Mercury Begins to Retrograde

mercury-retrograde-ani   The innermost planet Mercury is about 2 weeks after its Greatest Eastern Elongation on August 16th. As the name suggests Mercury has been on the east (left) side of the Sun and visible in the evening skies after sunset. From a combination of the Earth’s and Mercury’s orbital speed and angle relative to each other Mercury is now seen as moving westward toward Inferior Conjunction on September 12th. Mercury will continue moving westward until it reaches its Greatest Western Elongation on September 28th.
   Retrograde or westward motion is probably best known as it relates to the apparent motion an outer planet relative to the Earth has. mars-retrograde-aniThis apparent westward motion comes about as the Earth, moving faster than an outer planet, catches up with and passes the slower moving outer planet. As this happens the outer planet appears to slow down, pause, then move westward for a period of time lasting from days to weeks. At some point the angle between the Earth and the outer planet has shifted enough so that it appears as if the westward motion has ended and the outer planet resumes its direct or eastward motion around the Sun.
   For an outer planet this is an apparent motion relative to the stars in the background, while for the two inner planets they really do move westward as they orbit the Sun between eastern and western elongation, and along the way passing between the Earth and the Sun at inferior conjunction.

   
   
   

Click here to go to the Qué tal in the Current Skies web site for monthly observing information, or here to go to bobs-spaces.

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