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How to draw longitudinal lines on a circle as Earth?

  1. Sep 9, 2015 #1
    I have a circle, representing Earth with equator drawn on it. I need to draw longitudinal lines on 15 degree, 30 degree, and 60 degrees distance? I am not sure how and from where to measure these angular distances. Thanks.
  2. jcsd
  3. Sep 9, 2015 #2


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  4. Sep 9, 2015 #3

    The earth is a ball. The angle is measured at the center of the Earth.

    Imagine a point X on the equator. Imagine a line going directly north from that point. Imagine a point Y somewhere on that north-going line.

    Imagine a line x between point X and the center of the Earth. Imagine a line y between point Y and the center of the Earth. There is an angle between the two lines. That's the measure of longitude.

    To figure out where to draw the line of longitude in your circle on paper, you will need trigonometry.
  5. Sep 9, 2015 #4
    Project a line from the center of the circle straight down. This is 0 degrees.
    Rotate this line left or right 15 degrees, place a vertical line where it intersects the circle. Where this vertical line intersects the equator is where the longitude is.
    Repeat for 30, 60 degrees.
    If you want to draw in the entire longitude you'd have to draw parts of an ellipse.
  6. Sep 9, 2015 #5


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    What scientific601 is demonstrating is that you can start by drawing the sphere looking down on it from above. Then project those points to the face.

    scientific601 has eloquently done it without needing to use an extra circle.
    Last edited: Sep 9, 2015
  7. Sep 10, 2015 #6
    Thank you so much everyone for your help and your time. Your answers are very helpful for me. Thanks again!
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