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?
merc-venus-ani
   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.
   
   
   
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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.

Smooth With No Bumps

globe3   Yesterday I had the pleasure of spending nearly an hour with a 2nd grade class at one of our local elementary schools talking with them about the night sky, how to use a ‘Big Dipper’ star clock, and other topics that only a 2nd grader could come up with. Prior to my visit they had read the story ‘Follow the Drinking Gourd’ so that was the introduction into apparent sky motions and using the Big Dipper clock. I used the Starry Night program to show how the sky appears to move when facing north and for comparison east, south, and west. They already knew that it was the Earth rotating and not the sky so this led to me telling them that where we live (our latitude) we are moving at nearly 800 mph. I then asked how is it that we don’t feel that speed. After a few responses I had them think about their bus ride to school.
   How did they know the bus was moving? Answers included things moving past the window, bumpy road, and engine noise. I then asked them to imagine a bus ride where there were no windows to look out, a quiet engine, and a smooth ride. How would they know the bus was moving? I asked what they felt when the bus started moving or when it slowed down (changing its speed) and was told that your body moves in the opposite direction when the bus speeds up or slows down. My follow-up was to then ask how is that we do not feel the Earth rotating at nearly 800 mph expecting to hear that the speed of the Earth does not change and instead was told that “there are no bumps as the Earth rotates”. We all agreed that that was an ok answer and then finished by having them reason that there was no change in speed.
   So then we proceeded to use the Law of Cosines to determine the rotational speed of the Earth at different latitudes! Well, not really but just in case any one is interested in how to determine rotational speed at different latitudes here is a basic explanation.
calculator   A good starting point is to determine the rotational speed at the equator, 0 degrees latitude. Using rounded up values the Earth has a circumference of 24,902 miles. It rotates in 24 hours so the circumference divided by the time comes out to 1,038 mph at the Earth’s equator. You then multiply the cosine of your latitude times 1,038 mph.
   How to get the cosine of your latitude? If you do not have access to a calculator or a calculator app but are online you can use Google Search, for example by typing “cosine of 40 degrees”, or whatever latitude you want, and pressing Enter. You will get an answer in a calculator display as this graphic shows.
   So hang on! Oh wait, never mind – “no bumps”.
   
   
Click here to go to the Qué tal in the Current Skies web site for more observing information.