The heat capacity of synthetic sapphire is determined by adiabatic shield calorimetry at temperatures from 298.15 K to 1000 K. The calorimetric procedures are discussed in detail. Special attention is given to temperature calibration, and to transformation of temperatures measured on IPTS-48 and IPTS-68 to ITS-90. The accuracy of the present temperature determination is considered to be within 0.02 K to T = 903 K and within 0.10 K up to T = 1000 K. The effect of the temperature scale changes on the measured heat capacities is within +/-0.03 per cent up to T = 800 K. Above this temperature, adjustment of the heat capacities might be warranted, but it depends on the thermometer used. The present directly determined heat capacities for synthetic sapphire are presumably the most accurate so far covering the range from T = 300 K to T = 1000 K. The standard deviation of a fit of the experimental heat capacity determinations with a smoothed curve is 0.09 per cent. The resulting heat capacity values for synthetic sapphire deviate from those recommended by Archer by less than 0.05 per cent. They thereby confirm the recommended values, which were based on drop-calorimetric results only at T > 700 K.