Aligning to the stars

While the commander and LMP busied themselves with equipment checks, the third crewman prepared for his guidance role. When the stack was inserted into orbit, it did so with the apertures for the optical systems facing towards space. The command module pilot (CMP) could use the optics to take sightings on stars and thereby properly align their guidance platform, essentially refining the system’s sense of direction. Not only did this prepare their guidance system for the all-important engine burn to take them to the Moon, it also allowed the CMP to satisfy himself that the system was working well and that it could be trusted to enable the flight to proceed to the next stage.

The first task in the CMP’s alignment procedure was to jettison the covers that protected the exterior surfaces of the sextant and the telescope; two optical instruments on the spacecraft’s hull that were articulated and could be aimed to view

any object within the range of their movement. As he peered through the telescope to report what he saw. he pushed a control lever fully to the right to actuate the ejection mechanism. On Apollo 8, Jim Lovell w’as surprised at what appeared. ‘‘The optics cover jettison worked as advertised; however. when they are first ejected, there is so much debris ejected with them (little sparkles and floating objects in front of the optics) it is hard to tell exactly what occurred." A problem faced by the CMP was that the spacecraft was usually bathed in full sunlight yet he was expected to sight on relatively faint stars through complex optical systems. The tiny points of light he was trying to see were many magnitudes dimmer than the nearby Sun. When additional floating particles were illuminated by the Sun, it only made the task trickier. "It is very difficult at first to see stars through the optics because of the jettisoning of the covers and the putting out of quite a bit of dust with them. As a matter of fact, during the entire mission some of this dust would come out every time w;e rotated the shaft [of the instrument]."

Unsurprisingly, it became standard practice to align the guidance system while the spacecraft passed over the night hemisphere of Earth. Then the particles, although present, would not be lit up by the Sun, giving the CMP a better chance of finding his way around the constellations. Richard Gordon w’as particularly busy when Apollo 12 entered Earth’s shadow for the first time. The guidance platform on board Yankee Clipper had been completely knocked out of alignment by a lightning strike on the spacecraft soon after lift-off. This meant that Gordon not only had to carry out a fine alignment as per routine, but he first had to align the platform from the beginning and this coarse alignment proved troublesome. “When I looked in the telescope I couldn’t see anything," he later explained. "There was no light or anything coming from there. I thought it must be because I’m not dark-adapted and probably this was correct."

It generally takes half an hour for a person’s vision to adapt to dark surroundings. As stars are so dim, and as there was a substantial loss of light through the complex optical system, the CMP needed some help to know’ which star he was pointing at. In the normal course of things, the guidance system itself could point the optics at where it believed the star to be and even if this was a bit off, it was usually enough for the CMP to finesse the aim. On this occasion, the guidance system was completely misaligned and it could give no assistance.

"Fortunately Л1 [Bean] was helping me with this. He was looking out his window’ and could see Orion coming up on his side. So, I just waited until it came into the field of view of the optics." Orion, one of the best-known constellations, w’as of assistance to Gordon because it not only contains Lw’o bright stars, Betelgeuse and Rigel. but is also near to Sirius, the brightest star in the sky excluding our Sun. Usefully, Orion’s belt points roughly towards Sirius. "I saw’ the belt of Orion dimly in the very edge, and then 1 could pick up Rigel and Sirius. Once I had picked up Rigel, I could find Sirius. They were the only stars I could see in the entire field of view." Once he had aimed the optics at known stars, he used Program 51 in the computer to roughly align the guidance platform. Now’ the job became easier because the computer could aim the optics to approximately the right direction for a particular star and Gordon could then use Program 52 for the all-important fine alignment.

“The pressure was on and fortu­nately those two stars were the only ones I ever did recognize,” said Gordon. “They were Rigel and Sir­ius. They were just barely in the field of view. I grabbed those two quickly and got a P51 and did a quick P52. I think that one of the stars [that the P52] came up with was Acamar. I wouldn’t have been able to find that in any circumstances.”

Having aligned the platform dur­ing their first night-time pass over Australia, Gordon repeated the P52 exercise on the second revolution to check that the platform alignment was not drifting excessively. “The second P52 over Carnarvon [West Australia], just before TLI, indicated that we had a good platform. Drift angles were very low. Everybody breathed a sigh of relief that we had our platform back again.”