C20110725 Occultation July 25, 2011

(last updated 2011 07 15)


Across the globe pictured above, the three solid lines correspond to the northern limit, centerline, and southern limit of Charon's shadow. The northern and southern limits correspond to a radius of 605 km. The upper and lower dashed lines indicate 3-sigma errors. The shaded area represents where the sun is more than 12 degrees below the horizon.
Table 1: Prediction Details
Charon Geocentric Mid-time (yyyy month dd hh:mm:ss)

2011 July 25 01:57:38± 00:00:101 UT

Charon Minimum Geocentric Separation 0.346± 0.0111 arcsec
Position Angle (Charon relative to the star; measured north through east) -9.01 degrees
Geocentric Velocity 21.02 km/sec
Occultation Star UCAC2 magnitude 15.82

1One standard deviation of random error.
2The UCAC bandpass (579-642nm) is between V and R.

Table 2: Reference Star Position
Reference star position:
(UCAC2, at epoch of event)
RA (h:m:s; J2000) Dec (d:m:s; J2000) Notes
C20110725 Catalog 18 22 40.1225 ± 0.1602 –18 54 11.825 ± 0.198
C20110725 Measured3 18 22 40.1232 ± 0.0069 –18 54 11.921 ± 0.006
From 7 CTIO SMARTS 0.9-m Telescope frames. See note 4.
Table 3: Projected KBO Offsets from Reference Ephemeris at the Time of the Event
Body RA (arcsec) Dec (arcsec)  

–0.1737 ± 0.0070

+0.181± 0.008 See Notes 5 and 6

3Measured position corresponds to RA offset of +0.0103'' and Dec. offset of –0.0963''.

4All "offsets" are defined in the ("corrected" – "reference") or ("observed" – "calculated") sense. The offsets should be added to reference positions to get the measured positions, which we use to calculate the prediction.

4Data analyzed using UCAC2 reference network. A weighted average of the data from the two telescopes was used to calculate the RA and DEC. The errors given are 1 standard deviation.

5The reference positions for Pluto and Charon are those given by JPL Horizon's ephemeris (Pluto source file: PLU017; Earth center source file: DE405). Our residual model is based on the assumption that the same offsets in RA and Dec apply to both Pluto and Charon.

6Data from the du Pont 2.5m, USNO 61-inch, and Lowell Astrograph telescopes over the span of 6 years were reduced with respect to stars in the UCAC2 catalog. A model was developed to fit the residuals obtained from our measured positions compared to that of the object's JPL ephemeris. All residuals obtained from the different telescopes were consistent with the model. The model includes the first-order effects of errors in the orbital elements of Pluto: (i) constant offsets in RA and Dec, (ii) linear (in time) offsets in RA and Dec, (iii) sinusoidal terms with the Earth's orbital period, and (iv) sinusoidal terms with the Pluto-Charon mutual orbit. The model was propagated to obtain the predicted position and error of the Pluto at the time of the occultation. The errors listed for the Pluto are the RMS of the models fitted.


Table 4: Site Information

East Longitude

Site Altitude6


Charon Distance7




Clay Center
–71 08 14
42 18 27
2177 S. (Pluto)
–71 29 06
42 36 36
2160 S. (Pluto)
–73 12 06
42 42 42
2147 S. (Pluto)
center of Earth
9857 S.

6Altitude of each observatory is measured in kilometers above sea level.

7"Distance" refers to the closest approach distance of the "Site" to the center of Pluto's shadow in the shadow plane. The errors on all closest approach distances are ±713 km (one standard deviation). "S." means the site is south of the center of Pluto's shadow. "N." means the site is north of the center of Pluto's shadow.

Table 5: C20110725 Occultation Predictions for Individual Sites

Charon Immersion (UT)9
Charon Mid-Time (UT)
Charon Emersion (UT)9
Charon Altitude
Solar Altitude9
Clay Center

9The errors on all times are ±0:10 (10 seconds; one standard deviation). The solar altitude is given for locations where it is relevant (solar altitude greater than -18°). No entry in the immersion and emersion columns indicates that the occultation is not predicted to be visible at that site.


Last updated by Carlos Zuluaga (czuluaga@mit.edu) 2011-07-15 14:00

Please direct all inquiries to PAL (planetary-astronomy@mit.edu)