A historian who compares world events during the classical era to world events during the modern era is organizing history by?

Period

Explanation:

In history, period defines a specific time frame when relevant historical events were occurring rather than focusing on one single event.

One characteristic of historical organization using period is the use of the word 'Era' to distinguish one historical period to another. We can see this in the example above.

Classical Era happened between 8th century BC and the 6th century AD

Modern Era happened between the year of   1500 to around 1800