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The motivation for this question comes from this question. A star is named in the occultation prediction shown below, from AsteroidOccultations.com's News & Announcements for 2014 MU69 on 2017-Jun-03 03:11.

I believe that the star is in the Omega Centauri cluster, and I believe it is between +12 and +14 visual magnitude roughly. The name given in the prediction is NH -08194514G The location shown in the prediction is R.A: 19 03 34.493 DEC -20 34 39.34 but according to Wikipedia the coordinates of the cluster are roughly R.A. 13h 27m, DEC. -47deg, 29m.

I need help understanding why the two coordinates seem so different (am I misunderstanding the prediction?) and finding out how to look up this particular star’s visual magnitude by using its name. I’ve tried a few quick searches but realized that I was over my head and not likely to get the right information.


From the linked question:

People have fanned out over South America and South Africa with telescopes with cameras and GPS clocks and have attempted to watch the shadow of a roughly magnitude +13 star cast by a rock in the KUIPER BELT pass over the Earth!

Early Twitter reports reported clear skies and images of the Omega Centauri star cluster. There is a Twitter hash tag for these events, #mu69occ. Here is a 500 millisecond exposure from a few days before the occultation. The short exposure time is necessary to resolve small scale debris from the main occultation.

enter image description here

above: Omega Centari Star Cluster, 500 millisecond exposure. Tweeted by Alex Partker.

The goal is actually to look for shadows cast by even smaller debris, orbiting around the rock in the Kuiper belt. This is for New Horizons mission planning.

enter image description here

above: From AsteroidOccultations.com's News & Announcements for 2014 MU69

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  • $\begingroup$ There's no need to quote the other question here. It doesn't serve to improve to clarify your question. As for your actual question, I would hazard a guess that one coordinate is the J2000 position and the other is the position at the expected time of transit. Either that or someone is just wrong. $\endgroup$ – zephyr Jun 26 '17 at 14:23
  • $\begingroup$ @zephyr I don' think the entire cluster could have moved 27 degrees in declination in 17 years! $\endgroup$ – uhoh Jun 26 '17 at 14:29
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    $\begingroup$ Looking into this, but a declination of -20 seems way too far north for Centaurus-- they might be talking ecliptic coordinates or something. More soon maybe. $\endgroup$ – user21 Jun 28 '17 at 18:54
  • $\begingroup$ Not sure if this helps, but gea.esac.esa.int/archive is the New Horizons/GAIA catalog where these stars come from... I think. However, I can't find a connection between the numbers given and the stars in the catalog. At first, I thought it was the random_index field, but apparently not. You may want to ping the email address in the newsletter for more info. $\endgroup$ – user21 Jun 30 '17 at 1:10
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I believe this is a misinterpretation of this tweet: https://twitter.com/Alex_Parker/status/870169900970639360

If you look at the reply chain, it starts with https://twitter.com/Alex_Parker/status/870168057032921088 where Alex Parker states "Also, last night while we were packing up I took a very quick image of the Carina Nebula with one of our #MU69occ cameras."

The tweet in question states "This is what Omega Centauri looks like after just 500 milliseconds of exposure with one of the #MU69occ cameras."

In other words, Alex Parker was taking images of various clusters with the camera he plans to use for the MU69occ event. The image targets have nothing to do with the occultation itself.

I also went to https://ssd.jpl.nasa.gov/horizons.cgi and took a geocentric view of "2014MU69" for the month of June and it confirms this object is nowhere near Centaurus.

EDIT: I emailed Amanda M. Zangari (azangari@boulder.swri.edu) and her response was:

Hi Barry,

I do not know where the star numbers come from.

Gaia IDs for each of the stars are here:

http://www.boulder.swri.edu/MU69_occ/gaiainfo.html
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  • $\begingroup$ OK thanks for clearing that up! I'll go fix up any references to Omega Centauri in a bit. Good detective work! Still, I'm really asking for "Where and how to look up the coordinates and magnitude of the star in this occultation prediction?" $\endgroup$ – uhoh Jun 29 '17 at 4:20
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    $\begingroup$ I'm guessing NOMAD (usno.navy.mil/USNO/astrometry/optical-IR-prod/nomad), but will dig deeper later. $\endgroup$ – user21 Jun 29 '17 at 5:45
  • $\begingroup$ Updated w/ info on stars $\endgroup$ – user21 Jul 10 '17 at 13:15
  • $\begingroup$ OK now I understand! The linked list in the e-mail contains links to VizieR for the three stars with g-band (~0.52 um) mean magnitudes of +15.1, +15.4, and +12.6 which have occultation predictions by 2014 MU69 on... $\endgroup$ – uhoh Jul 10 '17 at 14:32
  • $\begingroup$ June 3, July 10 and July 17 respectively. This is great - thank you very much! $\endgroup$ – uhoh Jul 10 '17 at 14:33

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