How to learn celestial navigation
Learn it in this order: time and the almanac, the sextant reading, latitude from the noon Sun, the compass check, Polaris, then lines of position and sight reduction, and finally the twilight star fix. All you need to start is a pencil, a calculator, and practice problems that check your work; a sextant can wait until you know the method.
What celestial navigation is
Finding your position from the Sun, Moon, planets and stars. A sextant measures a body’s altitude above the horizon; the almanac says where on Earth the body is overhead at that second; and the difference puts you on a line. Two or three lines cross at your position. It needs no electronics beyond a watch, which is why ocean sailors still learn it as a backup to GPS, and why it is still taught for professional mariners.
The order to learn it in
- Time, angles and the almanac. Arc and time (15° is one hour), UT, and reading the Sun's GHA and declination from the Nautical Almanac.
- The sextant reading. Turning a sextant reading into a true altitude: index error, dip and the altitude correction.
- Latitude from the noon Sun. The classic first sight. At local noon the Sun is due north or south, and one altitude gives your latitude.
- Checking the compass by the Sun. The Sun's bearing at sunrise or sunset is known, so it checks your compass. On ships this is the celestial job done every day.
- Latitude from Polaris. At twilight the Pole Star's altitude is your latitude, give or take a small correction.
- Circles and lines of position. The big idea: one altitude puts you on a circle centered on the body's ground position; close to you it is a straight line.
- Sight reduction. The assumed position, LHA, and the intercept method: working out the line with Pub. 249 or Pub. 229 tables, or a formula.
- The star fix. Planning twilight star sights (with a star finder), then plotting three or more lines where they cross.
- Planets and the Moon. The same method for the planets, and the Moon, which gives a daytime fix with the Sun.
Some teachers start with the circle of equal altitude instead, so that every sight afterward is the same idea. Starting with the noon sight gives a working result sooner, and the circle makes more sense once you have done one.
Ways to learn it
- Online, interactive
- A course that checks your work step by step catches the mistakes a book cannot: a sign the wrong way, the wrong almanac column, a minute of time slipped. This site is one; it is free while it is being tested.
- Books
- The standard reference is Bowditch, The American Practical Navigator, free from the US National Geospatial-Intelligence Agency. For learning, many sailors start with Mary Blewitt's Celestial Navigation for Yachtsmen (short and practical), David Burch's Celestial Navigation: A Complete Home Study Course, or John Karl's Celestial Navigation in the GPS Age.
- A class
- In the US, America's Boating Club (the US Power Squadrons) teaches celestial navigation in its Junior Navigator and Navigator courses, and some sailing schools run their own. In the UK, the RYA's Yachtmaster Ocean shorebased course covers it. A class gives you a teacher to ask and a fixed pace.
- At sea, with a navigator
- Nothing beats taking sights on a passage with someone who already can. Most people learn the method ashore first and then practice it at sea.
Questions
Can you learn celestial navigation online?
Yes. The method is pencil-and-paper work with an almanac and tables, so it can be learned anywhere. What you need is practice with feedback, and real sights once you have a sextant.
What should I learn first?
The noon sight for latitude. It uses only the Sun's declination and one altitude, it is satisfying to get right, and it teaches the almanac and the sextant corrections that every later sight needs.
Do I need a sextant to start?
No. Learn the method from given sextant readings first. When you are ready, a plastic training sextant costs well under the price of a metal one and is good enough to learn with; the Davis Mark 3 and Mark 15 are the usual choices.
Can I practice without a boat?
Yes. On land the horizon is hidden by hills and buildings, so navigators use an artificial horizon: a tray of water that reflects the Sun. The reading is twice the altitude.
Tables or a calculator?
Both are used at sea. Sight reduction tables (Pub. 249 or Pub. 229) need no electronics and are what most courses and exams teach; the formula on a scientific calculator skips the interpolation in the tables. Learning the tables first means you can still navigate when the batteries are flat.
How long does it take?
Most people can work a noon sight within a few evenings, and a full star fix within a few weeks of regular practice. Speed and confidence at sea come with taking real sights.
What do I need to buy?
To learn: nothing but a pencil and a calculator. To navigate: a sextant, an accurate watch, the Nautical Almanac for the year (printed annually), and sight reduction tables (Pub. 249 is free from the US NGA as PDFs, and in print).
This course
Asterodos is an interactive course in exactly that order: short lessons with globes and diagrams you can turn, practice that checks every step of a sight and names the mistake, the almanac and Pub. 249 built in, passages across an ocean, and guides to the printed almanac and tables and to sights on land. Free while it is being tested. Later, the first module stays free and the full course is a one-time purchase.