from typing import Tuple
import math


def rotation_y_to_alpha(
    rotation_y: float, center: Tuple[float, float, float]
) -> float:
    """Convert rotation around y-axis to viewpoint angle (alpha)."""
    alpha = rotation_y - math.atan2(center[0], center[2])
    if alpha > math.pi:
        alpha -= 2 * math.pi
    if alpha <= -math.pi:
        alpha += 2 * math.pi
    return alpha