Lunar Occultations

Coordinator: Tim Haymes. 

                                      Thank you for your interest in Lunar Occultations.  


1) Total Occultations
      a)  Visual observation and timing with a stopwatch and time signal. (Classical method)
      b)  Video recording  with a GPS time inserter.  ( High timing accuracy )
      c)  Fast frame rate recording.   ( >100 fps with digital cameras, new methods )

2) Double Stars (Project Fade)
      a) Visual observers report any strange non-instantaneous behaviour to the coordinator
      b) Video observers record the event to resolve a step in the light curve
      c) Collaborate with observers via the UKoccultations Yahoo! Group to monitor the same doubles

3) Grazing Occultations  
      a) Visual observing from home if the observer is in the graze zone  
      b) Visual observing with mobile instrument(s) perhaps in a team, forming a graze cross-section
      c) Video or visual recording from optimised locations to maximise graze contacts

4) General monitoring of the Moon’s appearance while recording occultations
      – Please report anomalies to Dr Anthony Cook  (Lunar Changes Coordinator)

The subsection encourages occultation timing.  New observers should find their telescope location using Google Earth  to 0.1” arc.  GPS receivers should be used as check only.

There continues to be a very strong international interest [see Appendix A] in lunar occultations, and the Lunar Section plays a part by reporting high quality timing data to the International Occultation Timing Association (IOTA).  Results are made available to the whole observing community through regular data file updates to the Occult4 software application.  Individuals are sent a reduction of their timings from an IOTA coordinator.  This is important feedback for the observer. Timing occultations is scientifically important as well as being visually pleasing!

Ultimately your observation will be archived with the VizieR Astronomical Data Centres in catalogue VI/132A. The observer can be searched using the query string:  * Name  [e.g. *Taylor].  This link will take you to the catalogue search page.

What we observe
These timed observations continue a long history of contributions to the study of lunar motion and topography by professional research groups.  The rate of rotation of the Earth and frames of reference may also be refined using these observations by amateurs.  The use of video in preference to visual timing is encouraged and observers should consider this option.  Video is more accurate with minimal systematic error.

Total Occultations
With the aid of predictions, the identity of the star and time of disappearance or reappearance is logged.  If there are a lot of events in a short period, a good planetarium program is recommended to simulate the location of stars at the limb. Fainter stars can be timed at low phase or during Lunar Eclipse.  The Coordinator can help to identify stars if the observer is unsure, and provide predictions.

Double stars (Project Fade)
Video light curves [see Appendix B]of the ingress or egress of multiple stars at the lunar limb are used in the study of double stars. Consequently a small group within the subsection aim to specialise in multi-site observing of the same star.  We use LIMOVIE software for the analysis of occultation light curves to obtain step times and other parameters useful to double-star analysis. Results are sent to the double-star coordinator in New Zealand.   Unusual fading events are logged in the report.

Grazing occultations
These can be observed by individuals and groups near the graze regions when conditions allow. The BAA Handbook contains predictions.   In highly populated areas, observers based at home can produce interesting and useful results.  A new Graze Occultation software tool GRAZPREP-4 is being developed by Dr. E. Riedel (IOTA-ES) for mobile graze observers. This predicts where an observer should be positioned to maximise the number of graze contacts. The comparison of observed vs calculated graze timing adds a new dimension for the graze observer. For additional information, please see the links below :

Grazing Occultations of Stars by the Moon

Observing Technique – Needed Equipment

The GRAZPREP user interface for Aldebaran on 2017 Aug 16 at W  4 00 00, N 58 12 53.  Twelve contacts are predicted.


Stop-watch Timing
A digital stopwatch with lap mode is ideal for recording time intervals by the classic method. The observer starts the watch at the instant of occultation (ignoring reaction time for a moment). When the time signal is available, stop the watch in lap mode and record the UT and elapsed watch time . Now take the watch out of lap mode, and stop the watch for a second time on a time signal. Note the times again. If this is repeated an average interval can be obtained, thus minimising the error when stopping the watch at the correct moment.  Reaction time (PE) is an uncertainty, and can only be an estimate. For disappearances at the dark limb the coordinator used PE = 0.3sec +/- 0.1s  and an event time rounded to the nearest 0.05s.  Thus the reported time = Time Signal – mean watch interval – 0.3s  ( +/- 0.1 sec)

Video Timing
Since about 1990 low light video (25 or 30 fps) has been available to record total occultations,  double stars and graze phenomena.  More important though is the improvement in timing accuracy that can be achieved using GPS time insertion on the video frame. The IOTA-VTI has been designed by occultation observers for occultation observers. A good alternatively is the GPSBOXSPRITE3-U supplied by Blackboxcamera in London.

LOLA High resolution lunar limb profile
After reporting results, the observer receives a list of the submitted times and a value of (O-C). This is a measure of the difference between the observed time (O) and the calculated (C) time. The O-C value for good observation data usually fall within the range -0.10″ to +0.10″ arc. Occasionally the O-C can be a little larger (within the range -0.20″ to +0.20″).

Reporting timings
A standard report is used ( as Excel or a data file from Occult4). It records the same information as the old paper forms with a few additional fields.

BAA Journal
The subsection writes occasional reports for the JBAA highlighting successful observations. See links below, for a couple of examples:

“A Successful Lunar Graze..”

“Thirteen Contacts observed..”

Any telescope can be used.  Please indicate which parts(s) of the program you would to like to participate in and Contact the Director:  

Your message will be forwarded to the occultation coordinator who is pleased to advise on observing and reporting.   Customised predictions for your location are also available upon request.  Please ask, giving your Long. and Lat. Obtained from Google Earth.

Tim Haymes
May  2017        

Appendix A   Introduction to the VizieR Astronomical Data Centre  catalogue VI/132A.

Appendix B   Example of a double star “step” occultation recorded with video converted to AVI for analysis.

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