Like many names in Harrison's work, Halvmörk is taken from a European language, here Swedish. It literally means "half-dark". This may be related to the fact that due to extreme temperature differences human life is possible only in a small twilight zone round the wintry pole.
Halvmörk's orbiting period (year) is a little over eight Earth years. Its orbit is extremely elliptic, and the inclination of its axis is 49°, so seasonal changes have an enormous effect on the weather. The rotational axis is inclined in the direction of the small axis of the orbit, so the periastron and the apastron coincide with the equinoxes.
As a result the twilight zone remains largely constant for most of the year, when the planet is far from its sun. During this time a colony of humans grows grain in the twilight zone. When the planet approaches its periastron or its apastron the colony migrates to the other pole, using enormous insulated trucks and the planet's sole road, built especially to be used once in four years.
The novel does not seem to take into account that due to the ellipticity of the orbit the planet would be in a near-equinoxe position during most of the orbiting period, which would mean that both poles would enjoy a mild twilight. If the orbit were less elliptic there would be no arctic twilight zone at all but polar nights and days of varying length, like on Earth. So it is hard to imagine the astronomic circumstances needed for Harrison's interesting plot.