A stitch in time

What do these things have in common?

• Muses in Greek mythology
• the third square number
• the symbol of completeness in the Bahá’í faith
• the number of justices typically comprising the US Supreme Court
• circles of Hell
• choirs of angels
• the number of days it takes for an anvil to fall to Earth from Heaven
• universal substances in the Vaisheshika school of Hindu philosophy
• years Fancy Notions has been online as of today

They are all nine. Turn me on, dead man.