MAX EBB — "C
elestial navigation," the instructor began, "can be an important backup to your GPS." Everyone in the classroom was probably thinking the same thing: "Why not just bring another handheld GPS or three and a pile of spare batteries?" And yet, the room was full of prospective celestial navigators, here for the intro session to see if they wanted so sign up for the next nine weeks of classes. Clearly we were in it for the art, not the science or even the safety backup. Except possibly for one young man wearing camouflage, as I learned later in the evening. Lesson one was very elementary, dealing only with the definitions of latitude and longitude. I was about to write off the whole program as being much too basic for me. On the other hand, the instructor could tell a pretty good yarn when he wanted to make a point: "There we were," he recalled. "Running before the tail end of a nor'easter and approaching the coast of Ireland. It had been overcast for two days, ever since my last noon sight, and longer than that since our last good fix. We only had dead reckoning for longitude. The clouds were breaking up, but it was long past dusk and we wouldn't have a good longitude sight until dawn. Fortunately, the full moon came out through a hole in the clouds, and it lit up the horizon just long enough for me to get a shot of Mars setting. But I was new at this, and forgot that in the Western Hemisphere, the longitude scale goes up from right to left, not left to right like any normal graph. I plotted it all wrong, and put our position about 40 miles too far to the west. If the captain hadn't heard a sea lion barking and tacked the ship, by golly we would have bricked it up right there." It was a good technique for implanting some warnings against common errors, and he left us with a navigational puzzler that was very much on topic for the latitude and longitude lecture: "A bird flies 100 miles north from San Francisco, and then 100 miles east. Another bird flies 100 miles east, and then 100 miles north. Do the two birds arrive at the same nest?" "Of course not," I thought as I walked out of the dining room and up the stairs to the yacht club bar. "Otherwise it wouldn't make a very good puzzler." I ordered my usual from the bar, then turned around to see Lee Helm and another woman, whom I presumed to be a fellow graduate student, at one of the tables. They had books and notepads spread out before them. "Max!" she said. "We're ready for a Page 86 •
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break. Why don't you join us?" "Who let you in here this evening?" I asked, looking around to see if there were any club members who might be hosting her. "I'm like, signed in as your guest, as usual," she grinned. "Well okay, but really, Lee, you're supposed to be invited in advance."
"In 1859 there was a massive solar flare on the sun, a 'coronal mass ejection.' " "Max, I'm a future member of this club and as such I have certain rights." There was no arguing with Lee on that point. It turned out she was using the yacht club bar to conduct a tutoring session. "Bessel functions" was the subject. "The housing co-op is too noisy tonight, so we thought this would be a nice, quiet place to work," Lee's student explained. The bar was filling up with novice celestial navigation students, and the guy in the camo, doubtless deviating his course toward the two attractive grad students, asked if he could take the fourth chair at our table. "What's the deal in the dining room?" asked Lee. "Another one of those safe boating classes?" "Celestial navigation," answered our new tablemate. "I'm planning to buy a sailboat to use as an escape pod. You know, if things really go south politically, or if there's a natural disaster. Can't trust the GPS system to be up and running after TSHTF." "Are you afraid that a hostile power will disable our navigation satellites?" I asked. "Cyberwar is much more likely," Lee's friend suggested. "They would just hack into our GPS control system, and I'm sure we'd return the favor on the Russian's GLONAS satellites." "My thinking exactly," the guy con firmed. "No need to zap any birds to bring the system down. That's why I need to learn celestial. Even a dual-mode GPS/ GLONAS unit won't be reliable." I noticed that he was toting a fairly large camo backpack. Even his socks were camouflage pattern. "A newbie," Lee stage-whispered to me. "Like, he needs to do a Singlehanded TransPac to really learn how to turn his boat into a reliable ticket out."
"I know, I know," he said defensively. "I'm trying to learn. Most of my prepper buds don't realize how useful a small sailboat can be for getting the heck out of Dodge, post-apocalypse." "That might actually be a scenario in which the sextant is useful," I concluded. "If we believe that a cyberwar that brings down our GPS is actually a possibility." "That's one scenario," he added. "But don't forget about the possibility of an electromagnetic pulse attack! A nuke blast in high orbit can trash all the onboard electronics. No GPS, no radio, no computer." "You could keep all your backup electronics in a Faraday cage," said Lee's friend. "Use a plastic bag for insulation, then a tight wrap of aluminum foil, then another plastic insulating layer and another foil wrap." In response to this suggestion, the survivalist pulled out a metallic-looking plastic bag, also styled in the ubiquitous camouflage motif. "I always keep my phone in this hardened Faraday bag," he explained. "Still, these global conflict scenarios seem a little paranoid," I said. "Two words," said the survivalist.
HOW TO USE Modern digital cameras are surprisingly capable as devices to measure the angle of a celestial body above the horizon, and in a pinch they can be used for reasonably accurate celestial navigation. However, there are some tricks and techniques to make this work: • The focal length must be repeatable. That is, a lens setting somewhere in the middle of the zoom range will not work. A prime lens with a fixed focal length is best. If a zoom lens is used, it must be set up against the stop at the longest or shortest end of its range. • The ideal field of view is about 25-30 degrees. A 65mm (35mm equivalent) with a sensor 4000 pixels wide will result in a resolution of over two pixels per minute of angular measurement, which corresponds to about a half mile of position accuracy for each pixel. This is more than enough precision for useful celestial work. • Avoid wider angle lenses, to keep lens distortion to a minimum. This will limit the observable angular altitude, but other advantages compensate, such as the ability to detect the horizon under very low light conditions thanks to ultrahigh ISO and post-processing options.
