At orbital speed, spacecraft circle the Earth every 90 minutes, with a sunrise or sunset every 45 minutes, making it difficult to keep track of time as usual. Time in space is tracked more than one way, by reference to more than one location or standard, such as Houston time or mission elapsed time (MET). Another way is Greenwich Mean Time or Universal Time (GMT or UT), the time at the Greenwich Observatory near London, where by international agreement, each 24-hour day starts and ends. This is a GMT digital clock; it would be set to display Greenwich time in the Mission Control Center or on a spacecraft for comparison to “local” and mission elapsed time. From GMT, one can add or subtract hours to know the time elsewhere in the world. NASA sent this timer to the Museum when it was no longer needed in the Space Shuttle program.