<p>Having spent an inordinate amount of time comparing the LBR values of Meeus (Astronomical Algorithms 1998) vs. ELP82B (Chapront's ELP82b Fortran Program)
I have figured out the data set I'm using is according to J2000 to wit:</p>
<pre><code>Lambda for the Moon on Julian day: 1241387.69306 is 179°30'48.3&quot; acc. to Meeus
Lambda for the Moon on J2000  is 225°45'0.3&quot; acc. to ELP82b
</code></pre>
<p>I would like to convert the position for J2000 for day: 1241387.69306 to be the position at the date and not J2000.  I thought I might convert my jde: thus:</p>
<pre><code>jde = (jde-2451545.0) / 365.25
</code></pre>
<p>but alas I'm not getting the right numbers.... what am I missing?</p>

CC BY-SA 4.0
2021-09-09T22:11:10.413Z
FWIW, [Horizons](https://ssd.jpl.nasa.gov/horizons.cgi) gives `b1315-Sep-26 04:38:00.384 1241387.693060000 31055.468561 184.4309494 4.1332279`, where the last 3 fields are TDB-UT, Observer Ecliptic Longitude,  and Observer Ecliptic Latitude, respectively. That's for the Moon (301) as the target, with the observer at the centre of the Earth (500@399). TDB-UT is the difference between Barycentric Dynamical time and Universal Time, in seconds. Please see the Horizons docs for further details.
PM 2Ring
CC BY-SA 4.0
2021-09-10T06:16:15.653Z
Horizons uses data developed from DE441.  Meeus copying ELP82's low precision truncated formulae is based on DE200/201.  In addition, since Meeus/Chapront, the IAU has undergone substantial changes in determining constants and epoch ssetting.  e.g. switching the epoch to J2000, changing the definition of the AU and shifting the data to use FK5 referencing scheme.  So I'm well aware of the differences in the numbers that various calculation schemes are using.
Danny F
<time><ephemeris><ecliptic>
J2000 conversion to Position of date
Danny F
# J2000 conversion to Position of date

(Asked by: username_0 on 2021-09-09)

Having spent an inordinate amount of time comparing the LBR values of Meeus (Astronomical Algorithms 1998) vs. ELP82B (Chapront's ELP82b Fortran Program)
I have figured out the data set I'm using is according to J2000 to wit:

```
Lambda for the Moon on Julian day: 1241387.69306 is 179°30'48.3" acc. to Meeus
Lambda for the Moon on J2000  is 225°45'0.3" acc. to ELP82b

```

I would like to convert the position for J2000 for day: 1241387.69306 to be the position at the date and not J2000. I thought I might convert my jde: thus:

```
jde = (jde-2451545.0) / 365.25

```

but alas I'm not getting the right numbers.... what am I missing?

**Tags:** time, ephemeris, ecliptic

> **Comments:**
> **username_1**: FWIW, [Horizons](https://ssd.jpl.nasa.gov/horizons.cgi) gives `b1315-Sep-26 04:38:00.384 1241387.693060000 31055.468561 184.4309494 4.1332279`, where the last 3 fields are TDB-UT, Observer Ecliptic Longitude,  and Observer Ecliptic Latitude, respectively. That's for the Moon (301) as the target, with the observer at the centre of the Earth (500@399). TDB-UT is the difference between Barycentric Dynamical time and Universal Time, in seconds. Please see the Horizons docs for further details.  
> **username_0**: Horizons uses data developed from DE441.  Meeus copying ELP82's low precision truncated formulae is based on DE200/201.  In addition, since Meeus/Chapront, the IAU has undergone substantial changes in determining constants and epoch ssetting.  e.g. switching the epoch to J2000, changing the definition of the AU and shifting the data to use FK5 referencing scheme.  So I'm well aware of the differences in the numbers that various calculation schemes are using.  

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## 0 answers

