Right now Polaris has a declination of about 89° 12'. This means that it circles the true pole at a radius of 48 nautical miles. As it is so far away this circle is invisible to the naked eye but it does mean that if you measure the altitude of Polaris and take it as your latitude, you could be almost one degree out.
To find your true latitude by Polaris you must know where Polaris is on its daily circle. This is its hour angle and if you have no tables it can be found from the star clock.
A line joining the trailing stars in Cassiopeia and the Plough passes through Polaris and the true pole. Polaris is on the Cassiopeia side of the Pole. The easy corrections are:
1. If the line is at right angles to the horizon with Cassiopeia on top then the correction is -48'.
HOW TO DO IT
^ 0 CASSIOPEIA
Imagine a line joining Ruchbah and Alcor. Polaris is on the Cassiopeia side of this line spinning round the True Pole. Imagine a graticule round the True Pole with a vertical scale marked off in minutes. Draw a line in from Polaris to meet this scale. The correction will be positive when Cassiopeia is above Polaris and negative when it is below. It may help to draw out the graticule.
K THE BIG DIPPER
16.2 Polaris Correction
2. If the line is at right angles to the horizon with the Plough on top and Cassiopeia on the bottom, the correction is +48'.
It is more complicated when the line is at an angle to the horizon. Now you have to imagine where Polaris is on its journey round the Pole (see Figure 16.2). Once that is decided you have to add or subtract to find your true altitude.
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