<p>So I've been having fun with trying to determine where the perihelia of different planets are (using JPL Horizons).</p>
<p>And by the time I got to Saturn, something weird started happening. I've noticed that at some moments true anomaly (that I've used to track perihelion time) started falling rather than rising. I've also noticed that it seems to be only a slight effect, which also demonstrated ~ 17 days period. So I though: perhaps that's the effect of Titan, right? Problem solved.</p>
<p>Yet now I look at Neptune's true anomaly (see the last column on the picture) and <em>have absolutely no idea what's going on</em>.
Can somebody explain, please? How is something like that even possible? It's like the planet decided to go backwards for a change.</p>
<p>My settings are:</p>
<p>Ephemeris Type [change] :   OBSERVER</p>
<p>Target Body [change] :  Neptune [899]</p>
<p>Observer Location [change] :    Sun (body center) [500@10]</p>
<p>Time Span [change] :    Start=1900-01-01, Stop=2100-12-01, Step=1 Y</p>
<p>Table Settings [change] :   QUANTITIES=18,41</p>
<p>Display/Output [change] :   default (formatted HTML)</p>
<p><a href="https://i.sstatic.net/zQ4JG.png" rel="noreferrer"><img src="https://i.sstatic.net/zQ4JG.png" alt="enter image description here" /></a></p>

CC BY-SA 4.0
2018-12-11T08:18:39.410Z
barrycarter's [answer](https://astronomy.stackexchange.com/a/28704/7982) is right. You can read more about osculating elements and why you should use them carefuly or not at all in these answers: [1](https://space.stackexchange.com/a/24278/12102), [2](https://space.stackexchange.com/a/25224/12102), [3](https://space.stackexchange.com/a/23264/12102)
uhoh
<orbit><orbital-mechanics><orbital-elements><neptune>
Why is Neptune true anomaly decreasing?
Eugene B.
# Why is Neptune true anomaly decreasing?

(Asked by: username_0 on 2018-12-09)

So I've been having fun with trying to determine where the perihelia of different planets are (using JPL Horizons).

And by the time I got to Saturn, something weird started happening. I've noticed that at some moments true anomaly (that I've used to track perihelion time) started falling rather than rising. I've also noticed that it seems to be only a slight effect, which also demonstrated ~ 17 days period. So I though: perhaps that's the effect of Titan, right? Problem solved.

Yet now I look at Neptune's true anomaly (see the last column on the picture) and *have absolutely no idea what's going on*.
Can somebody explain, please? How is something like that even possible? It's like the planet decided to go backwards for a change.

My settings are:

Ephemeris Type [change] : OBSERVER

Target Body [change] : Neptune [899]

Observer Location [change] : Sun (body center) [500@10]

Time Span [change] : Start=1900-01-01, Stop=2100-12-01, Step=1 Y

Table Settings [change] : QUANTITIES=18,41

Display/Output [change] : default (formatted HTML)

[![enter image description here](https://i.sstatic.net/zQ4JG.png)](https://i.sstatic.net/zQ4JG.png)

**Tags:** orbit, orbital-mechanics, orbital-elements, neptune

> **Comments:**
> **username_1**: barrycarter's [answer](https://astronomy.stackexchange.com/a/28704/7982) is right. You can read more about osculating elements and why you should use them carefuly or not at all in these answers: [1](https://space.stackexchange.com/a/24278/12102), [2](https://space.stackexchange.com/a/25224/12102), [3](https://space.stackexchange.com/a/23264/12102)  

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## 0 answers

