RYA Yachtmaster Ocean. The Moon, Planets and Polaris

Transcription

1 RYA Yachtmaster Ocean The Moon, Planets and Polaris

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3 The Remaining Sights What will I learn in this lecture? This lecture covers topics 6 and 7 of the RYA syllabus on the planets, moon sights, Polaris and compass checking. We put them together in one lecture because if you have successfully worked through the sun and star sight lectures these really are minor variations on the main sight types. Working on this topic Work through the Tiller notes and Chapters 7, 8, 10 and 11 of 'Ocean Sailing' and then complete RYA Exercises 10, 11, 12 and 13. None are very long and many students send them to us as a set. It is up to you but it makes sense if you can work this way. Suggested time Based on the RYA syllabus we suggest you allow about 4 hours plus additional time for the RYA exercises. Finding your way around this lecture CHAPTER 1 - THE PLANETS, MOON, POLARIS AND COMPASS CHECKING... 2 CHAPTER 2 - PLANET SIGHTS... 3 SPOTTING THE PLANETS... 3 THE SIGHT CALCULATION PROCESS... 3 CHAPTER 3 - USING THE MOON... 4 CONSIDERATIONS... 4 MOON SIGHTS... 5 CHAPTER 4 - POLARIS... 7 LATITUDE BY POLARIS... 7 CHAPTER 5 - COMPASS CHECK... 9 WHY DO IT?... 9 QUICK REVISION... 9 TECHNIQUE... 9 NOW WHAT? TRY ONE USING YOUR SEXTANT TO GET THE TRUE BEARING CHAPTER 6 - ANSWERS The other sights Page 1 of 12

4 Chapter 1 - The Planets, Moon, Polaris and Compass Checking We can study these topics together because they are all variations on a theme. You should find few problems with them if you have managed to work with sun sights and they fall reasonably logically into a group of minor RYA exercises. There s not much to add. The sights are all straightforward and the differences between the planets and the moon essentially lie with the corrections applied to the sextant altitude. Both use the AP3270 Vol 3 tables to calculate an intercept and azimuth. Polaris is slightly different because it leads directly to a latitude. It is also one of the few bodies that we can use as a steering aid at night. Compass checking is an interesting and easy exercise. The problem is that, when out of sight of land, we have few ways to check the accuracy of the steering compass. One obvious way would be to use successive GPS positions and knowledge of the most probable ocean currents. It is a way to establish actual ground track, over time.?? What does it NOT tell us? 1 There is a technique that meets our needs, uses the sun, and is elegant, simple and easy. It involves one simple measurement of the vessel s heading at sunrise or sunset and, as you have probably come to expect, the use of a table. The other sights Page 2 of 12

5 Chapter 2 - Planet Sights These are extremely straightforward and, as with the stars, the problem is to identify them. The calculations are simply a variation on the general sun sight approach we used earlier. Spotting the planets The planets are bright bodies that make excellent sight targets when they are visible. The picture on page 11 of the RYA booklet tells us the time of Local Mer. Pass. for each plus its approximate bearing. The path of the planets is given on the back of the Planisphere (if you have one) and you can often pick them out as an extra star on a constellation. Their position, relative to the stars, varies each night. We would expect that the best time for sighting a planet will be during the observation periods for the day in question. The diagram helps us to identify when a planet should be visible. Put another way - we will not be able to see a planet if it is a) below or close to the horizon or b) obscured by the sun's brightness. The best times are close to the planet's meridian passage, when it has a suitable declination and during the relevant observation period.?? Open up Ocean Sailing at Chapter 7 now. It has some useful information about the planets. The sight calculation process Calculating the True Altitude Two corrections may have to be applied to the Apparent Altitude: For ALL sights there is the usual star and planet correction. For Venus and Mars there is an additional correction which is both seasonal and sextant altitude dependent. GHA and Declination The planets move relatively rapidly so we need to take care with the calculations.?? Look at page 16 of the RYA booklet. Check for yourself that: Each of the observable planets has its own column. At the bottom of each column is a value for v and d. They apply respectively to the rate of change of GHA and Declination and are valid for the particular tri-day in the almanac (September 21 to 23 in this example). We need to note the sign of v and d and use both in the same way - if you cannot remember then go back to the lecture on sun sights. Sight Reduction Once we have LHA and Declination for the body we can obtain the intercept and azimuth in the normal way. The other sights Page 3 of 12

6 Chapter 3 - Using the Moon The moon is quite bright and is an easily visible body for much of the time. We can use it for: Meridian Passage sights to calculate our latitude. A conventional sight using AP volumes 2 or 3 as appropriate. Considerations There are some peculiarities with this sight which relate to the fact that the moon is bright, nearby and has somewhat irregular habits! Brightness We need to be careful with a very bright moon - it can cause us to perceive a false horizon at night and thus create a potentially huge error. The brightness can also allow a moon sight to be crossed with a sun sight to give us a good fix if the angle of cut is acceptable Irregular habits Orbit time The moon orbits the earth in a period of around 23h 10 minutes. This means that the LMT is NOT the same on all meridians and we need to correct for this if we plan to use the moon for a Meridian Passage sight. It is a simple process and to find UT Mer. Pass for the moon: 1. Extract Moon Mer. Pass - usually the UPPER value is used - the LOWER one is only applicable to high latitudes and can generally be ignored Take the difference between LMTs. E longitude - compare with the previous day. W longitude - compare with the next day. 2. Multiply by your longitude to the nearest quarter degree or so, divide the result by 360 and round to the nearest minute in time. This gives the corrected LMT Mer. Pass for your longitude. Here s an example. If our longitude were 090 W and we want to find UT Mer. Pass on September 22 nd we would work as follows: Mer. Pass Moon on 22 September is 22h 34m and for 23 September it is 23h 29m. The difference is 55 minutes and this means that as the moon orbits the earth its LMT Mer. Pass is changing over the 23 hours by that amount. We can guess that ¼ way round the earth at Longitude 090 W it will be ¼ of the way through its time change or 13m 45s. so LMT Mer. Pass will be 22h 48m for planning purposes. Applying the formula gives the same result; i.e. 55 x (90 /360) = 13m 45s. Now correct as usual to get UT Mer. Pass. for the moon by applying the normal longitude in time correction. The other sights Page 4 of 12

7 The declination changes quite fast so the time of Mer. Pass and, indeed, the Sextant Altitude may be difficult to observe precisely. Moon sights The problem is that the moon is relatively close to the earth (1/4 million miles or so) and the distance varies. The correction factors are therefore slightly more complex. You corrected the AA to TA in a Lecture 1. Taking the sight It s an easy and big object which can be easily sighted: Take the upper or lower limb as with the sun. The difference is that the decision will depend on which limb is fully visible, unless the moon is full. At quarter moon one limb or the other will be visible. The only real constraints are at the time of new moon, when it cannot be seen at all, and during periods of cloud cover. Corrections for the moon?? If you cannot remember how to correct from AA to TA for the moon then now is the time to revisit Lecture 1. The sight reduction process is similar to that for the sun and planets. Here are the main differences: The rapid movement of the moon means that we can no longer use a single figure for v and d over a 3 day period (as in the planets). Instead they are tabulated for each hour. The v (GHA) and d corrections (Declination) are tabulated hourly (see RYA page 17) plus the HP value. 1. Extract all three into the sight form. 2. Follow the rules on page 24 of the RYA booklet for calculation of the TA. 3. Use the applicable v and d to calculate the moon s GHA and Declination. 4. Proceed as usual to obtain intercept and azimuth. If this has totally confused you then don t worry! There s a worked example overleaf. The other sights Page 5 of 12

8 ?? Try an example: Sight data Evening twilight on 21 September EP S, W Sight of moon s lower limb gave an S A of at UT 18h 35m 16s IE 2.8 ON the arc and height of eye 3.2m What were the chosen position, intercept and azimuth? Sextant Alt IE Obs Alt Dip 3.2m App Alt Alt Corr n 59.9 HP True Alt GHA Moon21d 18h inc 35m 16s v = GHA Chosen Long LHA 300 Chosen Lat 50 S Declination 21d 18h d = 35m 5.0 Declination S decreasing S Latitude and Declination have SAME name (Southerly), so use RYA pages 30 and 31 to find entry with LHA of 300. Hc = 29 49, d = +46 so correction from table (pages 46/7) is 28 and added because the Hc increases with each degree of declination. Hc is therefore We can check that this is correct by looking at the Hc for declination of 15 (see pages 38 and 39 of the RYA booklet and LHA of the value of is (value of d ). The tables work! Z =104 and (from the bottom of page 30) if LHA > 180 (it is) then Z N = Z Z N therefore = = 076 Intercept is (rounded to nearest minute) = 73 Away (GOAT). There s no better time to work on and submit RYA exercise 12! The other sights Page 6 of 12

9 Chapter 4 - Polaris You probably know that the Pole Star is a star which is in the North. In other words it is always visible at night in N Latitudes and is called a circumpolar star. Being circumpolar, we perceive it as rotating in a full circle every 24 hours with a diameter of about 1.5. So, incidentally, do all the stars but, of course, they spend a lot of their time below our visible horizon. Strictly speaking a circumpolar body is a heavenly body which is above the (observers) horizon at its lower meridian passage (i.e. its polar distance is less than the observer s latitude).?? Chapter 10 of Ocean Sailing also covers the reduction of a Polaris sight. Look at it now. Latitude by Polaris A Polaris sight, once reduced to its True Altitude, actually gives a latitude to within about two degrees without further calculation. As always we can obtain a much more accurate result by some simple arithmetic. Here s the background: If Polaris were exactly over the N Pole then its True Altitude would equal the latitude. It isn t, though it is close, and we have to apply some corrections. They are to be found in the Nautical Almanac (page 18 in our books). Taking the sight Set the sextant to estimated latitude and look North: Polaris is not a bright star and is most easily observed in the morning twilight. Locate it relative to the Plough or Ursa Major and get the pattern set in your mind. Polaris can often be acquired in the sextant s telescope before it can be seen with the naked eye in the morning. This gives more time to locate the star than waiting till the evening, when the horizon may be lost before the star is positively seen. If possible, as always, take more than one sight and pick the best. Processing the sight This is quite straightforward but there are three corrections to apply. Each is tabulated and easy to extract (RYA page 18). a0 applies a correction related to the LHA of FPA. a1 corrects for latitude. a2 corrects for the time of year. Here s how to work it out: 1. Calculate the GD and LHA: Check for Greenwich day change. Use the EP or DR longitude - we do not need a full CP. From tables extract GHA Aries to the nearest minute. Adjust for DR longitude to work out LHA. 2. Convert Sextant Altitude to True Altitude: Apply Polaris corrections. From the Polaris table (RYA page 18) extract the three a corrections. ADD them to the True Alt and then subtract 1 degree to give the latitude. You do not have to calculate 90 -TA to give the Zenith Distance (in contrast to the calculation of latitude using The other sights Page 7 of 12

10 the sun s Mer. Pass). Polaris is above the north pole rather than the equator and, hence, TA = ZD = Latitude.?? There s a worked example at the bottom of RYA page 18 to help you see how the process works so study it now. IN FACT there is still a small error. Look at the Azimuth table at the bottom of RYA page 18. Here s the problem. We assume that a parallel of latitude is at 90 to True North South. That holds true for all sights except Polaris. Depending on the LHA and Latitude the actual azimuth of Polaris can be up to 2 degrees off True North. Technically, therefore, the Polaris position line will have a bearing that differs from the parallel of latitude. In practice, this introduces a negligible error that can be ignored. Steering by Polaris is also straightforward For practical purposes Polaris lies due North of us. This means that if we ever damage our ship s compass we can use Polaris by steering on a RELATIVE angle. A couple of examples will make the point: 1. Keep Polaris a little aft of the Starboard lower shroud might give us a course of around 45 DIFFERENT from due North in my boat. 2. It might be worth carrying a Pelorus or sight bearing ring of some sort so we can measure angles relative to the boat with reasonable accuracy.?? Try a Polaris Latitude: June 20th in EP 31 N and W. Polaris sighted in evening twilight at 20h 28 m30 s UT. SA was IE 2.4 on the arc. Height of eye 2.5m. What was our latitude??? First work out the LHA Aries. 2?? Now work out the TA for the sight. 3?? Now use the Polaris table to apply the corrections and work out the latitude. 4 And now you can work on and submit RYA Exercise 11. The other sights Page 8 of 12

11 Chapter 5 - Compass Check Why do it? When sailing across the ocean there is no obvious way to be sure that the compass is accurate. Many things can influence it including damage and accidentally induced deviation. Even something as small as an electrical screwdriver carefully stowed for safekeeping next to a compass can induce a large error. There s an interesting and easy way to check the compass and it can be done twice per day if required. That is the topic of this chapter and Chapter 11 of Ocean Sailing also covers this topic, albeit fairly briefly. Quick revision Remember your variation and deviation - you may recall C D M V T and CADET. Alternatively, try 'TRAWMA'; i.e. TrueAddWestMAgnetic. Technique We are going to measure the bearing of the sun just as it rises or sets and then compare it with, you ve guessed it, a tabulated value. Allow for variation (from the chart) and the difference is the deviation on that heading. Sounds simple and it really is. You can try it anytime the sun is visible at, or very close to, the horizon. Measure the sun s bearing as it rises or sets This is the first step and there are several ways to achieve it: 1. Steer boat directly towards the sun for a few moments. 2. Read across the steering compass to get a bearing of the sun. 3. Anything else that gives an accurate bearing of the sun relative to the boat s steering compass. Take the bearing as the sun s Lower limb is about a half diameter above the horizon. Because of atmospheric refraction this is the point at which it is actually just on the horizon. We have now measured the sun s COMPASS bearing when it has an altitude of zero. Calculate the sun s true bearing at that time Tables give us EITHER the sun s AZIMUTH or its AMPLITUDE. Reeds gives the azimuth, whereas Nories tabulates the amplitude. What s the difference? Amplitude and Azimuth North Observer on yacht This is the Azimuth To sun at sunrise This is the Amplitude A. D. Thomson Amplitude - the sun s true bearing at sunrise or sunset measured from E (rising) or W (setting) towards N for N declination and S for S declination. The other sights Page 9 of 12

12 Azimuth - the bearing of a heavenly body measured from N or S. See page 25 of the RYA Booklet for an excerpt of the table. It gives us the Azimuth (the sun s true bearing - relative to North or South) for a limited range of latitudes and declinations. Use the table 1. From the assumed position, and knowing the Greenwich Date, we can find the declination. 2. From the table we determine the Tabulated Bearing of the sun. 3. We now need to convert it to the True Azimuth by taking into account the name of the declination and whether it was rising or setting when the bearing was taken. 4. The rules are simple: a) Rising and declination N True Azimuth = 000 plus Tabulated bearing. b) Rising and declination S True Azimuth = 180 minus Tabulated Bearing. c) Setting and declination N True Azimuth = 360 minus Tabulated Bearing. d) Setting and declination S True Azimuth = 180 plus Tabulated Bearing. Work out the variation 1. We will need a chart to work this out. Remember that variation changes significantly on an ocean passage. For example, in 1992 the variation changed from about 9 W near the West Indies to 17 W in the middle of the Atlantic to around 3 W in the English Channel. 2. If we ignore variation we will have some significant errors on both our actual course and when checking the compass. Now what? We know: The compass bearing of the sun. The true azimuth. The variation. One simple way to calculate the deviation is as follows: True Azimuth plus/minus variation = magnetic bearing. Magnetic bearing plus/minus deviation = compass bearing. The difference between the magnetic and compass bearings is what we are trying to determine - the DEVIATION ON THAT HEADING (i.e. as we steer towards the sun, if that was the course we chose, or the actual ship s head if we measured the sun s angle relative to the ship s head with a pelorus or similar device). Try one The bearing of the sun at sunrise was 081 C. Declination was 11 N and our EP was 50 N 12 W Variation 4 W. What is the compass deviation, what would the GPS ground track have been showing if there was no appreciable leeway or ocean current and the vessel was aimed at the sun as it rose? From the table the sun s bearing = 72.7 N and E. True Azimuth = = 73 (rounded). Magnetic azimuth = 077. Compass azimuth = 081 C. Deviation therefore = 4 W. GPS COG would have been T Using your sextant to get the true bearing?? Think back for a minute. What is Zn and how do we obtain it? 5 The other sights Page 10 of 12

13 If we can measure the sun s actual bearing and, at the same time, use the sextant to take a sight of it then we can carry out a compass check at any time. Actually the sun s altitude (or any other body come to that) must be sufficiently small that the errors in measuring its azimuth by the ship s compass are within a degree. This is real world stuff and we can accept some significant errors (in astro navigation terms) without affecting the answer. Armed with the MEASURED bearing of the sun, a rough knowledge of our position, and the sextant altitude of the sun at a known time and date we can work out the deviation of the steering compass. Sounds unlikely? Here s how to do it. 1. Work out the true azimuth of the body - Zn. Calculate LHA and declination for body. Enter table and look up Z - we can discard the rest of the sight information - and work out Zn. 2. Proceed as before to calculate the deviation. Now is the time to work on and submit RYA Exercise 13.?? There is no more new astro to learn. You should now study the lectures on World Weather and Passage Planning. The other sights Page 11 of 12

14 Chapter 6 - Answers 1 The words ground track say it all. This is the vessel s actual progress over the ground and, by definition, it includes both the compass error and the effect of any ocean currents (covered in a later lecture), not to mention the steering errors caused by the helmsmen and an autopilot if one is fitted. This technique will lead to very accurate navigation but cannot solve our need - to establish the accuracy of the steering compass. 2 FP Aries Inc 28m 30s GHA Aries Long LHA Aries W 199. We ve corrected to a whole no. LHA but it is not, strictly, necessary. 3 Sex Alt IE Dip App Alt Alt Corr True Alt From tables: a a1 0.6 a2 1.0 Total corr n Subtract 1 Correction Latitude We know this one - or at least we should by now! Zn is the true azimuth of the observed body at the time and date of the observation. Sounds rather like the true bearing of the sun doesn t it? The other sights Page 12 of 12