Understanding celestial theory is best accomplished through a series of mental exercises that puts your mind in the real world and gets your nose out of the books and off the charts. It’s assumed that the reader will have a basic understanding of latitude and longitude and coastal navigation.
We’ll focus again on the sun. Imagine the sun and the earth suspended in space. Now, imagine one of the sun’s rays, emanating from its core, piercing the earth, right through to its core. In celestial nav. terms, the point where the sun’s ray pierces the surface of the earth is called its geographic position (GP). In other words, if you were standing on the GP, the sun would be directly overhead. Of course, this position is not fixed, as the earth is always spinning. Hence the importance of keeping accurate time when taking sights – you must ‘fix’ the sun’s GP to a specific time in order to make sense of it. We express GP in terms of latitude and longitude, called declination and Greenwich Hour Angle (GHA), respectively.
Celestial Navigation Part III – Tips and Tricks on Sight Reduction
The sun’s declination defines the tropics – they lie at 23Â½Âº north and south of the equator and represent the furthest the sun’s GP will travel from season to season. On an imaginary picture of the globe, and over the course of a year, the sun’s declination will trace a sine curve between the Tropic of Cancer and the Tropic of Capricorn (at the summer and winter solstices) and, during the autumnal and vernal equinoxes, cross the equator twice, In the course of a day, the sun’s GHA will always travel from east to west, from sunrise to sunset. Hence the earth’s 24 time zones and the number of hours (24) in a day. Three hundred and sixty degrees of longitude divided by the 015Âº per hour of the sun’s westerly march, equals an even 24. Here then – and this is one of the ‘Ah ha!’ moments of celestial navigation – time and longitude are one in the same and easily convertible.
It’s fun and intuitive to predict the GP of the sun at any given time – for example, I’m writing from a cafÃ© in Stockholm. The date is May 30th and the local time is 1430. My approximate longitude is 018Âº east. I know the sun is to the west because it’s past noon, and it’s a ways south of me, as Stockholm sits at 59Âº north latitude. How far west? Two- and-a-half hours, or about 037Â½ Âº of longitude. (Recall the sun travels 015Âº per hour.) I can, therefore, guess that the sun’s GHA is about 019Â½Âº, somewhere in the western hemisphere. In reality, the GHA is closer to 022Â½Âº, which I’d discover in the Nautical Almanac. Why? Because Stockholm sits a full three degrees east of the center of its time zone, 1430 on my watch in Stockholm is slightly inaccurate in terms of the sun. Time zones are spread east to west over 015Âº of longitude (for modern convenience), and unless you are positioned exactly over the center of a time zone, the sun will be a bit ahead of or behind your watch. In the Stockholm example, the sun is 003Âº ahead of my watch, or approximately 12 minutes. Knowing the center of your particular time zone is also how you compute actual local noon, the time when the sun is directly overhead. All this confusion over time also underscores why it’s imperative to keep accurate Greenwich Mean Time (GMT) when taking real sights. The sun’s GHA, by the way, is always measured west, through 360Âº, unlike longitude, which is divided into two hemispheres, with 180Âº in each. The sun, after all, cannot travel east.
Now for declination – it’s past April but before June 21, so I know the sun is somewhere north of the equator and south of the Tropic of Cancer, though closer to the latter. I can also predict in which general direction the sun bears on the compass – about SW from Stockholm. Make sense? Do this exercise several times over, in different imaginary places on the globe for practice.
Making these mental predictions is often as far as one needs to go to make practical use of celestial navigation. Offshore, during a winter passage from Tortola to Bermuda, say, I’d know that in the mid-morning, the sun should be off my starboard quarter (its GHA is east of me – not yet noon – and its declination is somewhere in the southern hemisphere, as its winter. Therefore, its GP must bear to the SE). If I wake up a little groggy, a quick look out a portlight is all I need to confirm the watch keeper’s course. Not once would I have to consult a chart, GPS or even the compass, and the sextant has never left its box, yet I’m still using celestial navigation.
In Part 3 we’ll look at finding an accurate position using celestial, and delve into the books to reduce an actual sun sight, step by step.
Don’t miss the full Series on Celestial Navigation. Any questions?
- Celestial Navigation Part 1: Introduction to Celestial Navigation
- Celestial Navigation Part 2: Predicting the Sun’s Geographic Position
- Celestial Navigation Part 3: Tips and Tricks on Sight Reduction
What is your experience with Celestial Navigation?
Of course comments or questions to the author are welcome below.