 What is the twilight zone?
The Twilight Lab, near the northern pole of Mercury, was the obvious jumping-off place. Twilight Lab wasn’t particularly interested in the Brightside, of course—the Sun was Sanderson’s baby and he’d picked Mercury as the closest chunk of rock to the Sun that could hold his observatory. But with Mercury’s wobble, the twilight zone between Brightside and Darkside offers something closer to survival temperatures. But the only way to avoid it is to travel over the Pole, which would lose us days and offer us no guarantee of less activity to the west. There was an atmosphere and a constant atmospheric flow from Brightside to Darkside.
 The place between Brightside and Darkside.
 The place at the end of the Brightside Crossing.  A lab where they study the Sun.  The place between Brightside and Darkside.  A lab where they study Mercury.