Golf ball

1. A golf ball comprising a core and a cover of at least one layer, wherein the core has a cross-sectional hardness which, letting R (mm) be a radius of the core, A be a JIS-C hardness at a center of the core, B be a JIS-C hardness at a position R/3 mm from the core center, C be a JIS-C hardness at a position R/1.8 mm from the core center, D be a JIS-C hardness at a position R/1.3 mm from the core center, and E be a JIS-C hardness at a surface of the core, satisfies formulas (1) to (4) below:

D−C≧ 7  (1)

C−B≦ 7  (2)

( D−C )−( C−B )≧7  (3)

E−A≧ 16,  (4)

wherein the core is formed from a material molded under heat and is formed from a rubber composition comprising a base rubber, a crosslinking initiator, and water,

wherein the difference between the crosslink density at the core surface and the crosslink density at the core center, both of which are crosslink densities measured based on a toluene swelling test, is at least 9×10 2 mol/m 3 , and

wherein the base rubber consists of polybutadiene synthesized with a rare-earth catalyst and/or a polybutadiene synthesized with a nickel catalyst.

2. The golf ball of claim 1 , wherein the ball has a deflection, when compressed under a final load of 5,880 N (600 kgf) from an initial load state of 98 N (10 kgf), of from 7 to 10 mm.

3. The golf ball of claim 1 , wherein the cover comprises an outermost layer and an intermediate layer interposed between the outermost layer and the core, and the ball has, at a surface of a sphere composed of the core covered by the intermediate layer, a JIS-C hardness of at least 90.

4. The golf ball of claim 1 , wherein the outermost layer of the cover contains a base resin comprising a polyurethane material having a Shore D hardness of from 40 to 60.

5. The golf ball of claim 1 , wherein the core has a diameter of from 32 to 41 mm.

6. The golf ball of claim 1 , wherein the core is composed of a single layer.

7. The golf ball of claim 1 wherein, when the loss tangent of the core center is measured at a temperature of −12° C. and a frequency of 15 Hz, letting tan δ 1 be the loss tangent at a dynamic strain of 1% and tan δ 10 be the loss tangent at a dynamic strain of 10%, the slope of these tan δ values, expressed as [(tan δ 10 − tan δ 1 /(10%−1%)], is not more than 0.002.

8. The golf ball of claim 1 , wherein the amount of water per 100 parts by weight of the base rubber is from 0.1 to 5 parts by weight.

9. A golf ball comprising a core, an envelope layer, an intermediate layer and an outermost layer, wherein the core has a cross-sectional hardness which, letting R (mm) be a radius of the core, A be a JIS-C hardness at a center of the core, B be a JIS-C hardness at a position R/3 mm from the core center, C be a JIS-C hardness at a position R/1.8 mm from the core center, D be a JIS-C hardness at a position R/1.3 mm from the core center, and E be a JIS-C hardness at a surface of the core, satisfies formulas (1) to (4) below:

D−C≧ 7  (1)

C−B≦ 2  (2)

( D−C )−( C−B )≧7  (3)

E−A≧ 16, and  (4)

wherein the material hardness of the intermediate layer, expressed as the Shore D hardness, is from 62 to 70,

wherein the core is formed from a material molded under heat and is formed from a rubber composition comprising a base rubber, a crosslinking initiator, and water, and

wherein the difference between the crosslink density at the core surface and the crosslink density at the core center, both of which are crosslink densities measured based on a toluene swelling test, is at least 9×10 2 mol/m 3 , and

wherein the base rubber consists of polybutadiene synthesized with a rare-earth catalyst and/or a polybutadiene synthesized with a nickel catalyst.

10. The golf ball of claim 9 , wherein the JIS-C hardness at the core center is from 63 to 70.

11. The golf ball of claim 9 , wherein the JIS-C hardness at the core surface is from 84 to 100.

12. The golf ball of claim 9 , wherein the material hardness of the envelope layer is from 40 to 51 at Shore D hardness.

13. The golf ball of claim 9 , wherein the material hardness of the outermost layer is from 34 to 47 at Shore D hardness.

14. The golf ball of claim 9 wherein, when the loss tangent of the core center is measured at a temperature of −12° C. and a frequency of 15 Hz, letting tan δ 1 be the loss tangent at a dynamic strain of 1% and tan δ 10 be the loss tangent at a dynamic strain of 10%, the slope of these tan δ values, expressed as [(tan δ 10 − tan δ 1 )/(10%−1%)], is not more than 0.002.

15. The golf ball of claim 9 , wherein the amount of water per 100 parts by weight of the base rubber is from 0.1 to 5 parts by weight.