120178.20121227 Occultation December 27, 2012

(last updated 2012 11 26)



Across the globe pictured above, the three solid lines correspond to the northern limit, centerline, and southern limit of 120178's shadow. The northern and southern limits correspond to a radius of 111 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
120178 Geocentric Mid-time (yyyy month dd hh:mm:ss)

2012 December 27 23:36:53± 00:04:071 UT

120178 Minimum Geocentric Separation 0.049± 0.0691 arcsec
Position Angle (120178 relative to the star; measured north through east) –0.38 degrees
Geocentric Velocity 23.05 km/sec
Occultation Star UCAC2 magnitude 13.782

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
120178.20121227 Catalog 21 54 17.3593 ± 0.0702 +02 29 22.034 ± 0.069
120178.20121227 Measured3 21 54 17.3569 ± 0.0075 +02 29 22.104 ± 0.010
From 3 USNO 61-inch frames
Table 3: Projected KBO Offsets from Reference Ephemeris at the Time of the Event
Body RA (arcsec) Dec (arcsec)  

0.0810 ± 0.1842

–0.158 ± 0.068 See Notes 5 and 6

3Measured position corresponds to RA offset of –0.0353 and Dec. offset of 0.0696.

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 Haumea are those given by JPL Horizon's ephemeris (120178 source file: JPL9; Earth center source file: DE405).

6Data from the USNO 61-inch, SMARTS 0.9m, and Lowell 42-inch telescopes over the span of 3 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 Quaoar: (i) constant offsets in RA and Dec, (ii) linear (in time) offsets in RA and Dec, and (iii) sinusoidal terms with the Earth's orbital period. The model was propagated to obtain the predicted position and error of the KBO at the time of the occultation. The errors listed for the KBO are 1 standard deviation.


Table 4: Site Information

East Longitude

Site Altitude6






SVH Observatory
–74 56 48
40 57 37
2471 N.
–71 29 06
42 36 36
2620 N.
center of Earth
1516 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 120178's shadow in the shadow plane. The errors on all closest approach distances are ±2125 km (one standard deviation). "S." means the site is south of the center of 120178's shadow. "N." means the site is north of the center of 120178's shadow.

Table 5: 120178.20121227 Occultation Predictions for Individual Sites

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

KBO Emersion (UT)9

SVH Observatory

9The errors on all times are ±4:07 (4 minutes, 07 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) 2012-11-26 13:31

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