Question

What is the linear diameter (Bimba-vyasa) of the Sun's disk as specified in the Surya Siddhanta?

• A. 7000 yojanas
• B. 480 yojanas
• C. 6500 yojanas
• D. 100 yojanas

Hint:

Use this mnemonic: "Sun is huge (6500), Moon is small (480), and Earth is in between (1600)." These three numbers are frequently tested together in the 'Graha-Bimba' section.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
In Siddhantic astronomy, celestial bodies are not mere points of light; they have physical dimensions. The 'Bimba-vyasa' refers to the actual linear diameter of the planetary or solar disk measured in Yojanas. This is distinct from the 'Kala-vyasa' (angular diameter) measured in minutes of arc as seen from Earth.

Step 2: Detailed Explanation of Constants:
The Surya Siddhanta provides a list of diameters for the primary celestial bodies to facilitate the calculation of eclipses and occultations. These diameters are:
1. Sun (Surya): 6500 Yojanas.
2. Moon (Chandra): 480 Yojanas.
3. Earth (Bhu): 1600 Yojanas.
These values are "true" linear diameters. When these bodies are at different distances from the Earth, their angular sizes (how large they appear in the sky) change, but their physical size in Yojanas remains constant according to the text.

Step 3: Application in Eclipses:
To predict an eclipse, an astronomer needs to know the angular diameters of the Sun and Moon. This is calculated using the Rule of Three (Trairashika):
\[ \text{Angular Diameter} = \frac{\text{Linear Diameter} \times \text{Mean Angular Diameter}}{\text{Distance}} \]
In the Surya Siddhanta's model, the Sun is much larger than the Moon (6500 vs 480 yojanas), but because the Sun is much further away, they both appear to be roughly 32 minutes of arc in the sky, allowing for total solar eclipses to occur.
- Option Analysis: 480 is the Moon's diameter. 1600 is the Earth's diameter. 6500 is specifically the Sun's diameter.

Step 4: Final Answer:
The linear diameter of the Sun according to the Surya Siddhanta is 6500 yojanas.