The "observer of times" (KJV) is someone who reads and interprets omens, not a historian.

Walton, Matthews, & Chavalas (The IVP Bible Backgrounds Commentary) write: [quote]"One of the priestly classes mentioned in Mesopotamian texts are the baru-diviners. It was their task to perform extispicy (generally on lambs), examining the liver and interpreting this omen for the person who has asked for a reading of the future. The baru might be consulted by a king who wished to go to war (compare 1 Kings 22:6), a merchant about to send out a caravan, or a person who had become ill. Government officials often included the report of omens in their reports (Mari texts). However, since omens were not always clear, several groups of diviners might be used before action was taken. An entire body of omen texts (with descriptions of past events and predictions) were archived in temples and palaces for consultation by staff diviners. Even clay models of livers were used in schooling apprentices in the trade."[/quote]

Deuteronomy 18:10 Why is an observer of times, aka a historian seen as being sinful?

Observer of time is what we call historians. Why is this seen as sinful according to Christian lore?