A tracker rotating in the east-west direction to align the solar panel with the sun over a day is known as a single-axis tracker. At any fixed location, the sun travels close to 180 degrees between day and night. A solar panel in a fixed orientation between dawn and sunset could see a significant movement of the sun to either side (about 80° each way), and thus, could miss out on significant amounts of energy if it were not aligned suitably. Rotating the panels to the east and west can optimize such alignment over the day.
Single Axis Solar Trackers – Definition, Glossary, Details – Solar Mangoadmin