Editor’s note: We’re revisiting this series on navigating by the sun, moon, planets and stars in the age of GPS because celestial nav is not only a viable backup to satellite navigation, but it is also a skill that ocean voyagers should have in their toolkit. In this series, we’ll cover all the basic knowledge you’ll require to get up to speed on this elegant and rewarding technique for finding your way at sea. Click to read Part 1, Part 2, Part 3, Part 4, Part 5, Part 6, Part 7, Part 8, Part 9, Part 10, Part 11, Part 12 and Part 13.
In this installment, we’ll explore the celestial navigator’s day at sea as he or she uses various types of sights during the course of the day.
A hypothetical day at sea for the enthusiastic celestial navigator: sun LOPs, sun running fixes and a noon latitude all bookended with pinwheels of star, planet and moon sights at morning and evening twilight.
In our previous installments in this series, we’ve looked at the various types of celestial bodies and how we shoot and reduce them for any lines of position (LOP): everything from the basic sun sight to star sights, planet sights and moon sights, and multiple sights of various bodies during one sight-taking session that yields us not just an LOP but multiple crossing LOPs, and thus a fix position.
Now, that we have all of these arrows in our celestial quiver, how do we use them on a passage? If we were to shut off our GPS and navigate an ocean passage wholly by celestial, what might a day at sea look like? Let’s examine what a navigator might do during the course of a full day at sea.
In this case, let’s put ourselves aboard a sailboat with a 50-foot waterline on a course of 120° True. We’ll call our boat Pegasus. We’re on a long passage, so we’ll maintain that course throughout our day at sea. Just because we can — and because it opens up the plot a bit — we’ll say that we’re maintaining a speed of 7 knots throughout (hull speed for a boat with 50 feet of waterline is about 9.5 knots, so this isn’t outrageous). In this case, we’ll label our plot in local time. Let’s also assume we have good observing conditions with no rain, clouds or fog. That’s the best case for making a series of sextant sights, and it does happen at sea. Although, it’s also possible to be dogged by the celestial navigation curse: clear skies all day, and then just as the time comes for evening or morning twilight and star and planet sights, the sky clouds over only to clear again once the sight-taking window has closed!
A morning twilight fix followed by an early sun line and EP.
A sun line at 0930 gives us a north/south LOP, and we advance that to get a running fix at 1040.
We would prepare for our day at sea aboard Pegasus before the day begins. The night before, we use Vol. 1 of HO 249 to precalculate which stars will be available at morning twilight. Recall that Vol. 1 gives us seven possible stars that will be visible at our location. If you want to shoot fewer, the table marks three of those stars that are the best to use based on the cut of their LOPs. Another option would be to precalculate using the Rude Star Finder. This method also allows you to determine if the planets and moon will be visible. Or, there are computer programs and smartphone/tablet apps that we can use to precalculate the stars available for morning twilight. We can also calculate the time of morning twilight.
With our trusty list of available stars and planets, we go on deck shortly before morning twilight and get ready. We convince one of our Pegasus crewmembers to be the assistant and record sextant angles and sight times. When the horizon appears in the east, we start shooting, working our way around to the west where the sky will stay dark the longest.
Afterward, we go to the nav station and do the sight reductions. In our hypothetical case, let’s say we only got sights of three bodies: Saturn, Spica and Antares. We plot the LOPs from those sights and the point where they all cross is our position. In this case, three sights formed a small triangle or “cocked hat” instead of a single crossing point for all three LOPs. We place our fix in the center of the triangle and begin a new dead reckoning plot. We label it “C120” on top and “S7” on the bottom.
LOP of the sun
We continue our DR plot until, a few hours after sunrise, the sun is more than 10° above the horizon and we decide to get an LOP of the sun. We shoot the sun and plot our LOP, labeling it “Sun” and “0830.” Since it is summertime in the Northern Hemisphere, the sun rises north of east and sets north of west. We can use this fact to get plenty of sun sights over the course of the day.
A noon sight gives us our latitude, and we also advance our 0930 LOP to get a running fix with a 90° cut.
A sun sight when the sun bears to the southwest gives us an LOP parallel to our DR track and a new EP.
We compare our 0830 DR position and the 0830 LOP. The LOP suggests we are a little further down our track than our DR tells us. Maybe we’re sailing a little faster then 7 knots, or we’re being assisted by a current. Since we want to integrate the DR with the LOP, we establish an estimated position on the LOP and label it “0830 EP.”
An hour later, the sun bears due east, so we grab another sight. When we plot that LOP, because the sun was due east at 90° the LOP forms a vertical line on our plotting. It’s like a line of longitude, telling us where we are in an east/west sense. Again, our DR and the LOP don’t agree. Now it seems we are not making as good time as we thought. Perhaps we’ve sailed into a contrary current. So, we establish another EP and start our DR track anew.
A running fix
About an hour later at 1040, we take another sun sight and this time instead of making an EP, we advance our 0830 LOP to our 1040 DR position. We label this LOP “Sun,” with a time of 0830-1040, so we know this is an advanced LOP. Where this advanced LOP crosses our current 1040 LOP is our 1040 running fix. Again, we start a new DR track.
As local apparent noon (LAN) approaches, we use the meridian passage info in the Nautical Almanac to determine the time of LAN — in this case, let’s say it is 1210. At that time, we shoot the sun and then do the simple LAN calculation to get an LOP that tells us our latitude. We can also advance our 0930 sun line and get a running fix. Again, we start a new DR track.
At 1420, two hours after LAN, we get another sun line when the sun bears 210°. Note that this bearing of the sun is our course line, plus 90°. The LOP is parallel to our DR track. If we were right on our plotted DR track, the LOP would line up with the track, but here we notice the LOP is displaced a few miles to the southwest. We are a bit to the right of track. We start a new DR track from the EP.
At 1530, we get another sun line and EP and start our DR track anew. This is like clockwork to us now.
An afternoon sun sight and a later one gives us a shallow running fix.
Finishing with a four-body pinwheel fix.
The final LOP
At 1650 the sun bears due west, or 270°. We get an LOP that, again, like our 0930 LOP of the morning when the sun was bearing 090°, is like a longitude line that helps show us where we are east/west. We can also advance our 1530 LOP and get a running fix. This is our last sextant work of the day with the sun. We’ve gotten seven LOPs from the sun, which have given us regular updates on our position during the day.
Our last bit of sextant work aboard Pegasus is at evening twilight. Again, we’ve precalculated the time of evening twilight and the celestial bodies that will be available for us to shoot. We are lucky because we get four good LOPs from Venus, the moon, Capella and Arcturus. We plot the LOPs and get an excellent fix from the multiple bodies. We know exactly where we are. And, as we’ve done before, we make use of this great fresh information to start a new DR track that will take us through the night.
Practice makes perfect
That’s an example of a celestial navigator’s day at sea. While this may seem like a lot of work, in fact, once you’ve practiced your sight reductions a bit, the process goes surprisingly fast using just a pencil and paper. It’s also possible to take your sights and use a calculator, laptop program or smartphone/tablet app to do your sight reductions — although regular practice makes doing them by hand on paper second nature and incredibly satisfying.
This process can be repeated every day of your passage and you can cross oceans with your sextant, watch, almanac and sight reduction tables. Not only will you know your position, you’ll get a great feeling of self-sufficiency, involvement and independence by using celestial. You’ll also become attuned to the incredible light show in the sky, as the motions of all these celestial bodies become an integral part of your voyaging experience.