A lucky guess?
Vitamin K. It was discovered in the 1930's, by a Danish biochemist, Henrik Dam, who shared with Edward Doisy the 1943 Nobel Prize for their work on vitamin K. In the 1930's, vitamin K was thought to be a factor in foods that helped prevent hemorrhaging. Scientists now know vitamin K is used by the body to produce prothrombin. If the body has a vitamin K deficiency, there will be a prothrombin deficiency. If there is a prothrombin deficiency, hemorrhaging may occur. Prothrombin causes the blood to coagulate. Basically, without prothrombin, the blood doesn't clot. It takes the first five to seven days of a newborn male's life for his body to produce a sufficient amount of vitamin K for him not to hemorrhage. This is documented in a book entitled "Holt Pediatrics". Holt Pediatrics observed that newborns have a "peculiar susceptibility to bleeding between the second and fifth days of life . . . . Hemorrhages at this time, though often inconsequential, are sometimes exten sive; they may produce serious damage to internal organs, especially to the brain, and cause death from shock exsanguination."
What does this have to do with the Bible?
The eighth day of life is peculiar in regard to prothrombin. Under normal circumstances, the eighth day is the only time that a person will have more than 100% of the normal amount of prothrombin in their system. If you had to pick a day to schedule surgery, selecting the day when your concentration levels of prothrombin are at the highest point in your life, would be the perfect choice. The book "None of These Diseases" by S.I. McMillen, M.D. (1984 Old Tappan, N.J.) states
How did Moses know to write the eighth day? If you believe the Bible, it's pretty straightforward. Genesis 17 is the account of God confirming a covenant with Abraham. God is the one speaking in Genesis 17:12. Obviously, God knew all about prothrombin long before he told Abraham what day infants should be circumcised.If you don't believe the Bible, there is another logical conclusion: A lie and a lucky guess. According to "The Bible Timeline" by Thomas Robinson, the accounts of Genesis 17 occurred in 1896 BC. So, some 3800 years before anyone had ever heard of vitamin K, Abraham started spreading a false rumor about God giving him exact instructions for minor surgery, and out of all the dates available, Abraham luckily guessed the best date.