Why is it that JPL (Jet Propulsion Lab) at NASA gives astronomical unit values for the planets for two different time periods, namely 3000 B.C. to 3000 A.D., and 1850 to 2050?
$\begingroup$
$\endgroup$
5
-
$\begingroup$ It would be helpful if you could provide more context for your question and also cite where J.P.L. reports these values. $\endgroup$– David HCommented May 27, 2015 at 21:07
-
$\begingroup$ Keplerian elements jpl put into google seems to get there $\endgroup$– John ShanahanCommented May 28, 2015 at 6:44
-
$\begingroup$ It seems to work by putting keplerian elements jpl into google $\endgroup$– John ShanahanCommented May 28, 2015 at 6:51
-
$\begingroup$ @JohnShanahan - You should put the links in your question. Don't make us search, and maybe find something else. $\endgroup$– David HammenCommented May 28, 2015 at 16:42
-
$\begingroup$ Sorry but I am computer illiterate, I don`t know how to do links! $\endgroup$– John ShanahanCommented May 28, 2015 at 17:12
Add a comment
|
2 Answers
$\begingroup$
$\endgroup$
2
I believe the question is asking about the following tables, one for the period 1800 AD to 2050 AD, the other two for 3000 BC to 3000 AD. Note that in these tables, "EM Bary" refers to the Earth-Moon barycenter, the center of mass of the Earth-Moon system.
Table 1.
Keplerian elements and their rates, with respect to the mean ecliptic
and equinox of J2000, valid for the time-interval 1800 AD - 2050 AD.
a e I L long.peri. long.node.
AU, AU/Cy rad, rad/Cy deg, deg/Cy deg, deg/Cy deg, deg/Cy deg, deg/Cy
-----------------------------------------------------------------------------------------------------------
Mercury 0.38709927 0.20563593 7.00497902 252.25032350 77.45779628 48.33076593
0.00000037 0.00001906 -0.00594749 149472.67411175 0.16047689 -0.12534081
Venus 0.72333566 0.00677672 3.39467605 181.97909950 131.60246718 76.67984255
0.00000390 -0.00004107 -0.00078890 58517.81538729 0.00268329 -0.27769418
EM Bary 1.00000261 0.01671123 -0.00001531 100.46457166 102.93768193 0.0
0.00000562 -0.00004392 -0.01294668 35999.37244981 0.32327364 0.0
Mars 1.52371034 0.09339410 1.84969142 -4.55343205 -23.94362959 49.55953891
0.00001847 0.00007882 -0.00813131 19140.30268499 0.44441088 -0.29257343
Jupiter 5.20288700 0.04838624 1.30439695 34.39644051 14.72847983 100.47390909
-0.00011607 -0.00013253 -0.00183714 3034.74612775 0.21252668 0.20469106
Saturn 9.53667594 0.05386179 2.48599187 49.95424423 92.59887831 113.66242448
-0.00125060 -0.00050991 0.00193609 1222.49362201 -0.41897216 -0.28867794
Uranus 19.18916464 0.04725744 0.77263783 313.23810451 170.95427630 74.01692503
-0.00196176 -0.00004397 -0.00242939 428.48202785 0.40805281 0.04240589
Neptune 30.06992276 0.00859048 1.77004347 -55.12002969 44.96476227 131.78422574
0.00026291 0.00005105 0.00035372 218.45945325 -0.32241464 -0.00508664
Pluto 39.48211675 0.24882730 17.14001206 238.92903833 224.06891629 110.30393684
-0.00031596 0.00005170 0.00004818 145.20780515 -0.04062942 -0.01183482
Source: http://ssd.jpl.nasa.gov/txt/p_elem_t1.txt
Table 2a.
Keplerian elements and their rates, with respect to the mean ecliptic and equinox of J2000,
valid for the time-interval 3000 BC -- 3000 AD. NOTE: the computation of M for Jupiter through
Pluto *must* be augmented by the additional terms given in Table 2b (below).
a e I L long.peri. long.node.
AU, AU/Cy rad, rad/Cy deg, deg/Cy deg, deg/Cy deg, deg/Cy deg, deg/Cy
------------------------------------------------------------------------------------------------------
Mercury 0.38709843 0.20563661 7.00559432 252.25166724 77.45771895 48.33961819
0.00000000 0.00002123 -0.00590158 149472.67486623 0.15940013 -0.12214182
Venus 0.72332102 0.00676399 3.39777545 181.97970850 131.76755713 76.67261496
-0.00000026 -0.00005107 0.00043494 58517.81560260 0.05679648 -0.27274174
EM Bary 1.00000018 0.01673163 -0.00054346 100.46691572 102.93005885 -5.11260389
-0.00000003 -0.00003661 -0.01337178 35999.37306329 0.31795260 -0.24123856
Mars 1.52371243 0.09336511 1.85181869 -4.56813164 -23.91744784 49.71320984
0.00000097 0.00009149 -0.00724757 19140.29934243 0.45223625 -0.26852431
Jupiter 5.20248019 0.04853590 1.29861416 34.33479152 14.27495244 100.29282654
-0.00002864 0.00018026 -0.00322699 3034.90371757 0.18199196 0.13024619
Saturn 9.54149883 0.05550825 2.49424102 50.07571329 92.86136063 113.63998702
-0.00003065 -0.00032044 0.00451969 1222.11494724 0.54179478 -0.25015002
Uranus 19.18797948 0.04685740 0.77298127 314.20276625 172.43404441 73.96250215
-0.00020455 -0.00001550 -0.00180155 428.49512595 0.09266985 0.05739699
Neptune 30.06952752 0.00895439 1.77005520 304.22289287 46.68158724 131.78635853
0.00006447 0.00000818 0.00022400 218.46515314 0.01009938 -0.00606302
Pluto 39.48686035 0.24885238 17.14104260 238.96535011 224.09702598 110.30167986
0.00449751 0.00006016 0.00000501 145.18042903 -0.00968827 -0.00809981
------------------------------------------------------------------------------------------------------
Table 2b.
Additional terms which must be added to the computation of M
for Jupiter through Pluto, 3000 BC to 3000 AD, as described
in the related document.
b c s f
---------------------------------------------------------------
Jupiter -0.00012452 0.06064060 -0.35635438 38.35125000
Saturn 0.00025899 -0.13434469 0.87320147 38.35125000
Uranus 0.00058331 -0.97731848 0.17689245 7.67025000
Neptune -0.00041348 0.68346318 -0.10162547 7.67025000
Pluto -0.01262724
---------------------------------------------------------------
Source: http://ssd.jpl.nasa.gov/txt/p_elem_t2.txt
It's important to note that the above tables are approximations. More accurate techniques exist.
For details on how to use these tables, and on the errors that result, see http://ssd.jpl.nasa.gov/txt/aprx_pos_planets.pdf and http://ssd.jpl.nasa.gov/?planet_pos .
The latter shows the errors in the two: $$\begin{matrix} &&1800&\text{to}&2050 && 3000\ \text{BC}&\text{to} & 3000\ \text{AD} \\ \\ \text{Planet} &\ & \text{RA} & \text{Dec} & \text{r} &\quad& \text{RA} & \text{Dec} & \text{r} \\ && \text{arcsec} & \text{arcsec} & \text{km} &\quad& \text{arcsec} & \text{arcsec} & \text{km} \\ -----&&---&---&---&&---&---&---\\ \text{Mercury} && 15 & 1 & 1 && 20 & 15 & 1\\ \text{Venus} && 20 & 1 & 4 && 40 & 30 & 8\\ \text{Earth-Moon} && 20 & 8 & 6 && 40 & 15 & 15\\ \text{Mars} && 40 & 2 & 25 && 100 & 40 & 30\\ \text{Jupiter} && 400 & 10 & 600 && 600 & 100 & 1000\\ \text{Saturn} && 600 & 25 & 1500 && 1000 & 100 & 4000\\ \text{Uranus} && 50 & 2 & 1000 && 2000 & 30 & 8000\\ \text{Neptune} && 10 & 1 & 200 && 400 & 15 & 4000\\ \text{Pluto} && 5 & 2 & 300 && 400 & 100 & 2500 \end{matrix} $$ Source: http://ssd.jpl.nasa.gov/?planet_pos
Note that the errors are considerably greater for tables 2a and 2b than they are for table 1, even with the extra complexity involved with using tables 2a and 2b. This says you should use table 1 if the time point of interest is between 1800 AD and 2050 AD, tables 2a and 2b if the time point of interest is outside that interval but between 3000 BC and 3000 AD.
The reason for two tables is that a narrow span of time around the present is of interest to most people, but a broad span of time is of interest to some.
These tables were constructed using some kind of optimal fit (e.g., least squares) that best matched a set of observations. It is very bad form to extrapolate an optimal fit outside of the region over which the estimation was performed. The errors that result from using table 1 (1800 AD to 2050 AD) on the span 3000 BC to 3000 AD would be immense. Never extrapolate curve-fit data!
The data used to generate table 1 was based on (somewhat) modern instrumentation. The measurements are much more accurate and much more plentiful than the older data needed to generate the long ephemeris. The narrow time span combined with plentiful, accurate measurements is what makes the 1800 AD to 2050 AD table considerably more accurate than the long ephemeris.
Some of that older data goes back to 2800 BC. Chinese, Babylonian, and Egyptian astrologers granted immense powers in the stars and planets to influence kings and calamities such as war, famine, drought, and disease. That irrational behavior led to great record keeping. Those ancient astrological observations (particularly solar eclipses) provide a fantastic anchor for long ephemerides.
-
$\begingroup$ Could you, by chance, explain what EM Bary is, in this data? It seems to take Earth's place in the list of planets but Google returns nothing for the phrase.. $\endgroup$– SethCommented Oct 22, 2017 at 3:23
-
$\begingroup$ @Seth - It refers to the Earth-Moon barycenter. I updated the answer to incorporate this. $\endgroup$ Commented Oct 22, 2017 at 14:48
$\begingroup$
$\endgroup$
2
Because "the expected positions and distances of objects at an established time are calculated (in au) from these laws, and assembled into a collection of data called an ephemeris". According to Description of JPL Solar System Ephemeris "the computer calculations have been extended as far as 3000 BC to 3000 AD, but positions for the 1850-2050 range are the most accurate, of course... one needs to be very careful about comparing observations reduced on the basis of different ephemerides and coordinate systems."
There is a separate issue with the astronomical unit itself. Until 2012 IAU (International Astronomical Union) defined astronomical unit as the mean distance from the earth to the sun, and their best 2009 estimate "was still derived from observation and measurements subject to error, and based on techniques that did not yet standardize all relativistic effects, and thus were not constant for all observers." This should not be relevant anymore because in 2012 IAU finally saw the light, and "simply used the 2009 estimate to redefine the astronomical unit as a conventional unit of length directly tied to the meter (exactly 149597870700 m) and assigned it the official symbol au". The meter is currently tied to the speed of light in the vacuum, so that is as ironclad as it gets. But... stay tuned for JPL to get up to speed.
On Physics SE David Hammen describes a previous standardization "war" between JPL and Europeans, over relativisitic time measurements, which affected the astronomical unit under the old definition, by the way. "Relativistic religious wars were fought over this issue. Europeans (Russia and France, the only other countries with a stake in the game) wanted to use a (theoretical) clock far removed from the Sun. One second on that clock would always tick faster than an Earth-based clock. JPL's ephemeris time is essentially that far-removed clock, scaled to tick at Earth rate (on average). JPL won that religious war, but perhaps only because they got slightly better results than did the Russian Academy or the Paris Observatory."
-
$\begingroup$ Is it generaliy true that over millions of years as the Sun loses mass and the planets gain somewhat, that they all move further away from the Sun? $\endgroup$ Commented May 28, 2015 at 7:13
-
1$\begingroup$ @John Shanahan Sun does lose mass in emitting radiation, which causes planetary orbits to expand and speeds to decrease, this was one of the reasons IAU changed the definition of astronomical unit. newscientist.com/article/… However, this effect is not enough to explain observed 15cm/year drift of earth away from the sun, tidal repulsion was also suggested, the same effect that moves moon away from earth newscientist.com/article/… $\endgroup$– ConifoldCommented May 28, 2015 at 21:03