Using the stars that never set
Finding north THE NIGHT SKY
O ne of the advantages of sailing in the mid-northern latitudes is that you get to enjoy ‘circumpolar stars’ that neither rise nor set, but stay above the horizon 24 hours a day. As the name suggests, they rotate around the pole and unlike seasonal stars, you will see circumpolar stars every single night of the year, from anywhere in the world with a high enough latitude. This is because for a star to be circumpolar it must be in the same hemisphere as you and its declination, added to your latitude, must make a number over 90. For example, from latitude 50º north on the south coast of Britain, any star with a declination over 40° north will be circumpolar.
Dubhe is a useful star to familiarise yourself with. With a declination of 62° north it will be circumpolar anywhere above latitude 28°N (the Canary Islands) and from the English Channel it will culminate high at 78° above the horizon and be due north, directly above Polaris. This relationship with Polaris is partly what makes Dubhe so important; along with Merak it is the primary method for locating the North Star, which is not that bright or easy to find. Instead, the best way to find Polaris is to extend a line from Merak to Dubhe by five times and the only star in that vicinity is Polaris. With this guiding star you then have a reference point north all night, even more accurate than a moving compass that is vulnerable to deviation.
In addition to pinpointing north, Polaris provides a quick and easy method for finding your latitude. A star will culminate directly overhead if its declination is the same as your latitude – but for every degree difference the culmination is one degree lower. Without using a sextant you can use this technique with Polaris to estimate your latitude. If it is directly overhead you are at the north pole; if it is halfway up the sky your latitude is 45 north, and if it is only just visible above the horizon you are near the equator. If you want to be more exact, a sextant will allow you to measure its precise altitude and then by aligning the declination of Polaris with the red arrow on the ruler of your star calculator (free to print at www.tidalcompass. com), your latitude will be aligned with the ‘observed altitude’. The advantage of this technique is that it can be done at any time because Polaris is static.
Because the earth makes a complete anti-clockwise rotation on its axis each day, the effect is that circumpolar stars pivot around Polaris by 15° each hour. This creates a vast 24-hour clock-face in the sky and although the concept takes a bit of getting used to, once you have mastered the idea that every 6 hours Dubhe will move 90° anticlockwise around Polaris you can use this to tell the time. I like to imagine the line from Dubhe to Polaris as the hour hand. When it faces the 1200 position on a clock, Dubhe is culminating. So if you know the time it will be vertically above Polaris, you can compare the position you are seeing it in relation to 1200 position and work out the time. For example, Dubhe culminates around 1600 in July, so if your ‘hour hand’ is at the equivalent of 0900 on a 12-hour clockface, six hours must have passed since culmination so the time would be 2200. While this may not be as precise as an electronic clock, having this knowledge creates a sense of satisfaction and develops confidence that if all else fails you can still find north by looking at the night sky.