ADVICE

www.flickr.com/photos/fdctsevilla/4030994509/

Reader’s question

“How could I draw an ellipse on gypsum board?”

Our answer

The pin and string method we describe below is useful for constructing large ellipses.

You need to know or determine the length of the major and minor axes of the ellipse.

1. Using a pencil draw two perpendicular lines on the gypsum board; these will be the major and minor axes of the ellipse.

2. Find the location of the two foci of the ellipse, which lie on the major (longest) axis and are equidistant from the center using the formula below

c² = a²-b²  a> b

where

c is the distance from each focus to the center

a is the semi-major axis (major radius)

b is the semi-minor axis (minor radius)

The focal distance is 2c.

3. Nail a pin in the each focus E1 and E2 and tie a string that leaves slack with length (2c+b).

4. Pull the string taut with a pencil point away from the pins.

5. Drag the pencil to draw the ellipse on the gypsum board.