Quaoar.20120417 Occultation April 17, 2012

(last updated 2012 03 29)



Across the globe pictured above, the three solid lines correspond to the northern limit, centerline, and southern limit of Quaoar's shadow. The northern and southern limits correspond to a radius of 500 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
Quaoar Geocentric Mid-time (yyyy month dd hh:mm:ss)

2012 April 17 02:15:06± 00:01:111 UT

Quaoar Minimum Geocentric Separation 0.024± 0.0201 arcsec
Position Angle (Quaoar relative to the star; measured north through east) 19.78 degrees
Geocentric Velocity 11.92 km/sec
Occultation Star UCAC2 magnitude 11.422

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
Quaoar.20120417 Catalog 17 34 59.4820± 0.0702 –15 31 32.710± 0.074
Quaoar.20120417 Measured3 17 34 59.4816± 0.0254 –15 31 32.641± 0.015
From 11 SMARTS 0.9m frames
Table 3: Projected KBO Offsets from Reference Ephemeris at the Time of the Event
Body RA (arcsec) Dec (arcsec)  

–0.2907± 0.0151

–0.119± 0.010 See Notes 5 and 6

3Measured position corresponds to RA offset of –0.0057 and Dec. offset of +0.070.

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 (Quaoar source file: JPL16; 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






Canary Islands
–15 18 00
28 00 00
1889 N.
Namibia (ATOM)
17 06 00
–22 26 22
1719 S.
20 48 36
–32 22 54
2595 S.
center of Earth
746 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 Quaoar's shadow in the shadow plane. The errors on all closest approach distances are ±620 km (one standard deviation). "S." means the site is south of the center of Quaoar's shadow. "N." means the site is north of the center of Quaoar's shadow.

Table 5: Quaoar.20120417 Occultation Predictions for Individual Sites

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

KBO Emersion (UT)9

Canary Islands
Namibia (ATOM)

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) 2012-04-13 12:00

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