From: Brian Hehn
Sent: October 26, 2017 8:12 PM
To: Bob Goethe
Subject: One more thing ...
How often will one take sights at night? I am sure there are lots of “well that depends” responses. Reason for asking is that I am ordering the sextant travel case tomorrow and thought about adding a couple things to that order, including:
Mini flashlight: https://www.celestaire.com/sextant-accessories/photon-micro-lights.html
Only thing I will need after this is a good time piece. I have not wore for many years other than my dive computer which is far too bulky for regular wear. Any suggestions?
If I could design a wrist-watch, besides an accurate analog time display (not digital), it would also display tide cycles and/or a barometric pressure graph, and it would have two programmable alarms … one for wake up, another for an hourly or some other routine time period for log entries (say every three hours or four hours based on watch schedule).
I have found a few interesting options but they all cost a bazillion dollars or more. Also found digital time pieces but I need reading glasses every time I want to know the time.
Thanks so much for your help Bob. Your knowledge is always greatly appreciated.
Cheers … Brian.
From: Bob Goethe
Sent: October 27, 2017 4:21 PM
To: 'Brian Hehn'
Subject: NASA: Daily Rhythm of the Celestial Navigator
The answer to your question about “how often night sights?” is “all the time.”
A sight on the sun gives you a single line of position, just like a compass bearing on a single landmark.
For part of each month, you can see the sun and moon in the sky together, and you can shoot them both, get two lines of position that cross, and have a fix.
Lacking that, you need to do a running fix (familiar from your coastal navigation) with the sun, advancing – for example – your 9 AM line of position to a 12 noon line of position. If you are looking for a celestial fix, you take what accuracy you can get, and a running fix is better than no fix at all. But of course, if you don’t get precision in your boat speed and course (and both of those are always changing when at sea), then your running fix will be inaccurate to that degree.
However, there is a period of 30 or 40 minutes before dawn and 30 or 40 minutes after sunset (less than this in the tropics) called “nautical twilight”, when there is still enough light that you can see the horizon, and yet some of the brighter stars have come out. You can take sights of 2 or 3 stars/planets/moon and get a precise fix anytime the sky is clear enough to see them.
So for a working celestial navigator, it is best not to be standing watch with the rest of the crew, as you will really want to be awake before dawn and after sunset every day, and then through the day be taking a sun sight every 3 or 4 hours. Maintaining that pace of life after having stood watch from midnight to 4 AM is simply not feasible.
So in summary, we will begin our learning – as every celestial nav student does – on how to take sun sights and how to do running fixes. But we will advance to using the moon, planets and stars.
For the working navigator, the two periods of twilight per day are the primo times for taking sights. Those are the two times per day when you can actually pretend to some level of precision in your fixes.
As for the mechanics of taking a sextant sight during this dusk period, I have a light on a headband that can toggle between white light and red light. For taking sights during twilight, you need a red, hands-free light of some sort or another, so you can a) read your sextant, and b) read your timepiece, and c) write down the time and altitude in a notepad.
As I said on the phone, the sextant-lubricant is a good idea when you are using your instrument in a salt-water environment. Less so from me as a land-locked navigator.
I will send you a separate email about time.
"The only noble thing a man can do with money is to build a
Robert Louis Stevenson, 1889
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