I have just uploaded an update to Occult, and to the XZ80 catalogue of stars used for lunar occultations.

I have updated the XZ80 catalogue used for Lunar Occultations to use Gaia DR1 position when available. Over 80% of the stars in XZ80 are now based on Gaia.

As a result of the update to XZA80, the typical residual for a lunar occultation event is now smaller than 0.04".

Review of predictions and reductions showed that the (Observed - Predicted) time differences were larger than expected given the small residuals in the reductions. The major cause for this was identified (after much effort) as being due to how the position angle of the event was calculated - with the traditional method having an error of up to ~0.2°. This error resulted in an incorrect calculation of the correction due to the lunar limb; the great majority of predictions are now within 0.3sec of the observed time [providing the user has downloaded the LOLA lunar limb (Occult download #26)]. The same issue existed when generating a graze profile - with the orientation of the profile potentially being in error by up to ~0.2 °.

It is appropriate to note here that with this type of prediction accuracy, VISUAL observations to determine the EVENT TIME of ordinary occultations no longer have an accuracy that can lead to the observation being useful.

However this is not to say that lunar occultations are no longer of any value. Rather a lunar occultation observed with video provides an opportunity to scan the stars in the ecliptic at a resolution of about 0.01". This leads to discovery of new double stars, precise measurement of known double stars, exclusion of the presence of a companion star (within limits), and detections of stellar diameter. Observers are encouraged to observe lunar occultations and report the light curves (using the functionality in Occult) so that the light curves can be archived at VizieR. Just as important, all double star detections/measurements (including non-detections) should be reported to Brian Loader (in New Zealand) for reporting in the Journal of Double Star Observations (from where they are picked up by the US Naval Observatory for the Washington Double Star Catalogue).

Dave Herald
November 20, 2016