# Celestial navigation for beginners

Celestial navigation finds your position from the Sun, Moon, planets and stars with a sextant, a watch and an almanac. It looks like a wall of jargon from outside. Underneath is one simple idea, repeated.

## The one idea

At any instant, every body in the sky is directly over one spot on Earth: its [ground position](https://asterodos.com/glossary/ground-position). The almanac tells you where that spot is for any second of the year.

If the Sun were straight overhead, you would be at that spot. If it is lower, you are some distance away, and the angle tells you how far: every degree short of overhead is 60 nautical miles. A sextant measures that angle. So one sight puts you somewhere on a [circle](https://asterodos.com/m/circle) around the ground position, everyone on which sees the Sun at the same height.

A second sight, of another body or of the same one hours later, gives a second circle. You are where they cross. That is a fix.

## From a circle to a line on the chart

The circles are thousands of miles across, far too big to draw. Near you, though, a small piece of the circle is practically a straight line: a [line of position](https://asterodos.com/m/lop). The working of a sight, called sight reduction, finds where that line runs. The standard way is the [intercept method](https://asterodos.com/intercept-method): work out what the altitude would be at a position you choose, compare it with what you measured, and the difference in minutes is the number of miles to shift the line.

## What you need

- **A sextant.** A plastic one is enough to learn on and to cross an ocean with.
- **The time, to the second.** Four seconds of error is one minute of longitude, up to a mile. A quartz watch checked against a time signal is fine.
- **A Nautical Almanac** for the year: where each body is.
- **Sight reduction tables** ([Pub. 249 or Pub. 229](https://asterodos.com/pub-249-vs-pub-229)), or a calculator with trig functions.
- **A plotting sheet**, dividers and a protractor or plotter.

You do not need a boat to learn. The arithmetic is the same on a kitchen table, and an [artificial horizon](https://asterodos.com/land/1) lets you take real sights from a backyard.

## The order to learn it in

- [Latitude, longitude and the nautical mile](https://asterodos.com/m/onramp), then [time and angles](https://asterodos.com/m/time).
- [Reading the almanac](https://asterodos.com/m/almanac): GHA and declination.
- [Correcting the sextant reading](https://asterodos.com/m/sextant) to the true altitude.
- [The noon sight](https://asterodos.com/m/noon): latitude from one reading, no tables. Most people's first success.
- [Latitude from Polaris](https://asterodos.com/m/polaris).
- [Lines of position](https://asterodos.com/m/lop), the [assumed position](https://asterodos.com/m/ap) and the [intercept method](https://asterodos.com/m/intercept).
- [The fix](https://asterodos.com/m/fix): crossing two or three lines, and [planning star sights](https://asterodos.com/m/planning) at twilight.

## How long does it take?

A noon latitude is within reach in a weekend. A full star fix usually takes a few weeks of regular practice. The mathematics is addition and subtraction of angles; the tables do the trigonometry. What takes time is doing every step in the right order with the right sign, which is why practice that tells you exactly which step went wrong matters more than reading.

## Learn it properly, free

Asterodos is a free interactive course: short lessons you can see and move, and practice that checks every step of your working.

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From Asterodos, a free interactive course in celestial navigation: https://asterodos.com/celestial-navigation-for-beginners
For learning, not for navigating a vessel.