Function calculateSighraEquation

calculateSighraEquation(
    planetLong: number,
    sighraLong: number,
    circTuple: [number, number],
): number
Calculate the Sighra Equation (Equation of Conjunction).

[Ch. II, v.40-42] A more complex correction using the 'Chala-karna' (Variable Hypotenuse) to account for the planet's change in distance from Earth.
Parameters
- planetLong: number
  Current corrected position
- sighraLong: number
  Longitude of the conjunction point
- circTuple: [number, number]
  Sighra epicycle circumferences
Returns number
Sighra correction (Sighra-phala) in degrees