# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: getting GHA from MICA
From: Mike Burkes
Date: 2014 Mar 21, 12:51 -0700
Hi folks,cannot resist this;Was not MICA the sheriff played by Paul Fix on TV series The Rifleman? OK let me have it!!!
Mike Burkes

From: cfuhb-acdgw---.net
To: m_burkes---.com
Date: Wed, 19 Mar 2014 14:56:02 -0700
Subject: [NavList] Re: getting GHA from MICA

```Sean C wrote:
> Question: Is calculating the sidereal time and then the apparent geocentric equator of date position the only way to get the GHA of a body?

I don't have the latest MICA version, but 2.0 has "Apparent Topocentric
Equator of Date (LHA, Dec)" as one of the choices for position type. In
theory, that will give you GHA if the topocenter is very close to a pole
and at 0 longitude.

For instance, at 2014 March 19 at 1700 UT1, compare Moon LHA from N89 59
59 W000 00 00 to a GHA computed by subtracting the geocentric apparent
RA from Greenwich apparent sidereal time.

220 01 11.6  topocentric LHA
220 01 11.5  RA - GAST

I converted LHA from hours to degrees.

If you set the topocenter at exactly 90 north, MICA's LHA is a couple
degrees in error. Of course one could argue that LHA and azimuth are
undefined at a pole. However, my software does the common sense thing
and computes as if you were on the specified longitude and an
infinitesimal distance away from the pole. That happens with no extra
effort by the programmer if the coordinate transformations utilize
vectors and rotation matrices.

--
I filter out messages with attachments or HTML.
```
View and reply to this message: http://fer3.com/arc/m2.aspx?i=127287
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