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A satellite follows a polar orbit when it travels around a structure, such as a planet or star, on a path that crosses above both poles of the structure. A satellite is a body in space, either man-made or natural, that orbits another body. For example, the moon is a natural satellite that orbits the Earth. The moon, however, is not an example of a polar orbit because it does not pass over both the north and south poles of the Earth. Several man-made satellites, such as mapping satellites and reconnaissance satellites, do follow such a path.
Bodies in a polar orbit around the Earth stay roughly at roughly a ninety degree angle from the equator. A latitude is a location point in reference to its distance north or south of the equator and a longitude is a location point in reference to its distance from the prime meridian, or the midline that divides Earth into Eastern and Western hemispheres. Imagine the globe was perfectly centered on a grid so that the vertical Y axis runs from the North Pole to the South Pole on the prime meridian and the horizontal X axis runs along the equator. A longitudinal line, a line parallel to the Y axis or prime meridian, can cross every Y coordinate, but stays fixed on one X coordinate.
One would imagine that a satellite polar orbit would follow an exact longitudinal line from pole to pole, crossing every longitudinal point on the Y axis and staying on one X coordinate, or at a fixed distance from the prime meridian. Because the Earth is constantly spinning, however, the line traced by a satellite in polar orbit may move straight from pole to pole in space, but does not follow a straight longitudinal line on Earth. Imagine drawing a line from pole to pole on a still toy globe. Now imagine spinning the globe and trying to draw a straight line from pole to pole. The line would come out diagonal, crossing many longitudes.
Over the course of a day, a polar orbit around Earth will cross every longitude in its travels from pole to pole. This makes a polar orbit an attractive choice for man-made satellites that need to observe every point on Earth. Mapping satellites used to create images of the entire globe, are commonly launched into a polar orbit, as are spy satellites, also called reconnaissance satellites. Some weather satellites are also launched on this path, but polar orbits are not ideal for weather satellites that seek to observe a particular region continuously.
Sometimes the orbit of a satellite is structured in such a way that the satellite moves across the Earth at the same pace as the sun. This is called a sun-synchronous orbit. As a satellite in sun-synchronous orbit passes over any given point on Earth, it will be the same local time, making it possible to observe the entire globe at a constant solar time of day. This is often combined with a polar orbit, especially in satellites designed to measure the temperature in the atmosphere.
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