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- We know how to get a diameter, drawing a chord.
- We know some properties of the similar rectangle triangles.
- We can use the programm SALSAJ in order to take measurements on a picture given ¡

- We are asked to measure the Moon's diameter.

- Open SalsaJ and then open the picture below:

- Draw any line (a chord of the circumference) on the picture.
- This line intersects the edge of the circumference on two points.
- Measure (pixels units) the length of this line between these endpoints.
- Divide it up into equal parts.
- Now you can have the midpoint, M, find it helped by the programm SalsaJ.
- Draw a perpendicular to the line through the midpoint.
- Measure (on that line) the distance from M to the edge of the circunference of the Moon.
- Using your knoledges related with circumference's properties, find out the Moon's diameter.