The Introduction section of NASA Technical Memo TM-X-64628 Angular momentum desaturation for Skylab using gravity gradient torques begins:

The Skylab Apollo Telescope Mount (ATM) experiments require that the solar instruments remain inertially fixed (sun-oriented) during the day portion of the orbit. Gravity gradient, aerodynamic, venting, and other external torques acting on the vehicle during this time must be absorbed by an angular momentum storage device; in this instance, a system of three double-gimbaled control moment gyros (CMG’s) (references). Portions of the disturbance torques are noncyclic and tend to saturate the CMG system, which has a limited momentum storage capacity. A method for momentum desaturation that does not require mass expulsion is desired. The gravity gradient torques acting on Skylab are developed for small deviations from the sun-oriented reference coordinate system. These equations are used to show that maneuvers about the two axes of large moments of inertia are sufficient to desaturate the accumulated momenta about all axes. All attitude maneuvers for desaturation are made during the night portion of the orbit (unless an insufficient night portion is available, where part of the daylight portion is used), and the percentage of the orbit utilized for desaturation is selectable. [...]

This means that for at least some extended periods of time Skylab's attitude was predominantly Sun-oriented for the Skylab Apollo Telescope. See also How did Skylab's electrographic camera work? and How would the Apollo telescope have worked in the Apollo command module? Where would it be located and how would it be operated?

This is different than for the ISS, which maintains a fairly strict Earth-oriented attitude; even the signs inside the ISS are oriented with respect to "up" and "down" When reading “the writing on the wall” in the ISS, which way is up?.

Did Skylab ever assume an Earth-oriented attitude for a period of time? Perhaps for some Earth observation experiments?

If so, or if not, why, or why not?


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