# NavList:

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

**Re: Celestial Navigation Data from USNO--alternative**

**From:**Bill Ritchie

**Date:**2019 Nov 28, 11:01 +0000

re / Ritchie-nov-2019-g4624

Yes, Antoine, I suspect the difference is probably due to Delta T, or possibly oblate Earth handling.

For Delta T, Astron uses the NASA formulae in http://eclipse.gsfc.nasa.gov/SEhelp/deltatpoly2004.html.

Between 2005 and 2050, this is

ΔT = 62.92 + 0.32217 * (y – 2000) + 0.005589 * (y – 2000) * (y – 2000)

Using
this for 2037/11/24 14:00:00 Astron
gives **Delta T = ****83.139**.

For the
years 2019-2025, Astron overrides the above with the values from the United
States Naval Observatory __2019 REVIEW__. (No longer accessible on USNO
website). I created the polynomial

ΔT = 69.34 + 0.53 * (y - 2019) - 0.007 * (y - 2020) * (y - 2020)

which fits the review tabular values for those years within 0.01”.

For 2025/12/31 00:00:00 the NASA formula gives 75.049 and USNO review data 72.779, a difference of over 2 secs. I do not have the knowledge to extrapolate this review result to years after 2025, so Astron uses the NASA formula for the subject year, 2037.

Whilst testing this change, I made the following note.

*“**Separate checks on a 10 second reduction
in Delta T gave an increase in GHA of around 5 seconds of arc in Moon
GHA. This could become a rule of thumb! For the Moon, corresponding changes in declination
will vary depending on her declination. They are around zero when declination is
near maximum/minimum, increasing sinusoidally to a maximum of about 40% (occasionally
47%) of the GHA change when declination is zero.”*

For oblate earth correction to parallax correction, Astron code is

const FLATTENING = 1 / 298.257223563; // WGS84 value

const LatCorr = FLATTENING * HP / MINSinDEGREE * SineD(A_LAT) * SineD(A_LAT);

(SineD is bespoke function using degrees i/o radians)

Regards,

Bill Ritchie

50N 003W

re / Ritchie-nov-2019-g4624

Nice comparison indeed !

Which TT-UT (DeltaT) are each of these authors using ?

For the Moon, it seems worth knowing DeltaT in order to perform fully meaningful comparisons.

Kermit