This need to be converted to the ship’s time so that we know at what time (ship’s time) will be nautical twilight. The twilight time given in the Almanac are LMT. Or in other words, what is the time for start of nautical twilight?Ĭivil twilight time mentioned in the almanac is the time of “end of civil twilight” and “start of the nautical twilight”.įor evening star sights, this is the time we would be interested in. So for evening twilight, can you guess what time you should start looking for the stars for star sight?
That would be the time we need to start looking for the visible stars.įor the evening twilight, here is how the period from sunset to complete darkness is divided. So for morning star sights, we need to calculate the nautical twilight time from the almanac. With respect to times mentioned in the nautical almanac, this would be the period between the time of “nautical twilight” and “Civil twilight”. We are interested in the period of Nautical twilight. In the morning, the period from complete darkness to the sunrise is divided into three twilights. The twilight time is given in the nautical almanac.
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The name “Nautical twilight” is given because this is the time when mariners can see both the horizon and stars and is ideal for the star sight.īut how to know the time for the Nautical twilight? Nautical twilight is the time when some stars may be visible and during the hours of twilight there is still some daylight that horizon is visible too. Then how do we measure the sextant altitude of the stars? Or, when the horizon is clearly visible, there aren’t any stars in the sky and when the stars are visible, the horizon isn’t visible. For measuring the sextant altitude two things should be visible.īut the issue with the star sight is that, stars are visible at night and horizon is visible when there is daylight. Here is a video that explains the use of sextant. Sextant altitude is measured by bringing the celestial body on the user’s visible horizon and reading the altitude from the sextant. I won’t go more deep into the calculations part. Zenith distance + True Altitude = 90 Degrees.Īll we want is True altitude of the celestial body which can be calculated by measuring the altitude of the celestial body by a sextant and applying few corrections to the measured altitude by sextant. Zenith distance and altitude of the celestial body are inter-related. You can read this article by clicking on here.Īnd if you have read it you would know that celestial navigation is based on calculation of zenith distance and azimuth of the celestial body. I have covered the basics of the celestial navigation in a different article.
In this post, I will show you step wise procedure to use the star sight to calculate your position on board. But how can we use these to calculate the sight ? Most of us know all these calculations but only that much. Talking specifically about star sight, there are tons of resources that deals with the calculation part.īe it Longitude by chronometer or intercept method calculations. How do you apply all of this knowledge practically on board the ship. Which means there is also a lot of noise to sift through.Īnd when you do sift through it, you still have to answer one question: These days, there is an enormous amount of information available on celestial navigation.
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