Minor Planet Occultation
Example

• Enabling Stars/Proper Motion

• How accurate does TheSky plot minor planets?

• Stellar coordinates provided by the IOTA

• What about the minor planet?

• Where is the Star UCAC2 43092069?

• Why don't I see the occultation?

Background

This page will use the upcoming (at this writing) occultation on September 25, 2005 of the star GSC ID:2456:475 and found in the UCAC2 data by minor planet 139 Juewa.

To the best of my knowledge the UCAC2 stars do not have unique identifying numbers like the GSC, Hipparcos/Tycho-2, SAO, and other stellar datasets. Meaning - this is why you cannot search for a UCAC2 star in TheSky6.

Software Bisque Support Corner  - Please let us know if this is not correct!

The stellar ID "43092069' is therefore (based on the above assumption) not a UCAC2 designation but rather I believe the entry from the 2 MASS stellar catalog since it follows the 8 character format or possibly another designation created from the UCAC2 stellar data.

TheSky6 reports the 2 MASS ID as: 459765054 NOT 43092069. Since this discrepancy will take hours to verify AND since it is not relevant anyway I will not pursue it at this time. Anyone else able to shed some light here!?

In this particular example there are many ways to skin a cat so-to-speak. If the coordinates for the star are provided (they are and always will be provided by the IOTA page) use them to locate the star's most "accurate" coordinates for the occultation in question.

Stellar coordinates provided by the IOTA

Note the IOTA actually updates the minor planet's orbital elements and the stellar position to the highest degree of accuracy possible right up to the last minute!  Whenever possible use the coordinates from the IOTA site in addition to the stellar catalogs.

NOTE:  THIS IS NOT A LIMITATION OF THESKY6 or the programs' ability to compute accurate stellar and/or minor planet positions. Garbage in, Garbage out as they say.

Occultation work is one area where even the best stellar catalog positions are often problematic. So first add a reference marker using TheSky's Data | Add User-Defined Data to mark the exact position of the star from the best and most accurate source available, the IOTA page in this case.

Yes this is even better than using the coordinates provided by the UCAC2, Hipparcos/Tycho, USNO-B1, etc. The following demonstrates this.

Data From the IOTA occultation page for the UCAC2 star.

Data for the target star
------------------------
 * name: UCAC2 43092069
 * constellation : Gemini
 * J2000 position with proper motion  to date of event [h,m,s ; °,',"]
        RA: 07 28 57.6342  DE: +32 26 42.406
 * position source: UCAC2
 * standard error: RA,DE ["]: ( .017, .017 )
 * V mag [mag]: 11.4
 * B mag [mag]: 0.0
 * remarks:
 

NOTE: Look at the extremely high precision of the coordinates provided by the IOTA page!  Ra good to one 10,000th of an arc second. Dec good to 1,000th of an arc second. This will be more accurate than the UCAC2 stellar data. Compare these to TheSky6 J2000 coordinates below,

Equatorial 2000: RA: 07h 28m 57.63s Dec: +32°26'42.41"

The J2000 coordinates are nearly identical!

Object name: UCAC 2.0 Star
Magnitude: 11.41
Equatorial: RA: 07h 29m 18.79s Dec: +32°26'06.32"(current)
Equatorial 2000: RA: 07h 28m 57.63s Dec: +32°26'42.41"
Horizon: Azim: 65°10'11.73" Alt: +21°41'18.75"
Visibility: Rise 07:41, Set 00:02
Transit time: 15:52
Object type: Star
2 MASS ID: 459765054
2 MASS mag H: 10.270
2 MASS mag K: 10.232
2 MASS mag J: 10.535
Hour angle: -05h 50m 13.70s
Air mass: 2.70
Screen x,y: 613.00, 454.00
Proper Motion: 14.70, 31.30
Sidereal time: 01:39
Click distance: 3.0000
Celestial type: 0
 

Add a User-Defined object or X marks the spot!

Now add the reference marker at this very accurate position which can be trusted more than the catalogs themselves (based on my experience).

Add a User-Defined Data object

Add the coordinates here

With a reference marker "Magnitude" does not apply. However, if you have it enter it here so I have.

NOTE:  Using a reference marker allows a higher degree of accuracy compared to using another symbol!  The symbol defines precisely the center of the coordinates entered (assuming they are accurate and that is a BIG assumption!). Again I trust the IOTA coordinates first and foremost, use whenever possible!

When you have added the coordinates use the CENTER button highlighted above. That takes you directly to the position and shows the Reference Marker.

From here is easy to see which (if any star) corresponds to the coordinates. Using TheSky6 that should ALWAYS be the case.

If you do not see a reference marker indicating this position make certain the object type Reference Point has been enabled. That is done using View | Display Explorer.

In addition make certain you have all the stellar catalogs enabled. In my case all of them including the USNO A2.0 and full USNO-B1 which are optional datasets. By having the 1.5 billion stars available helps to further explain.

Enabling the stars/Proper Motion

Enable all STELLAR catalogs

And enable proper motion!

In this case make sure you have enabled the Proper Motion calculations as well. This should always be done for occultation work!  Now again make sure you are centered on the star. If not you can actually find and center on GSC ID:2456:475 keeping in mind these coordinates are not extremely accurate (as we all know!). However it puts you in the right field quickly and easily.

Or just use Move To and enter the IOTA coordinates in Epoch 2000 and center from there. Note you can slew to these coordinates as well. RIGHT + Left Mouse Click TheSky display and choose Move To.

Enter IOTA coordinates!

Epoch 200 and Press OK

Or knowing where the minor planet is for the date and time in question you can use TheSky's date and time for September 25, 2005 then find and center on the minor planet. By adding the minor planet to the local group you can use TheSky6 time skip tool and watch the position change every minute or every 5 minutes and watch the occultation occur (or not depending on EARTH location)!

NOTE: YOU MUST zoom all the way in as far as possible (30 arc second field) to see the discrepancies in these positions.

TheSky display

What?!

How can that be?

Are you saying there are 4 entries for the same star?

YES indeed!

The IOTA star coordinates marked by the little red Reference Marker are theoretically the best in that they are given to the highest precision. The red marker falls on the stellar position from the UCAC2 catalog, the next best position.

The RED line with arrow (vector line) indicates the stars motion and direction over a 20 year period by computing the effects of "proper motion". See above showing how to enable this.

The USNO-B1 entry USNO STAR is from the 80 gigabytes of stellar data and is next in line. Then next the GSC data, and lastly the very antiquated USNO A 2.0 11 CD-ROM dataset with 526,000 stars!

What about the minor planet?

Oh yeah.

First you must add the minor planet to TheSky6 in order to plot it. Again, best to use the IOTA data for the minor planet elements as well.

Again, several ways to skin a cat.

Please see the following for details regarding TheSky6 and minor planets!

By using the large orbital element datasets containing nearly all known minor planets to date (ALWAYS DOWNLOAD THE LATEST VERSION!) from the Lowell and or MPC page you can easily plot the predicted position for the minor planet including a 24 hour path prediction indicating direction and motion.

Minor planets plotted 275,201!

NOTE the IOTA orbital elements AND the MPCORB.DAT shown

In this case the Extended Minor Planet option will provide enough accuracy to plot the minor planet. There is only a difference of 0.18 arc seconds comparing the IOTA updated elements to the elements found in the MPCORB.DAT file from the Minor Planet Center or 0.17 arc seconds comparing the Lowell Observatory ASTORB.DAT elements to the IOTA provided.

You can of course Import the minor planet from other sources like the Lowell element file ASTORB.DAT or the MPC file MPCORB.DAT but again if you want the highest precision enter the orbital elements I recommend using the orbital elements provided on the IOTA page provided. BETTER YET use at least two sets!

 * orbital elements for (139) Juewa :
      Mean anomaly       = 297.96670632 deg
      Arg. of pericenter = 166.25593602 deg
      Long. of node      =   2.02084435 deg
      Inclination        =  10.90387668 deg
      Eccentricity       =   0.17229378
      Semimajor axis     =   2.78335433 AU
      Perihelion dist    =   2.30379970 AU
      Mag:             H = 7.77,  G = 0.15
      Epoch of elements  : MJD   53638.30800000 TDT
                           (2005 Sep 25.308)