From the NASA TIROS page on TIROS-1:
The craft was spin-stabilized and space-oriented (not Earth-oriented). Therefore, the cameras were only operated while they were pointing at the Earth when that portion of the Earth was in sunlight.
What does it mean that the craft was space-oriented (not Earth-oriented)?
The US Navy describes Earth orientation, but this appears to describe an axis definition and the orientation of planet Earth, whereas the NASA TIROS page describes something related to the actual spacecraft orientation.