The Copernican block system is a scheduling system for American high school classes. It differs in a number of ways from normal scheduling.
The normal method consists of approximately seven ~50 minute classes per school day (8 AM-3 PM; seven hours, with passing time and lunch). Each semester's work grants a semester's worth of credit for each class (provided one passes). It is the most commonly used scheduling system.
On the other hand, the Copernican schedule is four ~90 minute classes per school day. Each semester's work grants a full academic year's credit (thus, a 2-semester class under the normal system becomes a single-semester class with this one). The second semester comes with an entirely new set of classes. The assumption is that with fewer classes and more time per class, teaching becomes more effective.
This method has a number of drawbacks. The first, and foremost, is that the above assumption is usually invalid to one degree or another. Taking a full year's worth of history in one semester merely results in a very accelerated history class. It would seem that 90 minutes would provide almost double the teaching time; however, the productive time increase is not nearly as radical. The normal system knows that teenagers have short attention spans. Keeping them interested for 50 minutes is hard enough; by the end of 90, most of the class is slipping into the arms of Morpheus.
The second drawback is the difficulty of acclimating unfamiliar students to the new system. This is heightened by the issues regarding transfer of credits from one system to another, if switching mid-year. Some school districts, like LAUSD, use both systems concurrently (in different schools), which also creates problems.
Overall, the positives of a Copernican system are outweighed by the negatives. Several districts have switched or are switching to the normal system.