During their training, one of the tasks the Project Mercury astronauts needed to accomplish was knowledge of the constellations. Knowing the position of stars above Earth was vital in case the navigation system on their spacecraft malfunctioned. In the event of such an occurrence, an astronaut would resort to the use of the “Emergency Astro-Navigation Device” made by the Grumman Corporation.
In using the emergency device to determine his position above Earth, the astronaut needed to locate a star at its zenith through the spacecraft’s window, mark the star on the device, then position one of the red arrows over the current time as indicated by the vertical lines on the bottom portion of the slide rule. The marking made on the device then indicated the spacecraft’s position in case cloud cover or darkness obscured a visual siting. By knowing his position above Earth, the astronaut could then continue with his calculations for a safe reentry and recovery.
John Glenn carried such an emergency navigation device with him on his Friendship 7 space flight, but did not need to make use of it.
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