Time of Twilights

 


     Twilight Time is an important time of the day for the celestial navigator who is doing star observations. Getting the time right for twilight can mean being ready to take observations at the proper time, not too early or too late.

     There are four times that can be referred to as twilight. There is sunset, civil, nautical and astronomical twilight. It’s not complicated, but it can be confusing. It is essential to know the differences between the different times of twilights.

     Twilights is the relationship of the center of sun to the plane of the horizon. In order, at the end of the day, we have: Sunset - the center of the sun appears to be at the horizon, (though by the time the center of the sun appears to touch the horizon it is actually already below the horizon due to refraction,) Civil twilight - where the center of the sun is 6° below the horizon, Nautical twilight - when the center of the sun is 12° below the horizon, Astronomical twilight - when the sun is 18° below the horizon. In the morning the order is reversed and ends with Sunrise.

     For celestial navigator we will only concern ourselves with the times and importance of civil twilight and nautical twilight. At the start of the day, nautical twilight is the most important to get right. The reason is 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 just enough light to see the horizon At the end of the day, the most important to get right is the time of civil twilight when the sun has reached 6° below the horizon. Then it is light enough to still see the horizon and yet dark enough to see the navigational stars. These are the key times for the celestial navigator, actually it’s the time between civil twilight and nautical twilight that star sights are shot. This time can be short so it’s important to get it right.

     Since it takes place in the early morning or late evening, it is important to time it correctly. Show up on deck too late in the morning and the sky is already bright and no stars can be seen. Likewise, show up too late in the evening and it’s too dark to see the horizon. So in order to get it right you can calculate the time of nautical twilight and civil twilight so that you can arrive on deck confident in your calculations right before the event, thereby impressing your fellow crewmates with your navigational skills.

     Another reason that the correct time for twilight is so essential is that is can be used when converting to LHA of Aires, which can be used to calculate the altitudes and azimuths of the visible stars. Use the LHA of Aries set up your Star Finder to plan your sights in advance. You can then even pre-set your sextant for your first sight, making that little spec of light easier to locate.


An Example:

     Let’s say that we will be at a DR of 45° 35’ N by 122° 45’ W, (the location of The Compass Adjuster,) at about the time of Nautical Twilight, let’s say at about 0230 on June 6, 2012. I want to know at what time I should be up on deck, sextant in hand to get ready to take star sights. I 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 “Twilight – Nautical, Civil,” and “Sunrise.” I am looking for the time of nautical twilight in local mean time (LMT) and I see that at latitude 45° N the time of nautical twilight is 2:49. 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° … 120° etc. (15° of arc for every hour W or E of the Prime Meridian, Greenwich, England.) I am not at that position, but instead at 122° 45’ W - 2° 45’ west of the meridian of my time zone - 120° W. Next, I go to the arc to time conversion table (The Yellow or Grey pages at the back of the almanac,) and see that it takes the sun 8 hours 8 minutes of time to travel 122° and then get 3 minutes additional for the 45’ more of arc, so I add that time to two hours 49 minutes and get a time of eleven hours zero minutes GMT for Nautical Twilight. Subtract 7 hours, (120º ÷ 15 = 8, But remember your watch is set to Daylight Savings Time so don’t subtract 8 hours in the summer…), and you get 3 hours zero minutes LMT, and that is when you need to be up on deck ready to take sights 0300, or as some folks say "O dark thirty".

     Once I have this information, I go to the day in question in the Nautical Almanac and find the GHA Aires for June 6 at eleven hours GMT. I get 60° 15.6’. 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:


60° 15.6’ + 360° = 420° 15.6’ (or 419° 75.6”) - 122° 45’ = 297° 30.6’ LHA Aires


     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 when you do arrive on deck you’ll know just which stars to shoot and where to look for them.

      Follow the same process in the evening, just use the lower of the two tables in the Nautical Almanac and calculate for the time of Civil Twilight. Remember, you shoot you sights BETWEEN the times of Nautical Twilight and Civil Twilight in the morning and BETWEEN the times of Civil Twilight and Nautical Twilight in the evening.

      If you look at this time frame in the Nautical Almanac for June 6 you’ll see that you will have about 48 minutes to get your sights done at this latitude at this time of the year.  As the time gets closer to the Winter Solstice this time frame will become much shorter, only 37 minutes on December 21, 2012 at 45° N.

 

© 2012 Mark S. Anderson