Hello from the South Pacific gyre!
It’s Sophie again! I felt compelled to blog because I want to share an exciting navigational development on the Robert C Seamans. Just as we were making some serious headway on our leg from Nuku Hiva to Mangareva, the GPS stopped working…
Just kidding! The GPS is still working fine, but we’ve started plotting our location by celestial navigation only. For context, one task we have on watch is to record our GPS position and plot it on a navigational chart (a real paper chart, using a pencil, triangles, and dividers). But it turns out that with a sextant, a few celestial bodies (the sun, moon, and stars), some fancy books (nautical almanac of 2024 and our favorite 229er), and a fair amount of math, we don’t need the GPS at all. We’ve still been recording our GPS position at the top of every hour, but we are now exclusively using celestial sightings for navigation.
These sightings include:
1. Local Apparent Noon (LAN)
Every day, we calculate when the sun is going to reach the highest angle in the sky relative to us. This time is called local apparent noon or LAN. Today, it was at 11:32 in our +9:30 time zone. Ten minutes before LAN, navigators head up on deck with their sextants to “shoot the sun,” or bring it down to the horizon. When it reaches its peak, we take the reading on our sextant and apply some calculations to decipher our latitude with good accuracy.
2. Line of position (LOP) from sun lines
At any time of day when the sun is greater than 10 degrees above the horizon, we can shoot a sun line. For this, we still use sextants to bring the sun down to the horizon but add a few extra calculations and the help of the 229er book to decipher a line of position — a line on the chart that we have to be on according to the angle of the sun relative to us. We tend to shoot sun lines around 1000 and 1500.
3. Star Frenzies
Star frenzies are my favorite type of celestial navigation. To prepare for a star frenzy, we need a general sense of what stars are in the sky before the sun comes up and right after it goes down. We pick a few stars that we know we can see (the almanac is helpful for pre-calculating this, or I often just ask around on deck in the moment), and we bring these stars down to the horizon. At evening, I’ve been getting to know Spica, Sirius, Arcturus, Canopus, and Rigel K very well. Our time window to shoot the stars is short because we need to be able to see both the stars and the horizon, aka the twilight zone. When you have a star close to the horizon, you shout out “stand by” to alert your timekeeper that you’re close, and then “Fix,” when you want them to grab the time, all the way down to the second. With three to four stars, you can plot a very accurate star fix which gives you both latitude and longitude, so long as all of the LOPs intersect at the same place, forming a “pinwheel” at your location.
4. The moon
The moon is a calculation I have yet to do, but it can also get us a pretty accurate fix. Our Captain calculated a moon fix this morning, and it was helpful because the sky was too cloudy to see the stars at dawn, so we missed that fix, but the moon put us back on track.
To continue our dead reckoning (DR) line every hour, we apply the last hour’s increase in log (total nautical mileage traveled) along our DR line, which is determined by the course we are steering. Right now, this is 160 degrees true north. With all of these crazy calculations, we have a pretty decent sense of our position. It’s not perfect, but you really only need to be within 5 nautical miles of your actual position to call it good enough, as you will see land from much farther away than that. Our chart is also a little messy with us jumping around a bit, but it’s exciting to see how we can use the sky and some math to understand where we are in the world. It makes me appreciate how people navigated before GPS existed. Even if they weren’t using fancy books and sextants, the idea of navigating by the sky is not new to humans by any means. I’m not sure how much longer we will go without GPS, but it would be nice to make quite a bit of progress using celestial navigation. It’s a satisfying process, and I am gaining a new appreciation for math when it is applied.
Aside from celestial nav, things are going well on the ship. This is our longest leg so far, and I think we are settling into a good rhythm. Tomorrow is gyre today to celebrate the fact that we are entering the South Pacific gyre, and it’s also my assistant steward day, so I will be helping the stewards cook up some delicious food for the ship. Henry made amazing dumplings for dinner tonight, so I have a high bar to live up to. It’s time for me to go to bed and prepare for such an exciting day. I hope you enjoyed my navigational report!
All the best,
Celestial Sophie
