Explain why the first ionisation energy of Strontium is less than the first ionisation energy of Calcium

Both of these elements are part of the 2nd group of elements on the periodic table. ie in first ionisation, their electron configuration changes from nS2 -> nS1.

Strontium is a much larger element, so the outermost electron will be further away from the nucleus (larger atomic radius. This distance decreases the attration of the electron to the nucleus, decreasing the first ionisation energy.

Because Strontium's outermost electron is in 5s rather than 4s (in Calcium): it experiences more electron repulsion which decreases ionisation energy by reducing the electron's ability to get close to the nucleus.

These two factors (atomic radius and electron shielding) decrease the effective nuclear charge experienced by the outermost electron in Strontium, causing the first ionisation energy to be lower than that of Calcium.