Sight Reduction practice idea

[wooslehunter]

I decided to learn some astro nav this winter. I wondered how to practice sight reductions & found a simple way to get data to work with. Anyone got any ideas.

I came up with the idea of using a computer star chart to give me the altitudes to reduce and plot. You can simulate being anywhere in the world at any time.

The one I'm using is called Skymap and available from wwwskymap.com. There are lots of others around so if anyone nows of a freebee one I'd like to hear. I'm going to have to buy Skymap soon since the evaluation time is running out.

Just set up the time & position of the observer and you can get the apparent altitude for any body. The only correction you need to apply is for refraction & the positions seem quite accurate.

Now I need to get a sextant and do it for real but at least I know the reduction bit is OK now so when I reduce my first real sights the huge errors will be me and not the method.

 

[aitchw]

I have a spare copy of Redshift 3 you can have if you want. It's not a copy and totally legit. It cam efree with some bundle and I already had Redshift 4.

PM me if interested.

Howard

 

[AndrewB]

This is of limited usefulness, for three reasons. First, as you say there are a number of important corrections that have to be made to actual observations of time and altitude before the main part of sight reduction is done. These can't all be simulated, so really you are only doing part of the job. Second, unless you have a world set of sight reduction tables or are going to use your computer for this (which rather defeats the object), you will be limited in what you can simulate. Third, you will have to enter your supposed exact time and position in order to get an altitude, so you know the correct answer before you start - in my experience this tends to reduce mistakes which you may think an advantage, but coping with them under uncertainty is actually an important part of the training process.

All the standard primers for astronav are loaded with exercises which will be far more realistic. Plus they include the right bits of the tables that you are going to need, without giving too much away. I'd have thought this preferable.

Even if you live nowhere near the sea, it is possible to practice the whole process of taking a sight inland with your sextant, by using a reflector on the ground - a bowl of oil or a carefuly levelled mirror. The method was discussed fully on one of these forums some years back.

 

[alex_rogers]

Have a look at Ocean Navigator online.

http://www.oceannavigator.com/

They have a monthly astro-navigation problem which is normally based around some interesting historical nautical story. The answers are presented in the following month and they all use the current nautical almanac.

 

[stephenh]

I agree with AndrewB, but if you need an 'almanac' try "Navigator Light" - it's shareware. Free....permanent and very good.

and thanks to alex-rogers - thats a good site.

 

[wooslehunter]

Thanks for the info. I'll have a look at the links.

I agree it's only doing part of the job since you can't simulate things like index error, dip or the other corrections needed for the moon, sun and planets.

BUT you can "take the sights" at different times and if you really want at different positions anywhere on the globe.

It's just a way of getting some data to work with.

I also agree that if you know where you are it's easier to not make mistakes. BUT it is easy to know that you have made mistakes. If you do it right all of the position lines should cross very closely. Ive done quite a few now and from time to time one or two of the lines come out shifted a little - 5 miles or so. This would be acceptable in a real situation but perhaps not so when the altitude data is guarenteed to be close.

Anyone know what kind af accuracy you should get with the air nav tables? I do have the ones for this epoch.

I'm fully intending to practice this for real with an artificial horizon - bowl of old engine oil - once I get organised. Apart from doing this for fun, the day you really need it is the day when then everythings gone U/S and all you have left are the tables & a pencil. There's load of calculators available but I still believe the best way is to be able to do everything the old fashioned way - it's more fun and far more saticfying anyway.

 

[snowleopard]

you can practice in the garden at home by making your own artificial horizon - use a dish filled with oil and halve the angle. as you'll be making the sun line up with itself rather than a horizon you'll need to make appropriate adjustments for sun's semi-diameter.

 

[Gordonmc]

Artificial horizon devices are available for sextants. I live less than a couple of hundred metres from the beach, but I only have about 15 degrees of horizon because of headlands and islands, so the horizon is useful for practice.
I use a software star identification system from Nomad Electronics which works well on an iPaq. The cost was just under $40 for a year's licence.