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When is Mars easiest to observe from Earth?
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Mars
An especially serene view of Mars (Tharsis side), a composite of images taken by the Mars Global Surveyor spacecraft in April 1999. The northern polar cap and encircling dark dune field of Vastitas Borealis are visible at the top of the globe. White water-ice clouds surround the most prominent volcanic peaks, including Olympus Mons near the western limb, Alba Patera to its northeast, and the line of Tharsis volcanoes to the southeast. East of the Tharsis rise can be seen the enormous near-equatorial gash that marks the canyon system Valles Marineris.
Mars is easiest to observe from Earth when it reaches opposition, a configuration where the planet and the Sun are in opposite directions in the sky. During this time, Mars is high in the sky and displays a fully illuminated face.1
Successive oppositions occur approximately every 26 months. Because Mars follows an elliptical orbit, the most favorable conditions for viewing occur during "close oppositions"—which happen roughly every 15 years—when the planet is closest to the Sun and, consequently, closest to Earth. At these times, Mars appears at its largest and brightest.1