Whether you are hiking in the woods or walking on the Moon, you need to establish a course in order to get to your destination. Prior to modern era GPS systems, explorers used a map and a compass. When it came time to explore the Moon for the first time, NASA relied on what had guided earthbound explorers for centuries, the map and the compass.
Fig 1) David Scott took this 360 degree panorama of the Hadley landing site from a top the LM.
Fig 2) The “Overlay Map” used to identify landmarks during the SEVA
Fig 3) A training version of the Apollo 15 Sun Compass
After landing on the lunar surface at Hadley, the Apollo 15 Commander, David Scott, preformed the first ,and only, lunar “Stand Up EVA” (SEVA) during which CDR Scott stood up in the docking hatch of the LM Ascent stage and performed a visual survey of the surrounding area from a 18 foot high view point atop the spacecraft.
So what do you need to determine where you are anywhere in the Solar System? Well, you get out a map and a compass. Dave Scott did just that during the SEVA. As a portion of the NASA mission transcript of the SEVA reveals in the following conversation between Dave and Jim Irwin, his Lunar Module Pilot (LMP),
106:50:48 Irwin: Want the map?
106:50:50 Scott: Just the Sun compass first. Let's get a pick on our position.
You can ask Dave Scott1 about the Sun Compass or, better yet, read what Dave had to say during an interview and email exchange with Eric Jones2 of the Apollo Lunar Surface Journal (ALSJ) in the following journal excerpt;
[Jones - "Did the Sun compass have a little gnomon/shadow device so that you could get it properly aligned?"]
[Scott - "It was a card on which a circle was drawn with the highlights of the topographic features, the mountains around and the angles and a little flap that you would raise and align with the Sun so that the shadow fell within the marks on the map. And that would orient the card to where it should be and then you could look out at the mountains to see if they lined up. It was a backup navigation system for the Rover. If your Nav system didn't work and you couldn't see the LM, you could align this Sun compass with the mountains and you could point your way home. Great little device. Cheap. A piece of paper. Clever. I don't know who came up with it and, in fact, I often wonder did the other guys carry the same thing on the subsequent flights. (They did not.) It was a great get-me-home device."]
[Dave Scott commented in a December 2004 e-mail, "Not only was the sun compass a backup to an unproven -for A-15 - single-string nav system on the Rover (i.e. if the nav system failed, or in our case, since the first use, if it were not accurate), the sun compass could have been essential in the event of a time-critical return to the LM. Perhaps the following rough example will illustrate our thinking before the mission - and why a very clever chap came up with the sun compass. A simple piece of paper - the stiff back of a checklist - reduced the overall risk of the surface expedition. Why not? The only additional weight was the bubble level and the ink!"]
Fig 4) The photographs shown above portray the front and back of the Sun Compass
The Sun Compass was a stroke of genius that utilized an ancient navigation technique that was revived during the 20th Century race to the North Pole and later used to guide men on the Moon. While a Sundial uses the Sun’s shadow to determine the time, a Sun Compass uses time and the angle of the Sun’s shadow to determine direction. Through the use of time and Sun shadow angles, Dave Scott was able to determine the bearing (direction back to the LM) of various landmarks adjacent to the landing site.
The front of the Sun Compass, as shown in Figure 4, shows the topographical features that Dave Scott referred to, in the interview, illustrated on the compass card which provided the initial bearings for each of the major landmarks on the exploration site at Hadley. On the reverse side of the compass is a chart that presents key times of each EVA listed in Central Daylight Time (CDT) and the Sun shadow angles (Shadow <) for those times.
As the instructions on the Sun Compass say, "1.) Line the Pointer on the Target," "2.) Level the Compass (with the Bubble Level attached to the card)," "3.) Set the Shadow Angle (based on the chart on the reverse of the card)" and "4.) Read the Bearing." The instructions add that three bearings should be taken in order to triangulate the LM’s position on the Moon.
106:51:25 Scott: Okay, hand me the big overlay map, Jim.
Fig 5) Overlay Map with Course Heading and Distance Grid overlaying the entire landing area
The Overlay Map was designed by the U.S. Army Cartography Division based on Lunar Orbiter photographs of the Moon’s surface.
While the Sun Compass gave the bearing (reciprocal heading) back to the LM, the Overlay Map gave the heading or course to a landmark or lunar rover destination. The map covered the entire landing site. Included on the map were physical features and landmarks at the Hadley site. The term “overlay” comes from the circular grid that is superimposed over the map. The grid provided course heading and distance data for the explorers while on the lunar surface.
106:51:49 Scott: Okay. Good, Joe… Okay, Joe, our bearing to Icarus is 338.
During the SEVA, Dave Scott used the Sun Compass to take bearings on various landmarks. The reason behind making that task necessary was due to the trajectory changes caused by a mistake by Mission Control in Houston. It was essentially a double correction on a single navigation error that caused the ground controller to over-correct the trajectory by 1000 meters (3000 feet) to the North of the touchdown site. During pitch over, as Dave Scott would later describe, “The problem was, when we pitched over and began to look out the window , there was nothing there!” Dave was forced to redesignate his landing point via the Rotational Hand Controller (RHC) and the Landing Point Designator (LPD) eighteen times to bring the lunar module close to the correct point on the lunar surface.3 The actual landing point was just north of Last Crater within 300 meters (900 feet) north of Apollo 15’s planned landing target.
Fig 6) Two views of the Sun Compass showing the Sun Shadow Device on the dial
106:52:53 Scott: Okay… Another quick one. Bennett Peak is 255.
106:53:04 Allen: Roger.
The bearings that Dave Scott was taking during the SEVA would help Mission Control triangulate the actual position of the LM at the Hadley landing site. Since Apollo 15 was carrying a lunar rover, it was determined that any landing with 700 meters (2100 feet) of the planned site would not require any change in the planned EVAs.
"[In Houston, the Flight Director tells Joe that Dave should get one or two more bearings. "It won't take long to get 'em."]"
106:53:15 Allen: Rog, Dave; maybe one more bearing.
106:53:20 Scott: Okay, coming up… Make Hadley Delta at about 182.
106:53:27 Allen: Roger.
"[Because all of the features mentioned are fairly large, Dave must be sighting on some predetermined points.]"
Eric Jones’ comments shown above are, in all probability, correct. As can be seen in the photographs in figure 6, the compass card shows Bennett Hill and Hadley Delta. Icarus Crater is next to Chain Crater (also listed on the card at the 330 degree position) in the North Complex. One observation notes that the bearings listed on the compass card for Bennett Hill and Hadley Delta are a few degrees off the bearings taken by Dave Scott which probably relates to difference between the planned landing point and the actual landing point.
Dave Scott mentions the use of the Sun Compass in the Apollo 15 Debrief. "I guess the first thing we used was the Sun compass to try to get a relative bearing on three sites . We used Benefield 305 and Mount Hadley to get bearings... At this time, I wasn't sure where we were located. Although I could see prominent features, I was relying on the Sun compass to give us the data for triangulation to spot our point because there was nothing in the immediate vicinity which was recognizable."4
106:53:29 Scott: Here you go, Jim. (Handing down the compass)
Fig 7) Hand drawn range indicator on reverse of Sun Compass
Another simple device was actually hand drawn on the reverse of this training Sun Compass is a range finder that used the known height of the LM to determine the distance to the LM. By sighting pass the card at the LM and placing the LM within the various marks, the moonwalkers could gauge the distance back to the LM from up to one kilometer away. It is a simple and ingenious solution to the problem of estimating distances on the Moon due to lack of atmosphere and familiar reference points.
121:11:38 Scott: And, I'll put the Sun compass and the overlay map in my seat pan.
"[This is the same Sun Compass that Dave used during the SEVA at about 106:50:50.The TV image jiggles as Dave raises his seat.]"
Also nicknamed the “Walk Back Map” by later lunar explorers, when the Overlay Map was used in combination with the Sun Compass, the crew would be able to chart a course back to the LM in the event that the rover or it’s navigation system failed.
Studies by Bellcomm determined that the lunar rover could travel “…700 meters in about 5 minutes. Walking (700 meters), some ten to fifteen minutes would be required to complete the same distance."5 It could take the crew 20 minutes or more to travel one kilometer. If the lunar rover broke down at the furthest point of an EVA traverse, it might require as much as two and one half hours for the astronauts to walk back to the safety of the lunar module.
By placing the Sun Compass and the Overlay Map in the CDR’s seat on the lunar rover, Dave Scott was acting prudently in having a redundant portable navigation system at hand in the event that he and Jim Irwin were forced to trek back to the LM on foot. It is a case where two simple, old fashion instruments worked hand in hand to provide a route home for explorers on another world.
1.) Thanks to Dave Scott for his assistance in understanding the use of the Sun Compass and the Overlay Map.
2.) Thanks to Eric Jones, Editor of the "Apollo Lunar Surface Journal" (ALSJ). View the site here at https://www.hq.nasa.gov/alsj/
3.) Thanks to David Mindell, Author of "Digital Apollo" (MIT Press)
4.) "Apollo 15 Technical Crew Debriefing" NASA , Aug. 14, 1971, Prepared by Training Office Crew Training and Simulation Division
5.) "The Navigation System of the Lunar Rover Vehicle", Bellcomm Technical Memorandum, Dec, 1970, W.G. Heffron & F. LaPiana