Harstad is one of the better bases in Norway for northern lights (aurora borealis) viewing, with the season running from around late August through April. The key requirements — darkness and clear skies — are both achievable here. The nights are dark enough from late August, and the weather, while changeable, is not as relentlessly overcast as further south.
The lights are best away from town where light pollution is minimal. The Trondenes peninsula, or any of the minor roads heading into the hills around the city, will do. Local operators offer guided northern lights tours by minibus or snowmobile that chase clear skies within a reasonable radius. The intensity of the display varies — some nights it is a faint shimmer; other nights the whole sky moves in curtains of green and violet that people who have seen them describe as the most extraordinary natural spectacle of their lives.