 What was Sanderson studying?
He was a major in the Interplanetary Service for some years and hung onto the title after he gave up his commission. The Twilight Lab, near the northern pole of Mercury, was the obvious jumping-off place. Sanderson built the Lab up near the pole, where the zone is about five miles wide, so the temperature only varies 50 to 60 degrees with the libration. 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. The atmospheric tide moved toward the Darkside, where it condensed, carrying enough volcanic ash with it for Sanderson to estimate the depth and nature of the surface upheavals on Brightside from his samplings.
 The Sun
 The Brightside  The Darkside  The twilight zone  The Sun