Science feed
38 μs
GPS must account for relativity because satellite clocks and ground clocks do not naturally keep the same rate. For a nominal GPS orbit, the combined relativistic rate difference is about 38 microseconds per day, large enough to undermine accurate navigation if left untreated.
0.01°C
Water can coexist as ice, liquid and vapor near its triple point, about 0.01 °C and 611 pascals. In a vacuum demonstration, evaporative cooling can make liquid water freeze while it is still boiling.
Low contrast
The Great Wall is generally not discernible with the unaided eye from low Earth orbit, and it is not visible from the Moon. Saying it can never be seen from any altitude called space is too absolute: viewing conditions, equipment and identification all matter.