# Translations:Migration in the case of constant velocity/5/en

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We recall that an ellipse can be drawn with two pins, a loop of string, and a pencil. The pins are placed at the foci, and the ends of the string are attached to the pins. The pencil is placed on the paper inside the string, so the string is taut. The string will form a triangle. If the pencil is moved around so that the string stays taut, the sum of the distances from the pencil to the pins will remain constant, and the curve traced out by the pencil is an ellipse. Thus, if *vt* is the length of the string, then any point on the ellipse could be the depth point *D* that produced the reflection for that source *S*, that receiver *R*, and that traveltime *t*. We therefore take that event and move it out to each point on the ellipse.