala since there is an observed “fall back” eastward of the Sun with respect to the Nakshatra mandala. Over a period 365.25 days, the Nakshatra Mandala laps the Sun (like in a F1 race a relatively faster driver laps a slower driver) thus completing a solar year. In other words this eastward motion of the sun vis a vis the asterisms models the annular movement of the Sun around the earth that defines a solar year of 365 days.
• Hence, the Surya Siddhantha models both a westward daily (diurnal) motion and an eastward annular motion of the Sun.

To see this visually, try this animation of the orbital motion of the Sun. This animation simulates the daily westward motion of the sun (i.e. sunrise to sunset to next sunrise) at a point on the equator as per the above model. As the animation runs you can see the sun “falling back” eastward behind the asterisms thus illustrating the annular motion of the Sun. If you patiently watch the annual animation, you can see the asterism Mesha eventually catching up with the sun after 365 days.

I used the following parameters from Surya Siddhantha to create the above model:

“In a Yuga, the revolutions of the Sun, Mercury and Venus, and of the conjunctions of Mars, Saturn and Jupiter, moving eastward, are four million, three hundred and twenty thousand.”
- Surya Siddhantha, Chapter 1,  Verse 29

“Of the asterisms, one billion, five hundred and eight two million, two hundred and twenty seven thousand, eight hundred and twenty eight. The number of risings of the asterisms, diminished by the number of the revolutions of each planet respectively, gives the number of risings of the planets in a Yuga”
- Surya Siddhantha, Chapter 1, Verse 34

As Verse 34 says, the “number of risings of the sun”, i.e. actual westward (daily) revolutions of the sun, i.e. the number of sunrises, i.e. number of solar days is 1,582,237,828 – 4,320,000 = 1,577,917,828. Also note that the eastward annular revolutions of 4,320,000 mentioned in the verse is nothing but the duration of a Maha Yuga in solar years.