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.

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