Historians label an event as a turning point when it what

Marks the beginning of a distant historical period.

Explanation:

Historians set a turning point as a specific, significant period when decisive changes take place. It means more than a significant moment , since many events happened long time ago and sometimes debate arises how to classify turning points:

Nonetheless a turning point marks a new stage, take an example:

Soviet Sputnik rising above the sky commonly targets the new era of mankind entering the space, and as such a turning point in history, the turning point then means a lasting change that is revolutionary in essence and changes time and perspective for events to unfold.

A decisive change , makes us frequently to talk about After and Before Sputnik, and this can be applied to major events.

After and before History..

After and before Christ, After and before Printing Press,

This are all examples where most Historians agree that a decisive change made things to be reconceptualized.

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a historian researching the immigration policies of the early 1900s would expect a source written today,rather than in 1910 to :

( a ) be more interested in objective data and statistics.