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One reason for routinely launching GPS satellites is simply "constellation replenishment," said Col. Andrew Menschner, deputy commander of the Space Force's Space Systems Command. Old satellites degrade and die, and new ones need to go up and replace them. At least 24 GPS satellites are needed for global coverage, and having additional satellites in the fleet can improve navigation precision. Today, there are 31 GPS satellites in operational service, flying more than 12,000 miles (20,000 kilometers) above the Earth.
What are Moon phases?According to NASA, the Moon takes about 29.5 days to orbit the Earth. Over the course of this period, it moves through eight recognisable phases, what we call the lunar cycle. While the same side of the Moon always faces us, the amount of its surface lit by the Sun changes as it continues along its path. The shifts in sunlight create the different appearances we see from Earth, ranging from a fully illuminated Moon to a thin sliver or near darkness. The eight phases are: