# NavList:

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

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Re: Aldebaran occultation
Date: 2017 Mar 5, 22:40 -0500
Alex

I note that you provided your position to the nearest 0.1'.

Could you return to the spot and get a more precise answer?  With greater numbers of GPS satellites involved in the fix, the error in the fix diminishes.  Long term averaging, if your device provides it, also improves the accuracy.

I don't know how sensitive the parallax is in the occultation situation but it occurs to me that this could be an additional source of error.

0.1' = 182.5 meters

At 40°27.2'N, that's a variation of 139/2 meters in longitude (0.05' in E/W from your meridian)

In latitude, there is no reduction.

At the most extreme location, you can still conceivably be
Sqrt (139^2 +183^2) / 2 = 115 meters from the given position, and still not have to change the last digit.

That is 40°27.2' N +/- 0.05'  86°58.8'W +/- 0.05'

It may be worthy of experimentation to adjust the position of observation within those parameters to see the effect of parallax on the occultation.

Frank has indicated the error due to limb roughness could be as much as 0.3'. (If I have misquoted you Frank, please jump in!)  However, that's limb position non specific.  We would need to know the roughness at the specific limb location, given your observation of Alderbaran and Moon contact.

In 1916, when stranded on the ice in the Antarctic night, Worsley and Reginald James rated the chronometers using occultations. In Shackleton's 'South', Appendix 1 Reginald James makes the assertion.   I've always been quite interested in occultations as a result.  Not for probing the shape of the limb, but as a tool for determining longitude or time.

On Mar 5, 2017 8:29 PM, "Paul Hirose" <NoReply_Hirose@fer3.com> wrote:

```On 2017-03-05 14:03, I wrote:
> On 2017-03-04 22:48, Alexandre Eremenko wrote:
>> The moment of disappearance I timed as March 5, 3:56:48 (GMT)
>>
>> 40d27.2'N, 86d55.8'W
>
> My Lunar4 program says Aldebaran is .02′ inside the Moon's limb at that
> time. It puts the occultation 4 seconds earlier.

The USNO MICA program agrees with Lunar4. Topocentric coordinates of
both bodies from MICA:

267.63519° 61.59173° Moon az, ZD
267.71641° 61.33040° Aldebaran az, ZD

That equals .27090° separation angle. MICA says semidiameter is 16′
16.4″, which is .020′ greater than the separation angle.

However, neither program accounts for the irregularities in the Moon's
limb, which I believe are significant at this precision. My
computational tools are not accurate enough to find any error in the
observation by Alex.

```

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