Hello

[PeterB]

Peter,

Will you please explain the meridian t angle stuff.  I've read about it but it has always been too difficult to understand.  Why not just use LHA?

Thanks,

Carlos

Carlos

Both the local hour angle (LHA) and the meridian angle represent the angular difference in longitude between yourself (or your chosen assumed position) and the longitude of the geographic position (GP) of the body.  This is the angle at the pole vertex of the navigation triangle.

The only difference is that LHA is ALWAYS measured ONLY westward from 0 to 360 degrees
A meridian angle ( t ) can be measured either eastward or westward.  Usually they are from 0 to 180 degrees. In some well respected texts this ONLY WESTWARD requirement of all Hour Angles is dealt with a bit loosely which can add to confusion.

A westward LHA of 270 from the AP would ALSO be a meridian angle of 90 eastward.

If the meridian angle just so happens to be measured westward it is the same as the LHA.
A meridian angle of 45 degrees westward would be the same as a LHA of 45 degrees.

Some forms of sight reduction use meridian angles instead of LHA simply for compactness. H.O. 211 Ageton's being a good example. They can do this because the trig values used to solve the nav triangle repeat as you progress from 0 to 360 degrees -- but the plus and minus signs change -- so you need to apply certain "rules" to keep track of things properly when using those methods.

When working by calculator you must apply some simple deductive logic instead -- usually to figure out how to get a proper true azimuth (Zn) from your calculator result.

e.g. If the AP lat is north of the body's dec your Zn will be in a generally southward direction because the GP is south of your AP; AND if the LHA is less than 180 the Zn will be in a generally westward direction. Note that here I revert back to the LHA for reference, not the meridian angle since the meridian angle can go either east or west.

When I convert a LHA into a meridian angle I always make a note with the meridian angle if that is now being measured eastward or westward to save future confusion.

I hope this helps. I'm happy to answer more questions if you have them.

Peter