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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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Only 39 orbits were Earth-oriented. The overwhelming majority of the station's operating life was Sun-oriented.

The final Earth resources experiment package pass for Skylab 3 ended, In all, 39 Earth-oriented passes, six solar inertial passes, two Earth-limb surveys, and two lunar calibration sequences were completed.

Skylab: A Chronology, 1973 September 21

Starting with 13 hours after launch, the station was oriented "nose up" slightly off from a solar orientation to help control temperature (ibid, 1973 May 14). When the first crew arrived 10 days later, a solar shade was installed and proper sun-orientation restored.

These are the only mentions of other orientations in the Skylab literature. There were no other experiments that re-oriented the station from its normal solar orientation. The station normally maintained a solar orientation because

  1. power came from solar panels, which were fixed in place and needed to be pointed toward the Sun.

  2. a major component and purpose of the station was a solar observatory called the Apollo Telescope Mount, which needed to be pointed toward the Sun.

After the last crewed mission, the station was placed in an unspecified "stable attitude."

According to NASA, Skylab was crewed for 404 + 858 + 1214 = 2476 orbits, meaning that 1.6% of those were Earth-oriented. Wikipedia claims Skylab achieved 34,981 total orbits; 0.11% percent of those would be Earth-oriented.

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  • $\begingroup$ What fraction of the station's lifetime was 39 orbits? $\endgroup$ – Organic Marble Jun 17 at 17:52
  • $\begingroup$ @OrganicMarble: Depends on what you want to count as the denominator. Since launch? Since first occupation? Including the time between crewed flights? To the last crewed flight? To the station decomissioning? To loss of control? To re-entry? Considering all the possible denominators, I think the way I answered is more precise than a percentage that will likely be mis-interpreted. $\endgroup$ – DrSheldon Jun 17 at 18:14
  • $\begingroup$ I think any of those would be good as long as you said what it was. I'd just be curious to know if Earth-pointing was closer to 1% than 10% or 50%. $\endgroup$ – Organic Marble Jun 17 at 18:19
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    $\begingroup$ @OrganicMarble: Okay, I've done the calculation with counts from NASA and Wikipedia. You decide which number to trust. ;^D $\endgroup$ – DrSheldon Jun 17 at 18:29

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