Question: What is the date of birth of Ulbo De Sitter's father?
Answer: 6 May 1872

His father was the astronomer Willem de Sitter (1872–1934), and one of his sons was the Dutch sociologist Ulbo de Sitter (1930–2010). Willem de Sitter (6 May 1872 – 20 November 1934) was a Dutch mathematician, physicist, and astronomer.