NavList:

Frank Reed wrote: ...by the time of the Apollo flights, the sextant had largely been superseded for navigation. The sextant was used on one mission for "real" navigation to the Moon: by Jim Lovell on Apollo 8 (though even then, the ground's vectors were preferred in nearly every case), and it was tested out occasionally as a backup on each succeeding lunar flight. But by then it was redundant and un-necessary for actual navigation. The primary function of the sextant on the landing missions was as an astro-compass for orienting the spacecraft.

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THis use became critically important during the Apollo 13 crisis. NASA's ground radar could tell them their position and velocity, but they needed to know the space craft's orientation so that the rocket would point in the correct direction during burns. The inertial guidance platform provided that information, but needed to be periodically updated by a star sighting. The Command Module had both a sextant and a scanning telescope for that purpose, but after the explosion, its electrical systems were shut down. The Lunar Module had only a simple fixed telescope. Here is what happened, in Jim Lovell's words,

'We had many crises on Apollo 13, but the biggest heart- stopper has hardly been noticed, partly because the transcription released to the press was garbled, and partly because there wasn't much point in talking about a crisis that had been averted earlier. It occurred prior to the second maneuver I mentioned earlier; we called it P.C. + 2 (pericynthian 2 hours).

We had transferred the [Command Module] platform alignment to the [Lunar Module], but we had to make sure that this alignment was accurate before we made the long P.C. + 2 burn. Ordinarily it is simple to look through the sextant device, called the Alignment Optical Telescope, find a suitable navigation star, and with the help of our computer verify the guidance platform's alignment. But traveling with us was a swarm of debris from the ruptured service module. The sunlight glinting on these bits of junk- I called them false stars- made it impossible to sight a real star.

So what to do? If we couldn't verify the accuracy of the alignment, we didn't have a way to make an accurate burn, or to align the CM platform for reentry. In other words, the ground would have no accurate way to tell us the correct attitude to make the proper maneuvers to return home.

A genius in Mission Control came up with the idea of using the Sun to check the accuracy of our alignment. No amount of debris could blot out that star! Its large diameter could result in considerable error, but nobody had a better plan.

I rotated the spacecraft to the attitude Houston had requested. If our alignment was accurate, the Sun would be centered in the sextant.

When I looked through the AOT, the Sun just had to be there. It really had to be. And it was. At 73:46 hours the air-to-ground transcript sounds like a song from "My Fair Lady":

Lovell: O.K. We got it. I think we got it. What diameter was it?

Haise: Yes. It's coming back in. Just a second.

Lovell: Yes, yaw's coming back in. Just about it.

Haise: Yaw is in....

Lovell: What have you got?

Haise: Upper right corner of the Sun....

Lovell: We've got it!

If we raised our voices, I submit it was justified.

I'm told the cheer of the year went up in Mission Control. Flight Director Gerald Griffin, a man not easily shaken, recalls: "Some years later I went back to the log and looked up that mission. My writing was almost illegible I was so damned nervous. And I remember the exhilaration running through me: My God, that's kinds the last hurdle -- if we can do that, I know we can make it. It was funny, because only the people involved knew how important it was to have that platform properly aligned." Yet Gerry Griffin barely mentioned the alignment in his change-of- shift briefing -- "That check turned out real well" is all he said an hour after his penmanship failed him Neither did we, as crew members, refer to it as a crisis in our press conference nor in later articles.

The alignment with the Sun proved to be less than a half a degree off. Hallelujah Now we knew we could do the 5-minute P.C. + 2 burn with assurance, and that would cut the total time of our voyage to about 142 hours. We weren't exactly home free: we had a dead service module, a command module with no power, and a lunar module that was a wonderful vehicle to travel home in, but unfortunately didn't have a heat shield required to enter the Earth's atmosphere. But all we needed now was a continuation of the expertise we seemed blessed with, plus a little luck.'

Don Seltzer

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