Satellites can follow a polar orbit, passing through both poles of a planet or star. This is useful for mapping and reconnaissance, but not ideal for weather satellites. Sun synchronous orbit allows observation at constant solar time and is often combined with a polar orbit.
A satellite follows a polar orbit when it travels around a structure, such as a planet or star, on a path that passes through both poles of the structure. A satellite is a body in space, artificial 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 artificial satellites, such as mapping satellites and reconnaissance satellites, follow this path.
Bodies in a polar orbit around the Earth are at about a ninety-degree angle from the equator. A latitude is a locate point referring to its distance north or south of the equator, and a longitude is a locate point referring to its distance from the prime meridian, or midline dividing the Earth into the eastern and western hemispheres. Imagine that 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 traverse any Y coordinate, but remains fixed at one X coordinate.
One might imagine a polar orbit of a satellite following an exact longitudinal line from pole to pole, crossing each longitudinal point on the Y axis and remaining on an X coordinate, or at a fixed distance from the prime meridian. Since the Earth is constantly rotating, however, the line drawn by a polar-orbiting satellite can travel directly from pole to pole in space, but it does not follow a straight longitudinal line on the Earth. Imagine drawing a line from pole to pole on a motionless globe. Now imagine going around the globe and trying to draw a straight line from pole to pole. The line would have come out diagonally, crossing many longitudes.
Over the course of one day, a polar orbit around the Earth will traverse each longitude on its travels from pole to pole. This makes a polar orbit an attractive choice for artificial 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 aren’t ideal for weather satellites trying to continuously observe a particular region.
Sometimes a satellite’s orbit is structured so that the satellite moves across the Earth at the same rate as the sun. This is called sun synchronous orbit. When a satellite in sun-synchronous orbit passes over a given point on Earth, it will be the same local time, making it possible to observe the entire globe at constant solar time. This is often combined with a polar orbit, especially in satellites designed to measure the temperature in the atmosphere.
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