next up previous
Next: Radial Velocity and Light-Time Corrections
Previous: Geocentric Coordinates


SLALIB includes routines for generating positions and velocities of Solar-System bodies. The accuracy objectives are modest, and the SLALIB facilities do not attempt to compete with precomputed ephemerides such as those provided by JPL, or with models containing thousands of terms. It is also worth noting that SLALIB's very accurate star coordinate conversion routines are not strictly applicable to solar-system cases, though they are adequate for most practical purposes.

Earth/Sun ephemerides can be generated using the routine sla_EVP, which predicts Earth position and velocity with respect to both the solar-system barycentre and the Sun. Maximum velocity error is 0.42 metres per second; maximum heliocentric position error is 1600 km (about $2\hspace{-0.05em}^{'\hspace{-0.1em}'}$), with barycentric position errors about 4 times worse. (The Sun's position as seen from the Earth can, of course, be obtained simply by reversing the signs of the Cartesian components of the Earth:Sun vector.)

Geocentric Moon ephemerides are available from sla_DMOON, which predicts the Moon's position and velocity with respect to the Earth's centre. Direction accuracy is usually better than 10 km ($5\hspace{-0.05em}^{'\hspace{-0.1em}'}$) and distance accuracy a little worse.

Lower-precision but faster predictions for the Sun and Moon can be made by calling sla_EARTH and sla_MOON. Both are single precision and accept dates in the form of year, day-in-year and fraction of day (starting from a calendar date you need to call sla_CLYD or sla_CALYD to get the required year and day). The sla_EARTH routine returns the heliocentric position and velocity of the Earth's centre for the mean equator and equinox of date. The accuracy is better than 20,000 km in position and 10 metres per second in speed. The position and velocity of the Moon with respect to the Earth's centre for the mean equator and ecliptic of date can be obtained by calling sla_MOON. The positional accuracy is better than $30\hspace{-0.05em}^{'\hspace{-0.1em}'}$ in direction and 1000 km in distance.

Approximate ephemerides for all the major planets can be generated by calling sla_PLANET or sla_RDPLAN. These routines offer arcminute accuracy (much better for the inner planets and for Pluto) over a span of several millennia (but only $\pm100$ years for Pluto). The routine sla_PLANET produces heliocentric position and velocity in the form of equatorial $[\,x,y,z,\dot{x},\dot{y},\dot{z}\,]$ for the mean equator and equinox of J2000. The vectors produced by sla_PLANET can be used in a variety of ways according to the requirements of the application concerned. The routine sla_RDPLAN uses sla_PLANET and sla_DMOON to deal with the common case of predicting a planet's apparent $[\,\alpha,\delta\,]$ and angular size as seen by a terrestrial observer.

Note that in predicting the position in the sky of a solar-system body it is necessary to allow for geocentric parallax. This correction is essential in the case of the Moon, where the observer's position on the Earth can affect the Moon's $[\,\alpha,\delta\,]$ by up to $1^\circ$. The calculation can most conveniently be done by calling sla_PVOBS and subtracting the resulting 6-vector from the one produced by sla_DMOON, as is demonstrated by the following example:

      *  Demonstrate the size of the geocentric parallax correction
      *  in the case of the Moon.  The test example is for the AAT,
      *  before midnight, in summer, near first quarter.

            IMPLICIT NONE
            CHARACTER NAME*40,SH,SD
            INTEGER J,I,IHMSF(4),IDMSF(4)
           :                 RMATN(3,3),PMM(6),PMT(6),RM,DM,PVO(6),TL
            DOUBLE PRECISION sla_DTT,sla_GMST,sla_EQEQX,sla_DRANRM

      *  Get AAT longitude and latitude in radians and height in metres
            CALL sla_OBS(0,'AAT',NAME,SLONGW,SLAT,H)

      *  UTC (1992 January 13, 11 13 59) to MJD
            CALL sla_CLDJ(1992,1,13,DJUTC,J)
            CALL sla_DTF2D(11,13,59.0D0,FDUTC,J)

      *  UT1 (UT1-UTC value of -0.152 sec is from IERS Bulletin B)

      *  TT

      *  Local apparent sidereal time

      *  Geocentric position/velocity of Moon (mean of date)
            CALL sla_DMOON(DJTT,PMM)

      *  Nutation to true equinox of date
            CALL sla_NUT(DJTT,RMATN)
            CALL sla_DMXV(RMATN,PMM,PMT)
            CALL sla_DMXV(RMATN,PMM(4),PMT(4))

      *  Report geocentric HA,Dec
            CALL sla_DCC2S(PMT,RM,DM)
            CALL sla_DR2TF(2,sla_DRANRM(STL-RM),SH,IHMSF)
            CALL sla_DR2AF(1,DM,SD,IDMSF)
            WRITE (*,'(1X,'' geocentric:'',2X,A,I2.2,2I3.2,''.'',I2.2,'//
           :                              '1X,A,I2.2,2I3.2,''.'',I1)')
           :                                                SH,IHMSF,SD,IDMSF

      *  Geocentric position of observer (true equator and equinox of date)
            CALL sla_PVOBS(SLAT,H,STL,PVO)

      *  Place origin at observer
            DO I=1,6
            END DO

      *  Allow for planetary aberration
            DO I=1,3
            END DO

      *  Report topocentric HA,Dec
            CALL sla_DCC2S(PMT,RM,DM)
            CALL sla_DR2TF(2,sla_DRANRM(STL-RM),SH,IHMSF)
            CALL sla_DR2AF(1,DM,SD,IDMSF)
            WRITE (*,'(1X,''topocentric:'',2X,A,I2.2,2I3.2,''.'',I2.2,'//
           :                              '1X,A,I2.2,2I3.2,''.'',I1)')
           :                                                SH,IHMSF,SD,IDMSF

The output produced is as follows:

       geocentric:  +03 06 55.59 +15 03 39.0
      topocentric:  +03 09 23.79 +15 40 51.5

(An easier but less instructive method of estimating the topocentric apparent place of the Moon is to call the routine sla_RDPLAN.)

As an example of using sla_PLANET, the following program estimates the geocentric separation between Venus and Jupiter during a close conjunction in 2BC, which is a star-of-Bethlehem candidate:

      *  Compute time and minimum geocentric apparent separation
      *  between Venus and Jupiter during the close conjunction of 2 BC.

            IMPLICIT NONE

           :                 PVM(6),PVE(6),TL,RV,DV,RJ,DJ,SEP
            DOUBLE PRECISION sla_EPJ,sla_DSEP

      *  Search for closest approach on the given day
            DO IHOUR=20,22
               DO IMIN=0,59
                  CALL sla_DTF2D(IHOUR,IMIN,0D0,FD,J)

      *        Julian date and MJD

      *        Earth to Moon (mean of date)
                  CALL sla_DMOON(DJDM,PV)

      *        Precess Moon position to J2000
                  CALL sla_PRECL(sla_EPJ(DJDM),2000D0,RMATP)
                  CALL sla_DMXV(RMATP,PV,PVM)

      *        Sun to Earth-Moon Barycentre (mean J2000)
                  CALL sla_PLANET(DJDM,3,PVE,J)

      *        Correct from EMB to Earth
                  DO I=1,3
                  END DO

      *        Sun to Venus
                  CALL sla_PLANET(DJDM,2,PV,J)

      *        Earth to Venus
                  DO I=1,6
                  END DO

      *        Light time to Venus (sec)
           :                           (PV(2)-PVE(2))**2+
           :                           (PV(3)-PVE(3))**2)

      *        Extrapolate backwards in time by that much
                  DO I=1,3
                  END DO

      *       To RA,Dec
                  CALL sla_DCC2S(PV,RV,DV)

      *        Same for Jupiter
                  CALL sla_PLANET(DJDM,5,PV,J)
                  DO I=1,6
                  END DO
           :                           (PV(2)-PVE(2))**2+
           :                           (PV(3)-PVE(3))**2)
                  DO I=1,3
                  END DO
                  CALL sla_DCC2S(PV,RJ,DJ)

      *        Separation (arcsec)

      *        Keep if smallest so far
                  IF (SEP.LT.SEPMIN) THEN
                  END IF
               END DO
            END DO

      *  Report
            WRITE (*,'(1X,I2.2,'':'',I2.2,F6.1)') IHMIN,IMMIN,
           :                                      206264.8062D0*SEPMIN


The output produced (the Ephemeris Time on the day in question, and the closest approach in arcseconds) is as follows:

      21:19  33.7

For comparison, accurate predictions based on the JPL DE102 ephemeris give a separation about $8\hspace{-0.05em}^{'\hspace{-0.1em}'}$ less than the above estimate, occurring about half an hour earlier (see Sky and Telescope, April 1987, p357).

The following program demonstrates sla_RDPLAN.

      *  For a given date, time and geographical location, output
      *  a table of planetary positions and diameters.

            IMPLICIT NONE
            CHARACTER PNAMES(0:9)*7,B*80,S
            PARAMETER (R2AS=206264.80625D0)
            DATA PNAMES / 'Sun','Mercury','Venus','Moon','Mars','Jupiter',
           :              'Saturn','Uranus','Neptune', 'Pluto' /

      *  Loop until 'end' typed
            B=' '
            DO WHILE (B.NE.'END'.AND.B.NE.'end')

      *     Get date, time and observer's location
               PRINT *,'Date? (Y,M,D, Gregorian)'
               READ (*,'(A)') B
               IF (B.NE.'END'.AND.B.NE.'end') THEN
                  CALL sla_INTIN(B,I,IY,J)
                  CALL sla_INTIN(B,I,IM,J)
                  CALL sla_INTIN(B,I,ID,J)
                  PRINT *,'Time? (H,M,S, dynamical)'
                  READ (*,'(A)') B
                  CALL sla_DAFIN(B,I,FD,J)
                  CALL sla_CLDJ(IY,IM,ID,DJM,J)
                  PRINT *,'Longitude? (D,M,S, east +ve)'
                  READ (*,'(A)') B
                  CALL sla_DAFIN(B,I,ELONG,J)
                  PRINT *,'Latitude? (D,M,S, (geodetic)'
                  READ (*,'(A)') B
                  CALL sla_DAFIN(B,I,PHI,J)

      *        Loop planet by planet
                  DO NP=0,8

      *           Get RA,Dec and diameter
                     CALL sla_RDPLAN(DJM,NP,ELONG,PHI,RA,DEC,DIAM)

      *           One line of report
                     CALL sla_DR2TF(2,RA,S,IHMSF)
                     CALL sla_DR2AF(1,DEC,S,IDMSF)
                     WRITE (*,
           : '(1X,A,2X,3I3.2,''.'',I2.2,2X,A,I2.2,2I3.2,''.'',I1,F8.1)')
           :                          PNAMES(NP),IHMSF,S,IDMSF,R2AS*DIAM

      *           Next planet
                  END DO
                  PRINT *,' '
               END IF

      *     Next case
            END DO

Entering the following data (for 1927 June 29 at $5^{\rm h}\,25^{\rm m}$ ET and the position of Preston, UK.):
      1927 6 29
      5 25
      -2 42
      53 46
produces the following report:
      Sun       06 28 14.03  +23 17 17.5  1887.8
      Mercury   08 08 58.62  +19 20 57.3     9.3
      Venus     09 38 53.64  +15 35 32.9    22.8
      Moon      06 28 18.30  +23 18 37.3  1903.9
      Mars      09 06 49.34  +17 52 26.7     4.0
      Jupiter   00 11 12.06  -00 10 57.5    41.1
      Saturn    16 01 43.34  -18 36 55.9    18.2
      Uranus    00 13 33.53  +00 39 36.0     3.5
      Neptune   09 49 35.75  +13 38 40.8     2.2
      Pluto     07 05 29.50  +21 25 04.2      .1
Inspection of the Sun and Moon data reveals that a total solar eclipse is in progress.

SLALIB also provides for the case where orbital elements (with respect to the J2000 equinox and ecliptic) are available. This allows predictions to be made for minor-planets and (if you ignore non-gravitational effects) comets. Furthermore, if major-planet elements for an epoch close to the date in question are available, more accurate predictions can be made than are offered by sla_RDPLAN and sla_PLANET.

The SLALIB planetary-prediction routines that work with orbital elements are sla_PLANTE (the orbital-elements equivalent of sla_RDPLAN), which predicts the topocentric $[\,\alpha,\delta\,]$, and sla_PLANEL (the orbital-elements equivalent of sla_PLANET), which predicts the heliocentric $[\,x,y,z,\dot{x},\dot{y},\dot{z}\,]$ with respect to the J2000 equinox and equator. In addition, the routine sla_PV2EL does the inverse of sla_PLANEL, transforming $[\,x,y,z,\dot{x},\dot{y},\dot{z}\,]$ into osculating elements.

Osculating elements describe the unperturbed 2-body orbit. This is a good approximation to the actual orbit for a few weeks either side of the specified epoch, outside which perturbations due to the other bodies of the Solar System lead to increasing errors. Given a minor planet's osculating elements for a particular date, predictions for a date even just 100 days earlier or later are likely to be in error by several arcseconds. These errors can be reduced if new elements are generated which take account of the perturbations of the major planets, and this is what the routine sla_PERTEL does. Once sla_PERTEL has been called, to provide osculating elements close to the required date, the elements can be passed to sla_PLANEL or sla_PLANTE in the normal way. Predictions of arcsecond accuracy over a span of a decade or more are available using this technique.

Three different combinations of orbital elements are provided for, matching the usual conventions for major planets, minor planets and comets respectively. The choice is made through the argument JFORM:

t0 t0 T
i i i
$\Omega$ $\Omega$ $\Omega$
$\varpi$ $\omega$ $\omega$
a a q
e e e
L M  

The symbols have the following meanings:

		 t0 epoch at which the elements were correct
		 T epoch of perihelion passage
		 i inclination of the orbit
		 $\Omega$ longitude of the ascending node
		 $\varpi$ longitude of perihelion ($\varpi = \Omega + \omega$)		 $\omega$ argument of perihelion
		 a semi-major axis of the orbital ellipse
		 q perihelion distance
		 e orbital eccentricity
		 L mean longitude ($L = \varpi + M$)		 M mean anomaly
		 n mean motion 

The mean motion, n, tells sla_PLANEL the mass of the planet. If it is not available, it should be claculated from n2 a3 = k2 (1+m), where k = 0.01720209895 and m is the mass of the planet ($M_\odot = 1$); a is in AU.

Conventional elements are not the only way of specifying an orbit. The $[\,x,y,z,\dot{x},\dot{y},\dot{z}\,]$ state vector is an equally valid specification, and the so-called method of universal variables allows orbital calculations to be made directly, bypassing angular quantities and avoiding Kepler's Equation. The universal-variables approach has various advantages, including better handling of near-parabolic cases and greater efficiency. SLALIB uses universal variables for its internal calculations and also offers a number of routines which applications can call.

The universal elements are the $[\,x,y,z,\dot{x},\dot{y},\dot{z}\,]$ and its epoch, plus the mass of the body. The SLALIB routines supplement these elements with certain redundant values in order to avoid unnecessary recomputation when the elements are next used.

The routines sla_EL2UE and sla_UE2EL transform conventional elements into the universal form and vice versa. The routine sla_PV2UE takes an $[\,x,y,z,\dot{x},\dot{y},\dot{z}\,]$ and forms the set of universal elements; sla_UE2PV takes a set of universal elements and predicts the $[\,x,y,z,\dot{x},\dot{y},\dot{z}\,]$ for a specified epoch. The routine sla_PERTUE provides updated universal elements, taking into account perturbations from the major planets.

next up previous
Next: Radial Velocity and Light-Time Corrections
Previous: Geocentric Coordinates

SLALIB --- Positional Astronomy Library
Starlink User Note 67
P. T. Wallace
12 October 1999