Golf ball

A golf ball comprising a spherical core, an intermediate layer positioned outside the spherical core, and a cover positioned outside the intermediate layer, wherein:

• the spherical core includes an inner layer and an outer layer,
• a difference (HX+1−HX−1) between a hardness (HX+1) at a point outwardly away in a radial direction from a boundary between the inner layer and the outer layer of the spherical core by 1 mm and a hardness (HX−1) at a point inwardly away in the radial direction from the boundary between the inner layer and the outer layer of the spherical core by 1 mm is 0 or more in Shore C hardness,
• a surface hardness (HX+Y) of the spherical core is more than 70 in Shore C hardness,
• an angle α of a hardness gradient of the inner layer calculated by a formula (1) is 0° or more,
• a difference (α−β) between the angle α and an angle β of a hardness gradient of the outer layer calculated by a formula (2) is 0° or more,
• the intermediate layer has a material hardness (Hm) ranging from 65 to 80 in Shore D hardness,
• the cover has a material hardness (Hc) ranging from 57 to 72 in Shore D hardness,
• the intermediate layer has a highest hardness among the constituent members of the golf ball,
• a diameter of the spherical core ranges from 36.5 mm to 42.0 mm, and
• a ratio (Y/X) of the thickness Y (mm) of the outer layer to the radius X (mm) of the inner layer ranges from 0.2 to 0.63,
• α=(180/π)×a tan [{Hx−1−Ho}/(X−1)] (1)
• β=(180/π)×a tan [{HX+Y−Hx+1}/(Y−1)] (2)
• wherein X represents a radius (mm) of the inner layer, Y represents a thickness (mm) of the outer layer, Ho represents a center hardness (Shore C) of the spherical core, HX−1 represents the hardness (Shore C) at the point inwardly away in the radial direction from the boundary between the inner layer and the outer layer of the spherical core by 1 mm, HX+1 represents the hardness (Shore C) at the point outwardly away in the radial direction from the boundary between the inner layer and the outer layer of the spherical core by 1 mm, and HX+Y represents the surface hardness (Shore C) of the spherical core.