A golf ball 2 includes a spherical core 4 and a cover 6 positioned outside the core 4 . The golf ball 2 further has dimples 8 on a surface thereof. The core 4 is obtained by crosslinking a rubber composition. The difference between a hardness H(5.0) at a point that is located at a distance of 5 mm from the central point of the core 4 , and a hardness Ho at the central point is equal to or greater than 6.0. The difference between a hardness H(12.5) at a point that is located at a distance of 12.5 mm from the central point, and the hardness H(5.0) is equal to or less than 4.0. The difference between a hardness Hs at the surface of the core 4 and the hardness H(12.5) is equal to or greater than 10.0. The difference between the hardness Hs and the hardness Ho is equal to or greater than 22.0. There is no zone in which a hardness decreases from the central point to the surface. The rubber composition of the core 4 includes 2-naphthalenethiol. A cross-sectional shape of each dimple 8 is a wave-like curve.
1. A golf ball comprising a core and a cover positioned outside the core, and having dimples on a surface thereof, wherein
a difference between a JIS-C hardness H(5.0) at a point that is located at a distance of 5 mm from a central point of the core, and a JIS-C hardness Ho at the central point is equal to or greater than 6.0,
a difference between a JIS-C hardness H(12.5) at a point that is located at a distance of 12.5 mm from the central point, and the hardness H(5.0) is equal to or less than 4.0,
a difference between a JIS-C hardness Hs at a surface of the core and the hardness H(12.5) is equal to or greater than 10.0,
each dimple has a curved surface, and
a cross-sectional shape of the curved surface is a wave-like curve having:
(1) 3 to 7 projections located above a circular arc that passes through one dimple edge, a deepest point of the dimple, and another dimple edge; and
(2) two or more recesses located below the circular arc.
2. The golf ball according to claim 1 , wherein a ratio of a distance between the dimple edge and a peak of a projection closest to the dimple edge, to a radius of the dimple, is equal to or greater than 20% but equal to or less than 70%.
3. The golf ball according to claim 1 , wherein one recess is present between the dimple edge and a projection closest to the dimple edge.
4. The golf ball according to claim 1 , wherein the wave-like curve is obtained by combining a sine curve and a circular arc.
5. The golf ball according to claim 4 , wherein a number of cycles of the wave-like curve is equal to or greater than 2.0 but equal to or less than 6.0.
6. The golf ball according to claim 4 , wherein a ratio (WL/D) of a wavelength WL of the sine curve to a length D of a chord of the circular arc is equal to or greater than 1/6 but equal to or less than 1/2.
7. The golf ball according to claim 1 , wherein the wave-like curve is obtained by combining a cosine curve and a circular arc.
8. The golf ball according to claim 7 , wherein a number of cycles of the wave-like curve is equal to or greater than 2.5 but equal to or less than 7.0.
9. The golf ball according to claim 7 , wherein a ratio of an amplitude of the cosine curve to a depth of the circular arc is equal to or greater than 5% but equal to or less than 50%.
10. The golf ball according to claim 7 , wherein a ratio (WL/D) of a wavelength WL of the cosine curve to a length D of a chord of the circular arc is equal to or greater than 1/7 but equal to or less than 1/2.5.
11. The golf ball according to claim 1 , wherein a difference between the hardness Hs and the hardness Ho is equal to or greater than 22.0.
12. The golf ball according to claim 1 , wherein there is no zone in which a hardness decreases from the central point toward the surface of the core.
13. The golf ball according to claim 1 , wherein
the core is formed by crosslinking a rubber composition including a base rubber and an organic sulfur compound, and
the organic sulfur compound has a molecular weight of 150 or higher but 200 or lower and a melting point of 65° C. or higher but 90° C. or lower.
14. The golf ball according to claim 13 , wherein the rubber composition includes the base rubber in an amount of 100 parts by weight, and the organic sulfur compound in an amount that is equal to or greater than 0.03 parts by weight but equal to or less than 3.5 parts by weight.
15. The golf ball according to claim 13 , wherein the sulfur compound is 2-naphthalenethiol.
16. The golf ball according to claim 1 , wherein
the hardness Ho is equal to or greater than 40.0 but equal to or less than 70.0, and
the hardness Hs is equal to or greater than 78.0 but equal to or less than 95.0.
17. The golf ball according to claim 1 , wherein the hardness H(5.0) is equal to or greater than 63.0 but equal to or less than 73.0.
18. The golf ball according to claim 1 , wherein the hardness H(12.5) is equal to or greater than 64.0 but equal to or less than 76.0.
19. The golf ball according to claim 2 , wherein a ratio of a distance between the dimple edge and a peak of a projection closest to the dimple edge, to a radius of the dimple, is equal to or greater than 29% but equal to or less than 60%.