P20120718; July 18, 2012

³Measured position corresponds to RA offset of –0.2185'' and Dec. offset of +0.265''.

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 1: Prediction Details
Pluto Geocentric Mid-time (yyyy month dd hh:mm:ss)	2012 July 18 04:14:53± 00:01:02¹ UT
Pluto Minimum Geocentric Separation	0.062± 0.046¹ arcsec
Position Angle (Pluto relative to the star; measured north through east)	–8.98 degrees
Geocentric Velocity	22.56 km/sec
Occultation Star R magnitude	13.7²

Table 2: Reference Star Position
Reference star position: (UCAC2, at epoch of event)	RA (h:m:s; J2000)	Dec (d:m:s; J2000)	Notes
P20120718 Catalog	18 32 14.6840± 0.244	–19 24 19.550± 0.252
P20120718 Measured³	18 32 14.6689± 0.0267	–19 24 19.284± 0.041	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)
Pluto	–0.1025± 0.058	+0.253± 0.022	See Notes 5 and 6
³Measured position corresponds to RA offset of –0.2185'' and Dec. offset of +0.265''. 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.

Site	East Longitude	Latitude	Site Altitude⁶ (km)	Distance⁷ (km)	Velocity (km/s)
Lowell	-111 39 54	35 12 12	2.219	3889 N.	22.90
Magdalena Ridge Obs.	-107 11 05	33 58 36	2.22	3817 N.	22.92
McDonald Observatory	–104 01 21	30 40 17	2.08	3596 N.	22.94
Univ. of Central Florida	-81 22 32	28 32 10	0.	3246 N.	22.96
Wallace	-71 29 06	42 36 36	0.107	3983 N.	22.89
Williams	-73 12 06	42 42 42	0.215	4017 N.	22.89
Geocenter	----------	---------	center of Earth	1409 S.	22.56

Site	Pluto Immersion (UT)⁹	UT Mid-Time	Pluto Altitude	Solar Altitude⁹	PlutoEmersion (UT)⁹
Lowell	-----------	04:16:11	29°	–17°	-----------
Magdalena Ridge Obs.	-----------	04:15:57	32°	–21°	-----------
McDonald Observatory	-----------	04:15:50	36°	–24°	-----------
Univ. of Central Florida	-----------	04:14:19	42°	-----------	-----------
Wallace	-----------	04:13:39	27°	-----------	-----------
Williams	-----------	04:13:45	27°	-----------	-----------
Geocenter	-----------	04:14:53	-----------	-----------	-----------

P20120718 Occultation July 18, 2012