Expiation below in link below why a transit of Venus is the rarest astronomical event that can be predicted in just 2 minutes in the very short video in the link bellow. A transit of Mercury takes place quite frequently every 100 years. Before 20004 no one alive would have a transit of Venus. As the last transit of Venus Before 20004 took place in 1882!
4075.Predicting a Transit of Venus The Two-Minute Explanation.pdfT
Regards,
Ian.