On Earth, which latitudinal circles are half as long as the Equator?
The circumference of a circle is proportional to its radius, $\mathrm{s}$, so if the radius of the equator is $R$, the radius of the sought parallel circle is $R / 2$.

![](https://cdn.mathpix.com/cropped/2024_05_02_a1f343f0face7fcbcfefg-1.jpg?height=388&width=469&top_left_y=229&top_left_x=817)

A plane passing through the axis of rotation of the sphere intersects the sphere in a great circle, and the diameter of the sought parallel circle is the chord $A B$ of this great circle. The triangle $O A B$ is equilateral, so the $60^{\circ}$ parallel is half as long as the equator.

There are two such parallels: one in the northern hemisphere, passing through Sweden (Uppsala), while the $60^{\circ}$ parallel in the southern hemisphere is far from any land.