Within the earth's core, which is iron, pressure increases with depth  Because the temperature at which iron melts increases with pressure, the inner core is solid and the outer core is molten  Physicists can determine the melting temperature of iron at any given pressure and the pressure for any given depth in the earth
Ttherefore, the actual temperature at the boundary of the earth's outer and inner cores-the melting temperature of iron there -- can be determined, since
the temperatures within the earth's inner core are higher than in the outer core