I still have notes for calculating sunrise and sunset in my first cadetship on a tanker vessel.
We used paper charts back then, and the second mate suddenly asked me to determine the sunrise and sunset on our current latitude and longitude while at anchor.
That task opened up new methods connected to celestial navigation and the correct usage of the Nautical Almanac.
I’ll relive the computation I made during that time and show you the best and easiest method for finding the times of first light and sundown.
Promise. No fancy smartphones, computers, scientific calculators, or Google search to solve them. We’ll use the basic tools we can get because that’s what the old sailors did back in the day.
Key Takeaways
- Calculating sunrise and sunset using the nautical almanac is a valuable skill for seafarers and anyone interested in celestial navigation.
- The formula is straightforward – simply tabulated rise/set times adjusted by longitude and zone description.
- Interpolation is a technique used to estimate sunrise and sunset times for latitudes not explicitly listed in the nautical almanac.
What you needed for Sunrise/ Sunset Calculation
We only need a few things to find the sunrise and sunset. You can find everything inside the bridge and calculate it without the internet.
Here are the things you need:
- Ship’s position
- Zone Description
- Pen and Paper
- Basic Calculator (Optional)
A scientific calculator can be great, but where’s the fun in that?
Formula for calculating sunrise and sunset
The formula is straightforward, too. With a few rules to remember, you won’t go astray when adding or subtracting for Eastern and Western hemispheres.
Formula:
| SUNRISE | SUNSET |
| Sunrise = (Tabulated Sunrise +- Longitude in time) +- Zone Description | Sunset = (Tabulated Sunset +- Longitude in time) +- Zone Description |
That’s it. Straightforward.
We’ll only have a little legwork on tabulated sunrise and sunset and converting longitude to time.
After computing them, it’s gonna be a breeze.
Steps in determining sunrise/ sunset
Here’s the solution overview and steps for solving them.
1. Get your position in latitude and longitude, and take note of the date.
2. Get your nautical almanac.
3. Find the sunrise and sunset in the nautical almanac corresponding to your date and latitude.
4. Convert your longitude to time.
5. Add or subtract (West +, East -) your tabulated sunrise/ sunset (in step 3) to the longitude in time.
6. Apply zone description.
7. Double-check by actual observation.
How to Calculate Sunrise and Sunset
Let’s try to solve them and apply the steps mentioned above.
On December 06, 2023, your ship is anchored in Latitude 52° 30.6’ North and Longitude 004° 19.4’ East. Find the sunrise and sunset of your position.
Solution:
With the above data, let’s find the sunrise and sunset for that date and latitude using the nautical almanac.
Below is a snippet of tabulated data taken from the nautical almanac. You can already see the Date, latitude, sunrise, and sunset.
Interpolation
Since our latitude doesn’t fall exactly on the given tabulation, we can interpolate them to get the closest match for our sunrise and sunset.
Longitude to time
Now, let’s convert our longitude to time, in other words, converting arc to time. Check out my post on Arc and Time Conversion for more knowledge.
East longitude takes a minus sign, and West is a plus
Zone Description
You can use the Admiralty books to get the ZD or a map that shows them.
To find the zone description (ZD) for this tutorial, get the UTC and the local time. The difference is your ZD. The sign is PLUS if on the Eastern hemisphere, and MINUS if West.
Final Computation
Putting them all together, here is the solution.
| SUNRISE | SUNSET |
| Sunrise = (Tabulated Sunrise – Longitude in time) +- Zone Description Sunrise = (07:56 – 00:17.29) + 1 Sunrise = 08:38.31 or 08:38 | Sunset = (Tabulated Sunset +- Longitude in time) +- Zone Description Sunset = (15:46 – 00:17.29) + 1 Sunset = 16:28.31 or 16:28 |
Comparing it with another online tool from the National Research Council Canada, our answers are close.
The reasons vary from interpolation to the nautical almanac we use. Rounding off is also a contributing factor.
But as long as your results don’t stray widely from other sources, The method you use to calculate sunrise and sunset should be correct.
Can you do it?
Test your skills by calculating the sunrise and sunset for your current location using the method described in this article.
Compare your results to those obtained from an online tool or your actual observation.
May the winds be in your favor.