Skip Navigation

Announcement

Updating to Horizons v4.90 Next Week

Horizons version 4.90 will go into effect on the server late Monday, October 4 PDT, along with updated documentation. This e-mail is advance notice of new output options, redefinition, and format changes some might need to accomodate.

The redesigned web interface and new APIs are already public. Many thanks to A. Chamberlin for spearheading that big effort with excellent results.

OBSERVER tables

Quantity #47

1. New output quantities will be available to support observational astronomers. Selection of observer table quantity #47 will return three new columns of data with the column label:

Sky_motion Sky_mot_PA RelVel-ANG

Sky_motion

This is the total apparent angular rate of motion of the target in the plane-of-sky (POS); the combined (root-sum-square) of the linear RA*cos(DEC) and DEC angular rate components provided by quantity #3, but as a single angular rate in units of arcseconds/minute.

Sky_mot_PA

The position angle of the target’s direction of motion in the plane-of-sky, measured counter-clockwise from the apparent of-date north pole direction. Combined with the Sky_motion angular rate, this direction indicator can simplify tracking targets across the sky.

RelVel_ANG

This is the flight path angle of the target’s relative motion with respect to the observer’s line-of-sight (“RelVel-ANG”, -90 -> 0 -> +90). Positive values indicate motion away from the observer, negative values are toward the observer:

-90 = target is moving directly toward the observer 
  0 = target is moving at right angles to the observer's line-of-sight 
+90 = target is moving directly away from the observer

This information on the out-of-plane motion of an object can be useful to active sensors such as radar or simply by providing a sense of how the target’s motion is directed relative to the line-of-sight.

Quantity #48 will return one new column of data:

Lun_Sky_Brt

This gives sky brightness due to atmospheric-scattered moonlight; the difference in magnitudes per square-arcsecond between the observed sky brightness at the target’s sky position (when the Moon is above the horizon) and a reference dark sky.

This is a function of lunar phase, elevation of the Moon, elevation of the target, the separation angle between the Moon and target, and atmospheric extinction.

Output occurs only for topocentric Earth observers when both the Moon and the target are above the local horizon, the observer is in astronomical twilight/dawn or darker (i.e., Sun is below horizon), and the target is not the Moon. In other situations, “n.a.” is output.

This information can be used to inform exposure times and predict the visibility of a target having some brightness relative to the brightness of the background sky, much like a signal-to-noise ratio.

For example, suppose you requested output of observer table quantities 4, 9, 8, 48 for an object and obtained this output:

 Date__(UT)__HR:MN     Azi____(a-app)___Elev    APmag   S-brt  a-mass mag_ex  Lun_Sky_Brt
 2004-Dec-24 23:10 Am  186.279261  40.652895   17.634   4.990   1.532  0.371       -5.257
 2004-Dec-24 23:20  m  189.063178  40.345335   17.634   4.990   1.542  0.373       -5.288

The result indicates that, when at an elevation of 40.345 degrees (and just after astronomical twilight), the airless visual magnitude of the target is nominally expected to be 17.634, plus 0.373 magnitudes due to atmospheric extinction, so 17.634 + 0.373 = 18.007 magnitude.

If your normal dark-sky limiting magnitude is ~18, it might seem just barely possible. However, with the target seen against a sky brightness due to the Moon of -5.288 mag/arcsec^2, the target would need to be closer to 16th magnitude for a chance at visual-band detection under these circumstances.

In this case, the target radius is also known. This permits a surface brightness calculation of 4.990 magnitudes/arcsec^2 for the target, where no atmospheric extinction is considered. Comparison with the lunar sky brightness of -5.288 mag/arcsec^2 also indicates the target will be too dim to overcome lunar sky brightness and not be visible at that location and time, having an “SNR” of 4.990/|-5.288| = 0.944.

For OBSERVER & VECTOR table types:

2. The lunar rotation and cartographic model will be updated to use JPL's DE441 mean-Earth lunar solution (Park et al., The Astronomical Journal, 161:105 (15pp), 2021), replacing the IAU lunar model where possible. This will improve lunar surface coordinate accuracy from the typically 100-meter level to 10's of centimeters.

The new high-accuracy interval is used over A.D. 1550 - 2650. Outside that span, Horizons will automatically transition to use the low-precision IAU model. Models used are given in the output header. For example, output involving lunar cartographic models over 1500-2700 would contain the following summary line in the header:

>Target pole/equ : IAU_MOON, MOON_ME, IAU_MOON {East-longitude positive}

… indicating output begins with the IAU_MOON model, transitions to MOON_ME at 1550, then switches back to IAU_MOON after 2650.

If the output is entirely within the 1550-2650 interval of the high-precision model (typical usage), the label would instead be …

>Target pole/equ : MOON_ME {East-longitude positive}

“ME” indicates “mean Earth”. Like the IAU lunar model, the mean Earth/polar axis model uses a +Z axis aligned with the north mean lunar rotation axis, while the prime meridian contains the mean Earth direction. This system is also sometimes called the “mean Earth/mean rotation axis” system or just “mean Earth” system, and is a rotation from the principal axis (or PA) system sometimes also used in lunar studies (though not available in Horizons).

For OBSERVER table type:

3. & 4. The observer-table transit marker ('t') was changed to indicate the target crossed the observer's meridian since the prior output step (upper culmination).

Previously, the marker was applied to indicate maximum elevation angle. The prior maximum elevation event is still flagged, but now with a newly added ‘e’ marker.

The two conditions (upper culmination transit and maximum elevation) can occur at times differing by seconds or even minutes for close objects moving quickly.

Horizons gives priority to the transit ‘t’ marker if output resolution is too coarse to distinguish the times (no ‘e’ marker output).

Some objects such as orbiting spacecraft may not transit the observer’s meridian but still have a maximum elevation angle, therefore can have an ‘e’ marker but no ‘t’ marker.

If “rise-transit-set” ONLY output is selected, only those events will be returned. Maximum elevation ‘e’ will not be included in RTS-only output, only in general topocentric ephemeris tables with output steps less than 30 minutes.

Thanks to Tomasz Kluwak (K80) for bringing up the issue!

Formatting changes:

These changes could affect those who parse table contents (assuming fixed column positions) or column-header labels (exact string matches):

5. One space was added between output quantities to improve visual readability. This affects non-CSV (default) output only.

6. Quantity #4 azimuth & elevation

The gritty details on column header changes for azimuth & elevation are summarized below for those hardcore folks who need to know:

Default output (six figure precision case, no CSV):

                          Now                       (Was)
    airless   :   Azi____(a-app)___Elev        Azi_(a-appr)_Elev
    refraction:   Azi____(r-app)___Elev        Azi_(r-apprnt)_Elev 

  CSV output (six figure precision):     
    airless   :   Azi_(a-app), Elev_(a-app),   <unchanged>  
    refraction:   Azi_(r-app), Elev_(r-app),   Azi_(r-appr), Elev_(r-appr)

  NEW (extra precision output option w/nine figure precision)
    Standard (not CSV)
    airless   :   Azimuth__(a-app)__Elevation
    refraction:   Azimuth__(r-app)__Elevation

    CSV       
    airless   :   Azimuth_(a-app), Elevation_(a-app)
    refraction:   Azimuth_(r-app), Elevation_(r-app),