The impact, spotted by two telescopes in Spain, was the biggest yet observed on the moon and briefly almost as bright as the Pole Star.
Although the moon is frequently hit by small objects, the likelihood of seeing one by chance is low.
Scientists have set up networks of telescopes that can spot them automatically. The two telescopes in Spain are part of the Moon Impacts Detection and Analysis System (Midas) .
At 8.07pm UK time on September 11 last year an unusually long and bright flash was seen in Mare Nubium, an ancient lava-filled lunar basin.
A video made by Midas astronomer Professor Jose Madiedo shows an afterglow that was visible for eight seconds.
"At that moment I realised that I had seen a very rare and extraordinary event," said Prof Madiedo.
Scientists believe the flash was produced by an object weighing 400 kilograms and measuring between 0.6 and 1.4 metres across.
The rock struck the moon at around 61,000 kilometres per hour with an explosive force equivalent to 15 tons of TNT and created a new 40 metre-wide crater.