Wherever you are, the Sun stands overhead at one place: its ground position (GP). Right under it the Sun is 90° high. Walk away from the GP in any direction and the Sun sinks toward your horizon.

So all the places where the Sun stands at, say, 40° form a ring around the GP: a circle of equal altitude. When your sextant reads 40°, you are somewhere on that ring, but the sextant cannot say where.

Predict You measure a higher altitude of the Sun. What happens to your circle of position?

The shaded cap is where the Sun stands higher than Ho; the circle is where it stands exactly Ho. Drag "You" around it.

Ho (the altitude you measured): 40°00.0'
Radius: 90° − Ho: 50°00.0' = 3,000 nm
Where you are, the Sun stands: 40°00.0'
Your distance from the GP: 3,000 nm

You are on the circle: anywhere on it, the sextant reads the same. One altitude cannot tell you where on the circle you are.

Raise Ho and the circle shrinks toward the GP; lower it and the circle grows.

One altitude, a whole circle