As this answer mentions, there have been several questions about the data represented on the whereisroadster site within the Space Exploration Stack Exchange site.
One about wrong distances has been deleted (but is still viewable to higher rep users here), another about roadster standing-still for eleven days in chat, and a third Why does this plot show Starman's speed relative to the Sun fluctuating so often? has been very well received and highly up voted. There are many other Q&A referencing this site here as well.
I thought I'd do a spot check to see how it's doing.
I grabbed the Horizons output for the Roadster object as a target using Heliocentric and Geocentric positions as the origin getting the position and velocity vectors from those origins. 8PM (20:00:00
) here in UTC+8 is 12:00:00
UTC which is when Julian Date passes zero (within tens of seconds at least).
I missed the screen shot exactly at the hour, but nailed the one at 20:01:00
. I've squished the windows to make them both fit within a 640 pixel width so they show nicely in this SE site without manipulations. time.gov is from NIST and attempts to synchronize over the connection through ping timing.
I refreshed the windows a few minutes before just to make sure things were updated internally.
However, the whereisroadster site gets a significantly different distance between Earth and Roadster than Horizions gives, and this is supposed to be the data engine from which the site derives its data.
Why is it off by so much?
pos_rel_Earth = np.array([-1.064812011604053E+07, -8.608116821810877E+06, -2.355098230429459E+06])
r_rel_Earth = np.sqrt((pos_rel_Earth**2).sum())
print r_rel_Earth
13893474.1839
- That is 13,893,484 km (Horizons)
- versus 13,789,239 km (website)
That's a 1% difference in distance, ~100,000 kilometers
With respect to the Sun:
pos_rel_Sun = np.array([-1.591862416706663E+08, -2.299106450849950E+07, -2.353784956702188E+06])
r_rel_Sun = np.sqrt((pos_rel_Sun**2).sum())
print r_rel_Sun, v_rel_Sun
160855179.861 31.3143577616
- That is 160,855,179 km (Horizons)
- versus 160,696,811 km (website)
That's also an ~100,000 kilometer difference.
Raw Data:
Ephemeris / WWW_USER Mon Mar 26 11:34:36 2018 Pasadena, USA / Horizons
*******************************************************************************
Target body name: SpaceX Roadster (spacecraft) (-143205) {source: tesla_s9}
Center body name: Earth (399) {source: DE431mx}
Center-site name: BODY CENTER
*******************************************************************************
Start time : A.D. 2018-Mar-26 12:00:00.0000 TDB
Stop time : A.D. 2018-Mar-26 12:01:00.0000 TDB
Step-size : 1 minutes
*******************************************************************************
Center geodetic : 0.00000000,0.00000000,0.0000000 {E-lon(deg),Lat(deg),Alt(km)}
Center cylindric: 0.00000000,0.00000000,0.0000000 {E-lon(deg),Dxy(km),Dz(km)}
Center radii : 6378.1 x 6378.1 x 6356.8 km {Equator, meridian, pole}
Output units : KM-S
Output type : GEOMETRIC cartesian states
Output format : 2 (position and velocity)
Reference frame : ICRF/J2000.0
Coordinate systm: Ecliptic and Mean Equinox of Reference Epoch
$$SOE
2458204.000000000, A.D. 2018-Mar-26 12:00:00.0000, -1.064791354593495E+07, -8.608024506708045E+06, -2.355071522438325E+06, -3.442810011828843E+00, -1.538588913186599E+00, -4.451354722214971E-01,
2458204.000694444, A.D. 2018-Mar-26 12:01:00.0000, -1.064812011604053E+07, -8.608116821810877E+06, -2.355098230429459E+06, -3.442860173393305E+00, -1.538581181359120E+00, -4.451308989758778E-01,
$$EOE
Ephemeris / WWW_USER Mon Mar 26 11:55:30 2018 Pasadena, USA / Horizons
*******************************************************************************
Target body name: SpaceX Roadster (spacecraft) (-143205) {source: tesla_s9}
Center body name: Sun (10) {source: DE431mx}
Center-site name: BODY CENTER
*******************************************************************************
Start time : A.D. 2018-Mar-26 12:00:00.0000 TDB
Stop time : A.D. 2018-Mar-26 12:01:00.0000 TDB
Step-size : 1 minutes
*******************************************************************************
Center geodetic : 0.00000000,0.00000000,0.0000000 {E-lon(deg),Lat(deg),Alt(km)}
Center cylindric: 0.00000000,0.00000000,0.0000000 {E-lon(deg),Dxy(km),Dz(km)}
Center radii : 696000.0 x 696000.0 x 696000.0 k{Equator, meridian, pole}
Output units : KM-S
Output type : GEOMETRIC cartesian states
Output format : 2 (position and velocity)
Reference frame : ICRF/J2000.0
Coordinate systm: Ecliptic and Mean Equinox of Reference Epoch
$$SOE
2458204.000000000, A.D. 2018-Mar-26 12:00:00.0000, -1.591861789254073E+08, -2.298918688322970E+07, -2.353758256452293E+06, -1.045906639864063E+00, -3.129377652430419E+01, -4.450064271997949E-01,
2458204.000694444, A.D. 2018-Mar-26 12:01:00.0000, -1.591862416706663E+08, -2.299106450849950E+07, -2.353784956702188E+06, -1.045601993918644E+00, -3.129373246832911E+01, -4.450019025748677E-01,
$$EOE