1. The Marine Sextant Learning ObjectivesLearning Objectives Know the purpose of a marine sextant.Know the purpose of a marine sextant. Apply proper procedures to determine the observed altitude (Ho) of a celestial body.Apply proper procedures to determine the observed altitude (Ho) of a celestial body.

2. The Marine Sextant A marine sextant is nothing more than a device designed to measure the angle between two objects with a great deal of precision.A marine sextant is nothing more than a device designed to measure the angle between two objects with a great deal of precision. In celestial navigation, these two objects areIn celestial navigation, these two objects are –a celestial body (star, sun, moon, or planet) –the visible horizon

4. Use of the Sextant A sextant is used to determine the sextant altitude (hs) of a celestial body.A sextant is used to determine the sextant altitude (hs) of a celestial body. First, we have to decide which stars to observe; this is done using a Rude Starfinder or other methods.First, we have to decide which stars to observe; this is done using a Rude Starfinder or other methods. When making an observation, the star should look as shown in the next slide...When making an observation, the star should look as shown in the next slide...

6. Determination of Observed Altitude (Ho) We must make some corrections to hs to come up with the Ho, which we need to use the altitude-intercept method.We must make some corrections to hs to come up with the Ho, which we need to use the altitude-intercept method.

7. Determination of Observed Altitude (Ho) These corrections account forThese corrections account for –index error (error in the sextant itself) –difference between visible and celestial horizon, due to the observer’s height of eye –adjustment to equivalent reading at the center of the earth and the center of the body –refractive effects of the earth’s atmosphere

8. Index Correction There may be some error present in the sextant itself; this is known as index error.There may be some error present in the sextant itself; this is known as index error. This is easily determined by setting the sextant to zero and observing the horizon; if there is no error, the view looks like the next slide...This is easily determined by setting the sextant to zero and observing the horizon; if there is no error, the view looks like the next slide...

10. Index Correction However, often there is a slight error. In this case, the view looks a little different…However, often there is a slight error. In this case, the view looks a little different…

12. Index Correction To account for this error, we apply an index correction (IC).To account for this error, we apply an index correction (IC).

13. Dip Correction Next, we must account for the difference between the celestial horizon and the visible horizon, due to our height of eye.Next, we must account for the difference between the celestial horizon and the visible horizon, due to our height of eye. This is known as the dip correction.This is known as the dip correction. The need for this correction is illustrated on the next slide...The need for this correction is illustrated on the next slide...

15. Dip Correction The dip correction is dependent upon the observer’s height of eye.The dip correction is dependent upon the observer’s height of eye. Values of the dip correction are tabulated inside the front cover of the Nautical Almanac.Values of the dip correction are tabulated inside the front cover of the Nautical Almanac.

16. Apparent Altitude Now, by applying the IC and the dip correction, we can determine the apparent altitude (ha).Now, by applying the IC and the dip correction, we can determine the apparent altitude (ha). Simply put,Simply put, ha = hs + IC + dip ha = hs + IC + dip

17. Altitude Correction The last major correction accounts for the refractive effects of the earth’s atmosphere.The last major correction accounts for the refractive effects of the earth’s atmosphere. This correction is known as the altitude correction and is tabulated inside the front cover of the Nautical Almanac.This correction is known as the altitude correction and is tabulated inside the front cover of the Nautical Almanac. The next slide illustrates the need for this correction...The next slide illustrates the need for this correction...

18. Altitude Correction

19. Determination of Ho The corrections needed to convert from the sextant altitude (hs) to observed altitude (Ho) areThe corrections needed to convert from the sextant altitude (hs) to observed altitude (Ho) are –IC (sextant error) –Dip (height of eye) –Altitude (refractive effects)

20. Additional Corrections These corrections are all that are needed under normal circumstances to determine Ho of a star.These corrections are all that are needed under normal circumstances to determine Ho of a star. An additional correction is required if the observation is made under non-standard conditions of temperature or pressure.An additional correction is required if the observation is made under non-standard conditions of temperature or pressure.

21. Additional Corrections If we are using the sun, moon, or planets, the problem becomes a bit more complicated.If we are using the sun, moon, or planets, the problem becomes a bit more complicated. In addition to the corrections we already mentioned, we must also accout forIn addition to the corrections we already mentioned, we must also accout for –horizontal parallax (sun, moon, Venus, and Mars) –semidiameter of the body (sun and moon) –augmentation (moon)

22. Additional Corrections These additional corrections make determination of Ho for the sun, moon, and planets generally more difficult than those for a star.These additional corrections make determination of Ho for the sun, moon, and planets generally more difficult than those for a star. For simplicity’s sake, we’ll stick to determination of Ho for a starFor simplicity’s sake, we’ll stick to determination of Ho for a star

23. Use of a Strip Chart To walk us through the calculation, we normally use a form, called a strip chart.To walk us through the calculation, we normally use a form, called a strip chart. An example of a strip chart used for calculating Ho of Dubhe is shown on the next slide...An example of a strip chart used for calculating Ho of Dubhe is shown on the next slide...