Through the Vortex - Linear star trails !

Usually we come across star trails that are circular spinning around the north star (Polaris). The circular star trail is due to the effect of polar reference.
Polaris lies almost on the northern celestial pole. The southern celestial pole has no such visible star. Sigma Octantis is identified as the south pole star.

If you are standing at one of the poles, looking straight up, the celestial point would be right overhead, at the zenith, and all celestial bodies would seem to be moving around these points. These points differ with your latitude. It is right overhead if you are at the north or south poles and goes lower towards the horizon as you move towards the equator. If you are sufficiently close to the equator, you will see neither of the celestial poles. So when you are talking about completely linear movements, you will find the closest resemblance to this at the equator.

Now, if we look in the direction of the celestial equator (stars lying in almost same plane as our Equator) then startrails would almost resemble straight lines. As you can see in this image the central part shows linear trails while the parts above show motion of the stars around the North Star and the lower half shows motion around the South star.

Canon EOS 1300D
Canon kit lens 18-55mm

EXIF: 15 secs x 337 images
ISO 400, f/5.6, 18mm

Star stax used for combining all images
PS 2023 for processing