Two More Go Retro!

   Retrograde motion is an apparent motion of any Sun-orbiting object as viewed from one object to another object further from the Sun than the one the viewing is made from. Since we are based on the Earth our views of the planets and other objects orbiting the Sun come from that perspective. As I posted the other day, outer planets. for example, orbit the Sun more slowly than the Earth. So there are times when, as the Earth passes an outer planet, the outer planet appears to slow down and then move backward, to the west. After a period of time the planet resumes its regular eastward, or prograde, motion.
   The retrograde motion of an outer planet is easy to understand and even visualize, however the two inner planets also undergo retrograde motion. Half of each their respective orbit is eastward, prograde, but when they reach the opposite side of the Sun their orbit carries the inner planet around the Sun through inferior conjunction (between the Earth and Sun) toward the west, retrograde, for the other half of the orbit.
   For the record each ‘side’ of the orbit is known as an elongation. So at western elongation the inner planet is on the west side of the Sun and rises before the Sun rises – a morning planet. Half an orbit later the inner planet is at eastern elongation and rises and sets after the Sun – an evening planet.
   Venus is currently very prominent as an evening planet over the western horizon at sunset. The planet Mercury has recently passed through superior conjunction is gradually moving into the evening skies. Mercury is moving in prograde motion while Venus is moving in retrograde motion.


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Mercury at Eastern Elongation

orbital-positions    On Monday February 10th Mercury, the innermost planet, will reach its orbital position known as greatest eastern elongation. At that moment Mercury, the Sun, and the Earth, would be arranged in something close to approximating a right angle as this graphic shows.
   From our perspective the orbits of Mercury and Venus appear to move from one side of the Sun to the other – from superior conjunction, behind the Sun, out to the left (east) from the Sun to eastern elongation, then reverse and move westward through (inferior conjunction) between the Earth and the Sun to western elongation. From there the inner planet moves eastward going behind the Sun (superior conjunction) and eventually reappearing on the eastern side of the Sun for an eastern elongation. Repeat over and over – do not stop!

   The planet Mercury is currently over the western horizon at sunset local time and will remain visible for most of the month.

   
   
   

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Jupiter at Quadrature ——— Venus at Elongation

   Thursday January 12th the gas giant planet Jupiter reaches the point along its orbit around the Sun where Jupiter would be described as being at western quadrature. It is at a 90o angle relative to the Earth and the Sun.
   Also on this day the inner planet Venus reaches a point in its orbit known as eastern elongation. This puts Venus at somewhere around a 90o angle relative to the Earth and the Sun when viewed looking down from above the solar system but Venus is on the other side of the Sun from where Uranus is currently located.
   In terms of elongation Venus is 47.2o to the east of the Sun. In terms of viewing angle you are the point of a triangle with Venus at the left base corner and the Sun at the right base corner. The angle between the Sun and Venus is the elongation angle, and at 47.2o Venus sets around 3 hours after sunset as its seting motion or speed is (15o per hour based on Earth rotation rate.
   Where is Jupiter currently? Jupiter rises around midnight local time and is located high above the southern horizon at sunrise local time near the blue-white star Spica in Virgo the Harvest Maiden.
   And Venus? Can’t miss this planet as it is the brighest stellar object over the western horizon at sunset.
   
   
   

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