# Why does this plot show Starman's speed relative to the Sun fluctuating so often?

After the launch of Starman, during the inaugural flight of SpaceX's heavy lift vehicle, the Falcon Heavy, PearsonArtPhoto set up a website to track various stats about Starman and his Roadster.

A recent addition to the site was the distance from Starman to the three main astronomical bodies of interest – The Sun, The Earth and Mars – and the speed of Starman (and his Roadster) relative to those three bodies.

In the plot of the relative speed for Starman, the relative speed of Starman to the Sun, although it has the general shape of a sine wave, seems to fluctuate quite often. Why is this?

Plot (click to embiggen):

• I counted the oscillations of the yellow graph, divided the days by it and found out it has about a 25 day period. I googled that, (surprised by the result I appended ‚orbit‘) and found this. I guess they just picked a random spot on the surface of the sun as a reference point, that is rotating with the sun and thus fluctiating this strongly? Just a guess, though.
– Hans
Mar 20, 2018 at 22:58
• You know, that makes perfect sense. Huh... Mar 20, 2018 at 23:08
• @PearsonArtPhoto from here it looks like this could not have been an accident. Huh...
– uhoh
Mar 21, 2018 at 7:36

As was astutely noted by Hans, the period of the movement was about 25 days. It turns out that is the time it takes for the sun to rotate once. When I was grabbing the data from JPL Horizons, I listed the target (center) as "coord@10". I should have omitted the "coord", as that means coordinates, or in other words, a point on the surface. Without that, it assumes the center, with it, it will assume a point on the surface. Removed that, and the plots now looks as below. Note that the diameter of the Sun is very large, and thus the rotational velocity is significant, enough to create the motion that was observed.

• Wait, I'm having difficulty reproducing this explanation. i.stack.imgur.com/fJbKM.png How is it possible to enter "coord@10" and get results back? update: I've asked as a separate question: What does “coord@10” mean exactly as a target for JPL's Horizons? How to access it?
– uhoh
Mar 21, 2018 at 6:44
• Seeing as this is about as canonical as canonical can get I've accepted it. Thanks for the insight! Mar 21, 2018 at 8:00
• @uhoh One assumes that perhaps PearsonArtPhoto was using some custom template that takes something in the form "coord@10" and feeds an actual default coordinate value into NASA's system. Mar 21, 2018 at 18:06
• There are a lot of fields to enter in a target with the "Vector" format. I chose the wrong one, at the very bottom, which added the coord@ system. Looking more carefully, I'm realizing there is a better field, but... Horizons is not the most user friendly system in the world! Mar 21, 2018 at 18:25

A high frequency ~20% modulation of the velocity of an object in deep space in a heliocentric orbit with respect to the Sun should have been highly suspect. Clearly it is unphysical, and must be a programming oversight.

There have been other obvious bugs in the site discussed earlier as well; (wrong distances and Roadster stopping in its own tracks for eleven days), and so the information on this site should be considered qualitative and not yet necessarily accurate or reliable, or a negative reflection on NASA from whom it obtains its data.

A more subtle example of a website plotting NASA data beautifully but incorrectly is discussed in the question Why are these objects moving at Vastly Different Speeds along the same orbit?. Even though the website meteorshowers.org showed much better attention to graphics and astronomical and aesthetic detail than the Roadster site, it was also released to the public with glaring scientific errors.