Author Topic: CelNav Five - Shooting other Bodies  (Read 760 times)

Offline HenryC

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CelNav Five - Shooting other Bodies
« on: September 14, 2010, 03:06:21 PM »
CelNav Five  (Last Edit = 10/29/14)

By now, you've probably finished the previous exercise successfully and got the right answers, namely an Intercept of 39  miles (Away) and an Azimuth of 128 degrees.  No doubt, you made a few mistakes, but my notes helped you locate them quickly and correct them.  Now that you understand the process, go back and repeat the exercise from the beginning, without my notes.  You have to become familiar with this procedure because you won't have a "right answer" to check your work in the field, and you realize if you make a mistake anywhere every step following that will be wrong.  So do the exercise several times, as well as the examples in Pepperday and the Almanac.  You can buy books of "navigational exercises" to help you practice, and you can create your own exercises using the Naval Observatory online calculator. 

When you finally get to the point where Almanac and Sight Reduction work is second nature, you are ready to go to the beach and shoot real celestial bodies off a real horizon.  You need a  horizon, so the sea or a really big lake (one where the opposite shore is not visible) is necessary .  An eastern horizon is needed for morning sights, a western horizon for evening sights. And make no mistake, you do need a horizon.  You can't shoot any sights in the dark of night, and only the sun and moon can be shot during the day. For folks who live inland, you can use an "Artificial Horizon", which is a reflecting surface set up on the ground, allowing you to use the reflection of an object as a horizon, but that introduces additional issues in observation and calculation. You could use a bubble sextant, where a floating bubble visible through the telescope replaces the horizon, but they don't work well on a rolling platform, like a ship (they were originally developed for aircraft). For more about bubble sextants, read

 At any rate, in my opinion, learning to do the calculations is harder than picking up the physical skills needed to shoot a sight.

Also recall we have only discussed the sun shot (a "sunline", in the trade). You have to also master star, planet and moon shots.  Planet and moon GHA and Dec are listed in the Almanac daily pages alongside the sun's, although the moon has some tricky  parallax corrrections that have to be dealt with.  You may also need to shoot upper limb shots on the moon, depending on its phase. By the time you have mastered the sunline, you will be perfectly qualified to pick up the additional requirements by reading the instructions in the Almanac yourself. Both planets and moon may also have a "v" correction for GHA, similar to the "d" correction for Dec you already know about. Again, by the time you are checked out on the sun, you won't have any trouble teaching these additional wrinkles to yourself.  It's all covered in the "Explanation" pages of the Almanac. If you guys get that far, and need me to teach an advanced seminar on these topics, let me know and I'll be delighted to do so.  But there is no point getting into that untill you have mastered the sunline.

I should speak a little bit now about the star shot. There are 57 named navigational stars, and if we were to give each one its own column in the daily pages so you could read its GHA directly, the Nautical Almanac would be thousands of pages long.  Fortunately, unlike all other bodies,  stars are well outside the Solar System, light years away, and they do not appear to move around the sky, at least, their motions are not noticeable during a human lifetime..  Their only motions are due to the earth's daily rotation and its slow wobble on its axis.  Consequently, The declination of a star does not measurably change over the course of a year, and all their GHAs change together as the earth spins, as if they were all nailed onto a rigid celestial sphere surrounding the earth and spinning as a solid shell.

As a result, the Almanac gives us the GHA of "Aries" (a spot on the celestial equator designated by astronomers as the center of the celestial coordinate system), and each star's Sidereal Hour Angle (SHA), which is it's angular distance from Aries.  You add up the two to get the star's GHA. Again, the Almanac will explain the details on how to do this, and once you're at home with it you can easily read it for yourself.

It is tempting to learn just the sunline, and the related shot, the "noon sun"  for latitude, and forget the rest, and that is certainly better than nothing. (I will devote an entire chapter to the "nooner"  in the near future).  But once you've mastered the sun, the rest is a breeze and there is no good reason  for not expending a little extra effort and learning the other bodies as well.  Planets are bright, easy to locate, identify and shoot.  There are always stars there, plenty of them, enough for a good fix, and the hardest shot of all, the moon shot, is worth it because it is easy to find, even in light fog or overcast, visible during the day and a sun/moon combination is the only way you can get a crossing  fix in broad daylight.

There is one other topic we should talk about before we get back to work.  It should be obvious by now it takes time to shoot and reduce an observation.  And even under ideal conditions there is time between observations.  Under some circumstances, the LOPs that make up a fix may be taken hours, perhaps even days apart.  They may even be from different sources (I once saw a situation where a sunline, a starshot, a visual bearing off a lighthouse, a radar range off an island  and a LORAN line (all gathered over a period of two days, had to be combined to develop a fix. Ships, after all, do move, and sometimes the time elapsed and distance traveled between LOPs is substantial.  A typical celestial fix may be the result of a half dozen LOPs taken over a period of several hours, due to weather delays, or even interruptions from essential shipboard activity.  Even under routine circumstances, a daily fix often consists of a morning sunline, noon latitude, and an afternoon sunline, all DR'd to sunset (where hopefully, evening stars and horizon will be available to verify the estimated position). 

The morning sunline LOP will be a leaning line intersecting the DR track on the chart, like this: /.  The noon shot will be a horizontal, E-W line also crossing the DR track, like this: --.  The afternoon sunline will lean the opposite way of the morning line, like so: \.  Using the ship's known course and speed, and your parallel rulers to slide the lines and dividers to mark off the distance corresponding to your elapsed time, where all three lines cross is your afternoon position.  The size of the triangle formed by those three lines will tell you how reliable the position formed by your running fix is .

And if you couldn't follow what I was talking about in the paragraph above, maybe you aren't ready for a course in celestial navigation yet...
« Last Edit: October 28, 2014, 08:34:34 AM by HenryC »