Computing Sun position from Mars

# Computing Sun position from Mars

To compute planetary ephemerides from a given point located on the surface of another planet is something initially beyond the scope of the JPARSEC library. However, this question has recently attracted my attention since it is something interesting in certain projects like the Mars exploration rovers. Spirit and Opportunity were a real success, so the design of new rovers to explore Mars with better technology is something that can easy earn the attention (and the money) from USA goverment. The new rovers will be Curiosity and Phoenix, and are now in the final phase of construction. Curiosity will be launched in december, 2011, and will reach Mars on August, 2012.

In terms of software development, the most visible program produced by JPL during the Spirit and Opportunity adventure was Maestro. This software was designed to schedule the activities of the rovers and to analyze the data taken with their instruments, to obtain the maximum science from them. A public version of Maestro allowed everyone to download very interesting images and data just after they were taken and analyzed by scientists.

Mark W. Powell is one of the engineers working at JPL in the development team of the next version of Maestro. He recently contacted me asking for a program to obtain the ephemerides of the Sun from any given point on Mars. I always thought there should be a number of experts in ephemerides calculations at JPL (that's sure), but it seems that the different tools they have for ephemerides (let's say Horizons, NOVAS, among others) are not adequate since Maestro is developed in Java. I never thought to collaborate with JPL in the field of ephemerides calculation, but the characteristics of the JPARSEC library (open source, well documented and organized in Mark's words, combined with some lack of good Java software in this field) are appreciated in new projects.

The point in this case is to calculate if a given activity (like to capture an image of a given rock from a given direction and at a given time) is going to be made in shadow or not. The new rovers will be capable or shooting images at night and taking some decisions by themselves, but the presence of the Sun changes the lighting conditions and therefore the steps to be taken by the science investigation team, or the schedule to give priority to some scientific proposals. Once the position of the Sun is known, the lighting conditions on Mars can be simulated using 3d models for both the rovers and the terrain, giving a direct look of the shadows on the terrain, coming both from the rovers or from other rocks.

So I helped Mark (of course) in this interesting real case, providing him with a program that uses a reduced version of the JPARSEC library. The position of an object on Mars can be calculated in several ways. The most elaborated one is to use Mars as reference instead of the Earth to calculate the 'geocentric' coordinates of the objects, and then to use the Mars geoid for 'topocentric' positions. I discarded this since I realized I needed more than a few days to do it properly, with all necessary tests and developments to give him a clean code with updated javadoc. The second approach was to obtain the position of the Sun using the subsolar position from Earth that JPARSEC already calculates with excellent precision (using geometric coordinates instead of apparent ones), and doing a simple rotation. The third method was to implement the algorithm described in the program Mars24, done by a team at Goddard Space Flight Center (see http://www.giss.nasa.gov/tools/mars24/help/algorithm.html for a sketch of the algorithm, and http://pubs.giss.nasa.gov/docs/2000/2000_Allison_McEwen.pdf for the details). As reference for accuracy tests I used Horizons. After a complete check I concluded that the mean difference between JPARSEC and Horizons was in the 0.02 to 0.05 degrees range, and 0.15 to 0.25 relative to Mars24. Half of the difference with JPARSEC seems to be due to a different correction from UTC to TT (or maybe Horizons uses UT1?), and the other half to that simple rotation.

Both JPARSEC and Mars24 methods were suitable for Mark, but the better accuracy of JPARSEC combined with the organization, documentation, and simplicity of the distribution as a .jar file convinced him to use it for Maestro. I'm quite confident that the quality of my work is good enough, but now I feel somewhat responsible for such a large project involving so many engineers and scientists. Hopefully we will have a public version of Maestro even better than the one released for Spirit and Opportunity to allow people interested in this great project to participate in the exploration of the Mars surface.

Here is the code for the MarsEphem class I produced for Mark. The class implements a test over 5000 random locations on Mars and dates between 2010 and 2020 and returns maximum difference between both algorithms.

package jparsec.ephem;

import jparsec.ephem.planets.EphemElement;
import jparsec.math.Constant;
import jparsec.observer.LocationElement;
import jparsec.observer.ObserverElement;
import jparsec.time.AstroDate;
import jparsec.time.TimeElement;
import jparsec.time.TimeFormat;
import jparsec.time.TimeScale;

/**
* Ephemerides for the Sun from Mars.
* @author T. Alonso Albi - OAN (Spain)
* @version 1
*/
public class MarsEphem {

/**
* Test program.
* @param args Not used.
*/
public static void main(String args[]) {

try {
double maxDist = -1, maxDist_TT = 0.0;

for (int iii=0; iii< 5000; iii++) {

int year = 2010 + (int) (Math.random() * 10);
int month = 1 + (int) (Math.random() * 11);
int day = 1 + (int) (Math.random() * 27);
int hour = 0 + (int) (Math.random() * 23);
int minute = 0 + (int) (Math.random() * 59);
int second = 0 + (int) (Math.random() * 59);
double lon = Math.random() * 360, lat = 90 - Math.random() * 180;

// Example 1: 2000 Jan 6 00:00:00 UTC, from lon = lat = 0
// Example 2: 2004 Jan 3 13:46:31 UTC, from lon = 184.702, lat = -14.64
// Example 3: 2010 Jan 6 00:00:00 UTC, from lon = 184.702, lat = -14.64
/*
year = 2000;
month = 1;
day = 6;
hour = 0;
minute = 0;
second = 0;
lon = 0;
lat = 0;
*/
AstroDate astro = new AstroDate(year, month, day, hour, minute, second); // Example 1
TimeElement time = new TimeElement(astro.toGCalendar(), TimeElement.TERRESTRIAL_TIME);
EphemerisElement eph = new EphemerisElement(Target.MARS, EphemerisElement.EPHEM_GEOMETRIC,
EphemerisElement.EQUINOX_OF_DATE, EphemerisElement.GEOCENTRIC, EphemerisElement.APPLY_IAU2000,
EphemerisElement.FRAME_J2000, EphemerisElement.ALGORITHM_MOSHIER);
EphemElement ephem = Ephem.getEphemeris(time, observer, eph, false);

System.out.println("USING JPARSEC");

// Obtain Sun position in Mars knowing subsolar position
double dif = ephem.subsolarLatitude - (float) Math.atan(Math.tan(ephem.subsolarLatitude) / Math.pow(3376.2 / 3396.19, 2.0));
LocationElement subSolar = new LocationElement(ephem.subsolarLongitude, ephem.subsolarLatitude + dif, 1.0);
LocationElement refPos = new LocationElement(observer.longitude, observer.latitude, 1.0);
double elev = Math.PI * 0.5 - LocationElement.getAngularDistance(subSolar, refPos);
System.out.println("Sun azimuth     " + pa * Constant.RAD_TO_DEG);
System.out.println("Sun elevation   " + elev * Constant.RAD_TO_DEG);

// Obtain Earth position in Mars knowing sub-Earth position
/*				dif = ephem.positionAngleOfPole - (float) Math.atan(Math.tan(ephem.positionAngleOfPole) / Math.pow(3376.2 / 3396.19, 2.0));
LocationElement subEarth = new LocationElement(ephem.longitudeOfCentralMeridian, ephem.positionAngleOfPole + dif, 1.0);
elev = Math.PI * 0.5 - LocationElement.getAngularDistance(subEarth, refPos);
pa = LocationElement.getPositionAngle(refPos, subEarth);
*/
// Obtain Sun position in Mars using simple method by Goddard Space Flight Center
// See http://www.giss.nasa.gov/tools/mars24/help/algorithm.html and http://pubs.giss.nasa.gov/docs/2000/2000_Allison_McEwen.pdf
System.out.println("USING MARS24");
double JD_TT = TimeScale.getJD(time, observer, eph, TimeScale.JD_TT);
double dt = JD_TT - 2451545.0;
double M = Math.toRadians(19.387 + 0.52402075 * dt);
double afms = Math.toRadians(270.3863 + 0.5240384 * dt);
double pbs = 0.0;
double a[] = new double[] {0.0071, 0.0057, 0.0039, 0.0037, 0.0021, 0.0020, 0.0018};
double tau[] = new double[] {2.2353, 2.7543, 1.1177, 15.7866, 2.1354, 2.4694, 32.8493};
double phi[] = new double[] {49.409, 168.173, 191.837, 21.736, 15.704, 95.528, 49.095};
for (int i=0; i<7; i++) {
pbs += a[i] * Math.cos(Math.toRadians(phi[i] + 0.985626 * dt / tau[i]));
}
double vminusM = Math.toRadians((10.691 + 3.0E-7*dt) * Math.sin(M) + 0.623 * Math.sin(2.0 * M) +
0.050 * Math.sin(3.0 * M) + 0.005 * Math.sin(4.0 * M) + 0.0005 * Math.sin(5.0 * M) + pbs);
double ls = afms + vminusM;
double eot = Math.toRadians(2.861 * Math.sin(2.0 * ls) - 0.071 * Math.sin(4.0 * ls) + 0.002 * Math.sin(6.0 * ls)) - vminusM;
double mtc = 24.0 * ((JD_TT - 2451549.5) / 1.027491252 + 44796.0 - 0.00096);
mtc = (mtc - (int) (mtc / 24.0) * 24.0) * (2.0 * Math.PI) / 24.0;
double deltas = Math.asin(0.42565 * Math.sin(ls)) + Math.toRadians(0.25 * Math.sin(ls));
double lons = Functions.normalizeRadians((mtc + eot) + Math.PI);
double H = observer.longitude - lons;
double z = Math.acos(Math.sin(deltas) * Math.sin(observer.latitude) + Math.cos(deltas) * Math.cos(observer.latitude) * Math.cos(H));
// Azimuth = angle from north direction, measured clockwise
double azimuth = Functions.normalizeRadians(Math.atan2(Math.sin(H), (Math.cos(observer.latitude) * Math.tan(deltas) - Math.sin(observer.latitude) * Math.cos(H))));
double elevation = Math.PI * 0.5 - z;

// COMPARISON TEST
//                       JPL HORIZONS             JPARSEC            MARS24
// 2000-Jan-06 00:00     191.1564 -64.5049    191.1722  -64.5053     191.0398  -64.2616
// 2004-Jan-03 13:46:31  179.9952 -62.0741    180.0531  -62.0770     179.9890  -61.9392
// 2010-Jan-06 00:00     284.5225   1.1738    284.5392    1.2354     284.6525    1.1303
// RMS RELATIVE TO JPL HORIZONS:                     0.05                   0.15
// RMS RELATIVE TO JPL HORIZONS (with TT, no UT)     0.02                   0.15

/*
System.out.println(JD_TT);
System.out.println(dt);
System.out.println(Math.toDegrees(M));
System.out.println(Math.toDegrees(afms));
System.out.println("pbs "+pbs);
System.out.println(Math.toDegrees(vminusM));
System.out.println(Math.toDegrees(ls));
System.out.println(eot*24.0/(2.0*Math.PI));
System.out.println("mtc "+mtc*24.0/(2.0*Math.PI));
System.out.println(Math.toDegrees(lons)/15.0);
System.out.println(Math.toDegrees(deltas));
*/				System.out.println("Sun azimuth     " + Math.toDegrees(azimuth));
System.out.println("Sun elevation   " + Math.toDegrees(elevation));

LocationElement loc1 = new LocationElement(pa, elev, 1);
LocationElement loc2 = new LocationElement(azimuth, elevation, 1);
double dist = LocationElement.getAngularDistance(loc1, loc2) * Constant.RAD_TO_DEG;
if (dist > maxDist || maxDist == -1) {
maxDist = dist;
maxDist_TT = JD_TT;
}
}
System.out.println(maxDist);
System.out.println(maxDist_TT);
System.out.println(TimeFormat.formatJulianDayAsDateAndTime(maxDist_TT));
} catch (Exception exc) {
exc.printStackTrace();
}
}
}

# Update

This is al old post written before the implementation of vast improvements for computing planetary positions in JPARSEC for an observer located on the surface of any other body, and also many improvements in accuracy. Currently the position of a body from Mars is computed using an ObserverElement object for Mars (created from the class ExtraterrestrialObserverElement), and the azimuth and elevation returned agree with Horizons up to the fourth decimal place presented in this post.

Here is a test for the previous example 3:

public static void main(String args[]) {
System.out.println("Ephemerides from Mars");

try {
EphemerisElement eph = new EphemerisElement(TARGET.SUN, EphemerisElement.COORDINATES_TYPE.APPARENT,
EphemerisElement.EQUINOX_OF_DATE, EphemerisElement.TOPOCENTRIC, EphemerisElement.REDUCTION_METHOD.IAU_1976,
EphemerisElement.FRAME.DYNAMICAL_EQUINOX_J2000, EphemerisElement.ALGORITHM.MOSHIER);
TimeElement time = new TimeElement(new AstroDate(2010, AstroDate.JANUARY, 6, 0, 0, 0), SCALE.UNIVERSAL_TIME_UTC);
ObserverElement obs = ObserverElement.parseExtraterrestrialObserver(new ExtraterrestrialObserverElement("Mars", TARGET.MARS,

EphemElement ephem = Ephem.getEphemeris(time, obs, eph, false);
ConsoleReport.basicEphemReportToConsole(ephem);
System.out.println(Functions.formatAngleAsDegrees(ephem.azimuth, 4)+" "+Functions.formatAngleAsDegrees(ephem.elevation, 4));
} catch (Exception exc) {
exc.printStackTrace();
}
}

The output from the previous code to the console is:

Sun Right ascension: 02h 05m 11.1547s
Sun Declination: 13° 43' 57.384"
Sun Distance: 1.631840 AU
Sun Elongation: 0.000"
Sun Phase angle: 0.000"
Sun Constellation: Sagittarius
Sun Azimuth: -75° 28' 38.838"
Sun Elevation: 1° 10' 25.685"
Sun Magnitude: -25.679

-75.4775 1.1738


Comparing with Horizons there is perfect match for declination, and the two values at the bottom (azimuth and elevation in degrees), even without the need of JPL ephemeris files (Moshier method). For that match old reduction algorithms (IAU 1976) should be used. In some bodies the match is not perfect far from J2000 date due to a different rotation/orientation model (Horizons seems to mix modern orientation models with old reduction algorithms, something not allowed in JPARSEC). For the right ascension JPARSEC uses as origin (RA = 0) the meridian containing the body (Mars here) spring equinox of date, while Horizons uses the Earth equinox as origin. As an example of another publication using the Mars equinox as reference (thus giving similar right ascensions as JPARSEC) you can look at table II, page 70 (86 in the pdf) and previous ones, of the free ebook Relative Geometries of the Earth, Sun, and Mars, from the Year 1973 to the Year 2000 by Sue W. Souders.

## Discussion

, 2019/07/01 14:15

Execuse me, I am a student who is interested in the sub-solar point of the Mars. I recently used the way provided by this page to calculate the sub-solar longitude of the Mars. But I find the sub-solar longitude is different from the result of Mars Ephemeris Generator 2.6, which is derived from MAR097 + DE430 (https://pds-rings.seti.org/tools/ephem2_mar.html). The difference is about 3 degrees. May I have some advice about the difference?

, 2019/07/01 19:40

Hi Liu,

An accurate response is difficult because I don't know in detail how these values are computed in the web page you mention. I can only try to reproduce them. First, notice the author mentions the result are referred to J2000 equinox, so the precesion of the ecliptic is not considered. I have also checked that the right ascension and declination are astrometric, not apparent. Using JPARSEC I can suggest the following code:

AstroDate astro = new AstroDate(2019, 1, 1, 0, 0, 0);
TimeElement time = new TimeElement(astro, TimeElement.SCALE.UNIVERSAL_TIME_UTC);
EphemerisElement eph = new EphemerisElement(TARGET.MARS, EphemerisElement.COORDINATES_TYPE.ASTROMETRIC,
EphemerisElement.EQUINOX_J2000, EphemerisElement.GEOCENTRIC, EphemerisElement.REDUCTION_METHOD.IAU_2006,
EphemerisElement.FRAME.ICRF, EphemerisElement.ALGORITHM.MOSHIER);

EphemElement ephem = Ephem.getEphemeris(time, obs, eph, false);

String sep = "  ";
String ra = Functions.formatValue(ephem.rightAscension * Constant.RAD_TO_HOUR, 6);
String dec = Functions.formatValue(ephem.declination * Constant.RAD_TO_DEG, 5);
String subLat = Functions.formatValue(ephem.subsolarLatitude * Constant.RAD_TO_DEG, 3);
String subLon = Functions.formatValue(ephem.subsolarLongitude * Constant.RAD_TO_DEG, 3);
System.out.println(ra + sep + dec + sep + subLat + sep + subLon);

The output is:

23.988003  -0.40315  -17.176  326.558


The same example with the page you provide yields:

mjd         year mo dy hr mi sc Planet RA PlanetDec subslat subslon
58484.00000 2019  1  1  0  0  0 23.988003  -0.40315 -17.080 326.811


By the way, The horizons server at JPL gives this output:

2019-Jan-01 00:00     359.8194791  -0.4044568 326.82 -17.27


As you see, although I'm not using JPL DE ephemerides the RA and DEC are exactly the same for those decimal places. The difference is 0.1 deg in the subsolar latitude and 0.25 deg in the subsolar longitude, but as I said at the beginning I can't provide more details to explain this little discrepancy. Probably related to a little different TT-UT1 correction and Mars rotation/axis model. The results from Horizon are closer to the page you mention for the subsolar longitude, and closer to my results for the latitude.

Regards,

Tomás.

, 2019/07/02 08:59

Thank you very much for your prompt reply and useful advice. I thought that the subsolar longitude is derived from the equation (C-5) in (https://www.giss.nasa.gov/tools/mars24/help/algorithm.html). It seems that the subsolar longitude is derived from the package jparsec.ephem. I will try your method. Thank you very much.

, 2019/07/02 11:35

The method on that page is also correct, but light time is not considered. The example uses Earth's time Jan 6, 2000, 00h UT, and computes the angles for that specific time on Earth, without considering that Mars was 21s of light time away that moment. This is adequate when you have a rover there but you prefer to use the time on Earth to compute what's going on there. To use Earth's time you only have to change EphemerisElement.COORDINATES_TYPE.ASTROMETRIC to EphemerisElement.COORDINATES_TYPE.GEOMETRIC in the previous code to obtain with JPARSEC:

22.257001  -11.89809  -25.241  174.738


Which agrees with that page within 0.02 degrees. The rest of the stuff including the Sun position from Mars can be obtained using an observer on Mars, for lon = lat = 0 deg would be something like:

obs = ObserverElement.parseExtraterrestrialObserver(new ExtraterrestrialObserverElement("Mars (lon = lat = 0)", TARGET.MARS,
new LocationElement(0 * Constant.DEG_TO_RAD, 0 * Constant.DEG_TO_RAD, 0.0)));

Regards

, 2019/07/02 12:18

Hello Tomás

Thank you very much for answering my question! I used the Equation (C-5) to calculate the subsolar longitude. The time is 2019-Jan-01 00:00. The Julian date is 2458484.5. The calculated sub-solar longitude is 329.090198. It is different from the 326.811 by about 2 degrees. The time of light, 21 seconds, may not account for this error. The distance between the Mars and the Sun are in good aggrement. My code is written by Matlab. It is easy to read. I have checked the parameters with the website and the paper (Allion, 1997 and Allion et al. 2000). I am not sure where my mistake is. The code is as follows. May be I need to check it more carefully. Thank you very much!

Best Regards

Liu, N

clear all;clc;
format long;
JD=	2458484.5;  % 2019-Jan-01 00:00
t=JD-2451545;
M=19.3871+0.52402075*t; % unit degree
aFMS=270.3863+0.5240384*t; % unit degree
A=[0.0071, 0.0057, 0.0039, 0.0037, 0.0021, 0.0020, 0.0018];
tao=[2.2353, 2.7543, 1.1177, 15.7866, 2.1354, 2.4694, 32.8493];
phi=[49.409, 168.173, 191.837, 21.736, 15.704, 95.528, 49.095];

PBS=0;
for i=1:7  % perturbation
PBS=PBS+A(i)*cosd((0.985626*t/tao(i))+phi(i)); %  where 0.985626° = 360° / 365.25
end

v_M=(10.691+3e-7*t).*sind(M)+0.623*sind(2*M)+0.050*sind(3*M)+0.005*sind(4*M)+0.0005*sind(5*M)+PBS; %The equation of center is the true anomaly minus mean anomaly
Ls=aFMS+v_M; % unit degree
rM=1.52367934*(1.00436-0.09309*cosd(M)-0.00436*cosd(2*M)-0.00031*cosd(3*M)-0.00003*cosd(4*M)) %unit AU
EOT=2.861*sind(2*Ls)-0.071*sind(4*Ls)+0.002*sind(6*Ls)-v_M; % unit degree
EOT=EOT*24/360; % unit hour
MTC=mod(24*((JD-2451549.5)/ 1.0274912517+44796.0-0.0009626),24); %unit hour
Clon=(MTC+EOT)*360/24+180;
Clon=mod(Clon,360) % Normalize to (0°，360°)

, 2019/07/02 22:07

I suggest to apply the TT-UT1 correction in section A, since you are using the UT date (JD) in equations that should use the date in TT scale. The rest is just testing the result of each line of code until it is correct.

, 2019/07/03 02:45

Thank you for your reply. I have checked my code When the time on Earth was 00:00:00 on Jan. 6, 2000 (UTC). The result is also provided in the part II of the website(https://www.giss.nasa.gov/tools/mars24/help/algorithm.html). My sub-solar Longitude is the same with the website, 174.72600 degree. But the result in another website (https://pds-rings.seti.org/tools/ephem2_mar.html) is 170.938. The difference is about 4 degrees. The result really confuse me. Maybe there is some difference between the formula and the ephemeris. Thank you again for your useful advice.

, 2019/07/05 15:31

Hello Tomás

Execuse me, sir. I want to use the code provided above to obtain the sub-solar latitude and longitude of the Mars because it matches the DE430. I have some problems when installing the code. There are so many packages provided at (http://conga.oan.es/~alonso/jparsec/lib/). Error occurs that package org.stathissideris.ascii2image.graphics does not exist when I download the package org already. In addition, I can not find the package 'gov.noaa.pmel.sgt' in the website above. Symbol JPlotLayout can not be found at jparsec.garph.CreateGridChart although I have download the package jparsec.jar and added it to the directory. I am new in Java. May I have some advice from you.

Regards Liu N.

, 2019/07/05 18:21

Hello Liu. For (basic) ephemerides calculations you don't need anything from the lib directory, jparsec.jar is enough. You could need other files like eop.jar for Earth Orientation Parameters, or jpl_ephem.jar for some DE4xx integrations (or vsop.jar, elp2000 for other theories), but they are not required when the greatest accuracy is not needed. In case you install the code instead of simply adding jparsec.jar to the classpath of your program, you will see a lot of errors because many libraries from the lib directory need to be downloaded and added to the classpath of the project, but none of them are required for ephemerides, you can just ignore the compilation errors. In case of problems just add jparsec.jar to the classpath instead of creating a project with the source code of the library.

Here is a basic program that only uses jparsec.jar. The algorithm is set to Moshier which uses data already integrated in jparsec.jar, is a low precision fit to DE405. The optimization for speed in the eph object will not use eop.jar, so that only jparsec.jar is needed.

import jparsec.ephem.Ephem;
import jparsec.ephem.EphemerisElement;
import jparsec.ephem.Target.TARGET;
import jparsec.ephem.planets.EphemElement;
import jparsec.io.ConsoleReport;
import jparsec.observer.City;
import jparsec.observer.CityElement;
import jparsec.observer.ObserverElement;
import jparsec.time.AstroDate;
import jparsec.time.TimeElement;

public class minimumExample {

/**
* Main test program.
* @param args Not used
*/
public static void main(String[] args) {
System.out.println("Ephemerides test");

try {
CityElement city = City.findCity("Beijing");
ObserverElement obs = ObserverElement.parseCity(city);
AstroDate astro = new AstroDate(); // Now, you can also set here year, month, day, ...
TimeElement time = new TimeElement(astro, TimeElement.SCALE.LOCAL_TIME); // Assume CPU clock in local time
EphemerisElement eph = new EphemerisElement(TARGET.MARS, EphemerisElement.COORDINATES_TYPE.APPARENT,
EphemerisElement.EQUINOX_OF_DATE, EphemerisElement.TOPOCENTRIC, EphemerisElement.REDUCTION_METHOD.IAU_2006,
EphemerisElement.FRAME.DYNAMICAL_EQUINOX_J2000, EphemerisElement.ALGORITHM.MOSHIER);
eph.optimizeForSpeed();

EphemElement ephem = Ephem.getEphemeris(time, obs, eph, true);
ConsoleReport.fullEphemReportToConsole(ephem);
} catch (Exception exc) {
exc.printStackTrace();
}
}
}
, 2019/07/06 04:41

Hello Tomás Your advice is really useful. The problem has been solved. Thank you very much.

Regards Liu N

, 2019/11/11 19:36

I'm playing with NASA Horizons to get sun sub-longitude on various bodies, but I don't understand why, for example, results for 2019/11/11 from Horizons differ from result from this page: http://www-mars.lmd.jussieu.fr/mars/time/martian_time.html

, 2019/11/11 20:17

Because they are different quantities, Ls is solar ecliptic longitude from Mars, depends on Mars movement around the Sun, sub-solar longitude dependes also on Mars rotation. Ls is equation B5 and sub-solar longitude equation C5 in https://www.giss.nasa.gov/tools/mars24/help/algorithm.html

, 2019/11/11 20:46

Thanks. Do you think you could explain that with a couple of images? I am trying to draw an image per each NASA Horizons output quantity: http://win98.altervista.org/space/exploration/quantities.html

…but now I found FOUR different values of solar longitude for same date (2000/1/6 00:00)!

Result:

* sunlon: 141.169

* subslon: 170.938

Result: 284.96643

Result: 277.1

NASA Horizons: 170.94

, 2019/11/11 22:00

I think you are confused about the numbers you find and their meanings.

REF-1: sunlon there is inertial, referred to a reference system which is not rotating with the planet (ficticious) and for J2000 equinox. Forget that number, subslon is what you want.

REF-2: that calc gives the geocentric (Earth centered) ecliptic longitude of the Sun, nothing to do with subsolar longitude.

REF-3: again, this is the areocentric (Mars centered) ecliptic longitude of the Sun, which is different from the previous since they are different bodies/orbits.

REF-4 (Horizons): this is ok, Horizons is a source you can trust.

You can also go to http://www.oan.es/servidorEfem/form.php, a page developed by me, and you will find subslon = 170.939.

Respect the charts, nice work what you are trying to do, but I'm too busy with my own things to help with such time demanding tasks.

, 2019/11/12 19:34

And how is it named, or where do I find it, the angular position of a Planet w.r.t. Sun, vernal equinox and ecliptic? Is it the J2000 Right Ascension?

, 2019/11/13 17:37

Right ascension belongs to equatorial coordinates, I think you want ecliptic ones, and for that I would recommend Horizons. In case you want the position of the Sun as seen from each planet, use a reference system centered in each body and compute the planetocentric ecliptic longitude of the Sun. You can use J2000 or ecliptic of date for the ecliptic to use as reference. To use the same reference system for all bodies consider the Sun as the common center and compute the heliocentric ecliptic longitude of the planets.

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