Backfield in Motion

   This animated graphic shows the morning sky at 5 am CDT facing toward the east. It is set to 1-day per image starting with yesterday the 14th and ending on the 20th. If you watch the animation for a while you will see the two planets are lower above the horizon as each day passes. Additionally you should notice the stars are higher as each day passes. So what is going on?
   The animated graphic is simulating the orbital motions of the two planets as they are both moving toward the east. The animated graphic is also showing the effect the Earth’s motion on sky, and even the Sun although it is still below the horizon. orbital-positions
   As inner planets, Mercury and Venus orbit the Sun faster than the Earth. Each day Mercury moves approximately 4o, Venus moves approximately 1.6o each day, and the Earth moves approximately 1o daily. Which in turn means that the sun has an apparent eastward motion the same as the Earth’s 1o daily. The net result is that when either of the inner planets moves eastward they are traveling eastward faster than the Sun and will eventually catch up with and pass by the Sun as they move through superior conjunction to eastern elongation. Mercury moving much more quickly than Venus.
   The motion of the sky, like the Sun, is an apparent motion caused by both Earth rotation and revolution. Obviously as the Earth rotates toward the east celestial objects appear to rise in the east and move toward setting in the west. As time passes during the day and night celestial objects will have traveled 360o. star-aniRevolution also causes an apparent westward motion of the sky but each day, since the Earth only moves about 1o, the sky likewise only appears to move about 1o each day. This translates into celestial objects rising approximately 4 minutes earlier each day, or about 2 hours earlier each month. In terms of daily observing if you watch the same object at the same time each day that object will be slightly further to the west, and higher above the horizon as this animated graphic shows.
Caution: Objects viewed with an optical aid are further than they appear.
   Click here to go to the Qué tal in the Current Skies web site for more observing information for this month.

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