I once read up about astronauts measuring their position on the moon, or perhaps it was on the way to the moon and I would like to know more.

What I remember reading was they used an instrument that by using some star as some fixed point then twisting it so that a marked line eventually crosses this star. Based on the angle needed to be twisted the position was then read off. Crucially, the marked line was some form of spiral.

What instrument was this, how was it used and what are the advantages of using a spiral instead of simply a straight line. I assume the use of a spiral is for increased accuracy.


1 Answer 1


You're remembering the Apollo Lunar Module Alignment Optical Telescope which had that spiral reticle.

enter image description here

The alignment optical telescope is used for in-flight and lunar surface sightings.

For in-flight sightings, the telescope may be placed in any of the usable detent positions. However, when the LM is attached to the CSM, only the forward position is used. The astronaut selects a detent and the particular star he wishes to use. He then maneuvers the LM so that the selected star falls within the telescope field of view. The specific detent position and a code associated with the selected star are entered into the guidance computer by the astronaut using the DSKY. The LM is then maneuvered so that the star image crosses the reticle crosshairs. When the star image is coincident with the Y-line, the astronaut presses the mark Y pushbutton; when it is coincident with the X-line, he presses the mark X pushbutton. The astronaut may do this in either order and, if desired, he may erase the latest mark by pressing the reject pushbutton. When a mark pushbutton is pressed, a discrete is sent to the guidance computer. The guidance computer then records the time of mark and the inertial measurement unit gimbal angles at the instant of the mark.

Crossing of a reticle line by the star image defines a plane containing the star. Crossing of the other reticle line defines another plane containing the same star. The intersection of these planes forms a line that defines the direction of the star. To define the inertial orientation of the stable member, sightings on at least two stars are required. Each star sighting requires the same procedure. Multiple reticle crossings and their corresponding marks can be made on either or both stars to improve the accuracy of the sightings. Upon completion of the second star sightings, the guidance computer calculates the orientation of the stable member with respect to a predefined reference coordinate system.

On the lunar surface, the LM cannot be maneuvered to obtain a star-image that crosses the reticle crosshairs. The astronaut using the reticle control knob, adjusts the reticle to superimpose the orientation (Y) line on the target star. The reticle angle display on the reticle counter, is then inserted into the computer by the astronaut. This provides the computer with the star orientation angle (shaft angle). The astronaut then continues rotating the reticle until a point on the spirals is superimposed on the target star. This second angular readout (reticle angle) is then entered into the computer along with the detent position and the code of the observed star. The computer can now calculate the angular displacement of the star from the center of the field of view by computing the difference between the two counter readings. Due to the characteristics of the reticle spirals, the angle is proportional to the distance of the star from the center of the field-of-view. Using this angle and a proportionality equation, the computer can calculate the trunnion angle. At least two star sightings are required for determination of the inertial orientation of the stable member.

Note however, this procedure does not (and can't) measure their position, it measures their orientation.


  • CSM Command and Service Module
  • DSKY Display and Keyboard
  • LM Lunar Module
  • $\begingroup$ So while this answer says the sextant "was used for the lunar landing missions to get more accurate coordinates of the landing site, before and after the actual landing." I'm wondering if there is some mixing of the sextant and the alignment telescope in that answer? They are separate devices, right? Or am I (more) confused (than usual)? $\endgroup$
    – uhoh
    Feb 25, 2021 at 0:50
  • 2
    $\begingroup$ @uhoh without looking, ISTR the sextant was in the command module. The device asked about here was in the lunar module. $\endgroup$ Feb 25, 2021 at 1:00

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