A core 4 of a golf ball 2 includes a center 10 and an envelope layer 12 positioned outside the center 10 . The ratio of the volume of the core 4 to the volume of a phantom sphere of the golf ball 2 is equal to or greater than 78%. The difference (He−Ho) between a JIS-C hardness He at the surface of the core 4 and a JIS-C hardness Ho at the central point of the core 4 is equal to or greater than 15 but equal to or less than 40. The difference (H 1 −Ho) between a JIS-C hardness H 1 at a point P 1 that is located radially outward of the boundary between the center 10 and the envelope layer 12 and whose distance from the boundary is 1 mm and the JIS-C hardness Ho is equal to or greater than 0 but equal to or less than 2.
1. A golf ball comprising a core, a mid layer positioned outside the core, and a cover positioned outside the mid layer, wherein
the core comprises a center and an envelope layer positioned outside the center,
a ratio of a volume of the core to a volume of a phantom sphere of the golf ball is equal to or greater than 78%,
a difference (He−Ho) between a JIS-C hardness He at a surface of the core and a JIS-C hardness Ho at a central point of the core is equal to or greater than 15 but equal to or less than 40, and
a difference (H 1 −Ho) between a JIS-C hardness H 1 at a point P 1 that is located radially outward of a boundary between the center and the envelope layer and whose distance from the boundary is 1 mm and the JIS-C hardness Ho at the central point of the core is equal to or greater than 0 but equal to or less than 2.
2. The golf ball according to claim 1 , wherein a JIS-C hardness Hc of the cover is less than 65.
3. The golf ball according to claim 1 , wherein a thickness of the cover is less than 0.8 mm.
4. The golf ball according to claim 1 , wherein a JIS-C hardness Hm of the mid layer is equal to or greater than 90.
5. The golf ball according to claim 1 , wherein a thickness of the mid layer is equal to or less than 1.5 mm.
6. The golf ball according to claim 1 , wherein a JIS-C hardness Hc of the cover is less than the JIS-C hardness Ho at the central point of the core.
7. The golf ball according to claim 1 , wherein a JIS-C hardness Hm of the mid layer is greater than the JIS-C hardness He at the surface of the core.
8. The golf ball according to claim 1 , wherein
the cover is formed from a resin composition,
a shear loss elastic modulus G″ of the resin composition, which is measured under conditions of a vibration frequency of 10 Hz and a temperature of 0° C., is equal to or less than 1.95×10 7 Pa, and
a ratio (E″/G″) of a tensile loss elastic modulus E″ of the resin composition, which is measured under conditions of a vibration frequency of 10 Hz and a temperature of 0° C., to the shear loss elastic modulus G″ is equal to or greater than 1.76.
9. The golf ball according to claim 8 , wherein
a principal component of a base material of the resin composition is a thermoplastic polyurethane, and
a polyol component of the thermoplastic polyurethane is polytetramethylene ether glycol having a number average molecular weight of 1500 or less.
10. The golf ball according to claim 1 , wherein the JIS-C hardness Ho at the central point of the core is equal to or greater than 40 but equal to or less than 80.
11. The golf ball according to claim 1 , wherein a diameter of the center is equal to or greater than 10 mm but equal to or less than 20 mm.
12. The golf ball according to claim 1 , wherein the envelope layer is obtained by a rubber composition being crosslinked.
13. The golf ball according to claim 1 , wherein a JIS-C hardness at a surface of the envelope layer is equal to or greater than 75 but equal to or less than 95.
14. The golf ball according to claim 1 , wherein a thickness of the envelope layer is equal to or greater than 8 mm but equal to or less than 18 mm.
15. The golf ball according to claim 1 , wherein a difference (Ho−Hc) between a JIS-C hardness Hc of the cover and the JIS-C hardness Ho at the central point of the core is equal to or greater than 3 but equal to or less than 40.