Golf ball

1. 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 (H X+1 −H X−1 ) between a hardness (H X+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 (H X−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 (H X+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 hardness that is highest among the the spherical core and cover of the golf ball,

a difference (Hm−Hb) between a hardness (Hm) of the intermediate layer and a surface hardness (Hb) of the golf ball is less than 9 in Shore D hardness, and

a difference (Hb−Hc) between the surface hardness (Hb) of the golf ball and a hardness of the cover (Hc) is 22 or more in Shore D hardness,

α=(180/π)× a tan [{ H x−1 −Hoc }/( X− 1)]  (1)

β=(180/π)× a tan [{ H X+Y −H x+1 }/( Y− 1)]  (2)

where X represents a radius (mm) of the inner layer, Y represents a thickness (mm) of the outer layer, Hoc represents a center hardness (Shore C) of the spherical core, H X−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, H X+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 H X+Y represents the surface hardness (Shore C) of the spherical core.

2. The golf ball according to claim 1 , wherein the hardness (Hc) of the cover is 49 or less in Shore D hardness.

3. The golf ball according to claim 1 , wherein a difference (Hm−Hc) between the hardness (Hm) of the intermediate layer and the hardness (Hc) of the cover is 15 or more in Shore D hardness.

4. The golf ball according to claim 1 , wherein the center hardness (Hoc) of the spherical core is less than 70 in Shore C hardness.

5. The golf ball according to claim 1 , wherein the angle β ranges from −20° to +20°.

6. The golf ball according to claim 1 , wherein 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 2.0.

7. The golf ball according to claim 1 , wherein a ratio (S 2 /S 1 ) of a cross-sectional area S 2 (mm 2 ) of the outer layer to a cross-sectional area S 1 (mm 2 ) of the inner layer on a cut plane of the spherical core obtained by cutting the spherical core into two hemispheres ranges from 0.5 to 6.0.

8. The golf ball according to claim 1 , wherein a ratio (V 2 /V 1 ) of a volume V 2 (mm 3 ) of the outer layer to a volume V 1 (mm 3 ) of the inner layer ranges from 1.0 to 20.0.

9. The golf ball according to claim 1 , wherein the intermediate layer includes an inner intermediate layer and an outer intermediate layer positioned outside the inner intermediate layer.

10. The golf ball according to claim 1 , wherein the inner layer and the outer layer of the spherical core are formed from a rubber composition.

11. The golf ball according to claim 1 , wherein the intermediate layer is formed from a resin composition.

12. The golf ball according to claim 1 , wherein the hardness (Hm) of the intermediate layer is 55 or more in Shore D hardness.

13. The golf ball according to claim 1 , wherein a thickness Tm of the intermediate layer ranges from 0.8 mm to 1.7 mm.

14. The golf ball according to claim 1 , wherein the cover is formed from a resin composition containing a polyurethane as a resin component.

15. The golf ball according to claim 1 , wherein a thickness Tc of the cover is 0.9 mm or less.

16. The golf ball according to claim 1 , wherein the hardness (Hm) of the intermediate layer, a center hardness (Hod) of the spherical core, and the hardness (Hc) of the cover in Shore D hardness satisfy a following mathematical formula (3):

0.6≦( Hm−Hod )/ Hc   (3).

17. The golf ball according to claim 1 , wherein a difference (Hm−Hod) between the hardness (Hm) of the intermediate layer and a center hardness (Hod) of the spherical core is 20 or more in Shore D hardness.

18. The golf ball according to claim 1 , wherein the hardness H X−1 ranges from 63 to 82 in Shore C hardness, and the hardness H X+1 ranges from 72 to 90 in Shore C hardness.

19. The golf ball according to claim 1 , wherein the hardness difference (H X+1 −H X−1 ) between the hardness H X−1 and the hardness H X+1 is 5 or more in Shore C hardness.

20. The golf ball according to claim 18 , wherein a hardness difference (H X−1 −Hoc) between the center hardness Hoc and the hardness H X−1 ranges from 4 to 27 in Shore C hardness,

the hardness difference (H X+1 −H X−1 ) between the hardness H X−1 and the hardness H X+1 ranges from 5 to 20 in Shore C hardness, and

a hardness difference (H X+Y −H X+1 ) between the hardness H X+1 and the surface hardness H X+Y ranges from −7 to 10 in Shore C hardness.