Seamanship & Navigation: Times of twilight

The Platters sang about it in a song called Twilight Time. Something about “heavenly shades of night are falling” etc. For practitioners of celestial navigation, twilight time is a whole lot more than just a tune sung in the dark. It is an important time of the day for the celestial navigator who is doing star observations. Getting the time right for twilight can mean more sleep. And on a boat there is nothing more important than that, unless of course it is coffee.

There are four times that can be referred to as twilight. There is sunset, civil, nautical and astronomical twilight. Who would have thought it could be so complicated. Well it is essential to know the differences between all of these flavors of twilight especially if you are navigating aboard ship.

Twilights can be described as the relationship of the center of sun to the plane of the horizon. So at sunset we have the sun at the horizon, though by the time the center of the sun appears to touch the horizon it is already below the horizon due to refraction, but we will let that go for the nonce. Then we have civil twilight, where the center of the sun is 6° below the horizon. After that there is nautical twilight when the center of the orb is 12° below the horizon. Finally, astronomical twilight is when the sun is 18° below. For the sake of this article we will only concern ourselves with the times and importance of civil and nautical twilight. Of these two, nautical twilight is the most important to get right. The reason being that morning stars are observed at nautical twilight when the sun is still 12° below the horizon and the sky is brightening slowly. It is still dark enough to see the stars easily yet there is still enough light to see the horizon. This is a key time for the celestial navigator.

Since it takes place in the early morning, it is important to time it correctly. Show up on deck too late and the sky is already bright and no stars can be seen. Show up too early and you just hang around feeling a bit foolish. So in order to get it right you can calculate the time of nautical twilight so that you can arrive on deck in an unhurried professional fashion right before the event, thereby impressing your fellow crewmates with your consummate skill. The reason that the correct time for twilight is so essential is that is can be converted to LHA of Aires, which can be used to calculate the altitudes and azimuths of the visible stars. Let’s see how this is done.

Let’s say that we will be at a DR of 40° N by 70° W at about the time of twilight, let’s say at about 0400 on May 26, 2012. I want to know at what time I should be up on deck, sextant in hand to get ready to take star sights. Here is what I do. First I go to the daily sun pages of the Nautical Almanac and in the top right hand of the page I see the columns “sunrise,” “twilight nautical” and “twilight civil.” I am looking for the time of nautical twilight in local mean time (LMT) and I see that at latitude 40° the time of nautical twilight is 3:26. Now remember that is the time in LMT of nautical twilight if the observer is at the meridian of the time zone, i.e. 0°, 15°, 30°, etc. I am not at that position, but instead at 70°. Next, I go to the arc to time conversion table and see that it takes the sun four hours 40 minutes of time to travel 70° of arc so I add that time to three hours 26 minutes and get a time of eight hours six minutes GMT.

Once I have this information I go to the day in question in the Nautical Almanac and find the GHA Aires for May 26 at eight hours. I get 4° 17.8’. I add this to 360° and from that number subtract my west longitude. Remember the formula: In order to find LHA Aires GHA Aires – west longitude = LHA Aires. So we have:

364° 17.8’
-70° 17.8’
LHA Aires 294°

I can then use this LHA Aires on my star finder and I will know what stars are available, their altitudes and azimuths. This work doesn’t have to be spot on. There is room for a slight margin of error. The point is that you can calculate the times of twilight so that you can sleep longer, and when you do arrive on deck you’ll know just which stars to shoot. That is not a bad thing. And if you are old enough to remember the song that we began with, why you can even hum it as you work.

Contributing Editor David Berson writes the Nav Problem page in every issue of Ocean Navigator. He is also the owner and operator of Glory, an electrically powered excursion boat, in Greenport, N.Y.

By Ocean Navigator