P20120721 Occultation July 21, 2012

(last updated 2012 05 31)


Across the globe pictured above, the three solid lines correspond to the northern limit, centerline, and southern limit of Pluto's shadow. The northern and southern limits correspond to a radius of 1400 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
Pluto Geocentric Mid-time (yyyy month dd hh:mm:ss)

2012 July 21 19:29:17± 00:01:111 UT

Pluto Minimum Geocentric Separation 0.272± 0.0331 arcsec
Position Angle (Pluto relative to the star; measured north through east) 170.61 degrees
Geocentric Velocity 21.90 km/sec
Occultation Star R magnitude 11.52

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
P20120721 Catalog 18 31 53.1953± 0.072 –19 25 05.930± 0.503
P20120721 Measured3 18 31 53.1854± 0.0403 –19 25 07.439± 0.023
From 10 Lowell 42-inch Telescope frames. See note 4.
Table 3: Projected KBO Offsets from Reference Ephemeris at the Time of the Event
Body RA (arcsec) Dec (arcsec)  

–0.0985± 0.041

+0.254± 0.023 See Notes 5 and 6

3Measured position corresponds to RA offset of –0.1394'' and Dec. offset of –1.510''.

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






20 48 36
–32 22 54
4973 N.
center of Earth
6161 N.

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 ±755 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: P20120721 Occultation Predictions for Individual Sites

Pluto Immersion (UT)9
UT Mid-Time
Pluto Altitude
Solar Altitude9

PlutoEmersion (UT)9


9The errors on all times are ±1:11 (1 minute and 11 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) 2012-05-31 15:20

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