Planning a Lunar Graze Observation

How to use graze occultation predictions and plan to observe them.

Example of a Graze Occultation:

ZC 678 on 2017 Feb 05

Predicted using Occult-4

Fig-1 ...............................Fig-2................................Fig-3...............................Fig-4

1) This example is of a 5th magnitude star (ZC678 = 81 Tau) grazing the Southern limb of the Moon from a location in Oxfordshire UK. The output is from Occult-4 program which is displayed above at two expansion levels in Google Earth (GE, Fig-1 and Fig-2) to indicate the line of the graze at the mean limb.(Green Line)

2) From GE you can find suitable locations from which to observe the event, like a public car park. The limb profile in Fig- is the edge of the irregular Moon measured by the Lunar Reconnaissance Orbiter (LRO) laser ranging experiment. I have added extra colour to the diagram and the red lines. The blue histogram (selectable in Occult) indicates the relative number of contacts to expect. The red lines then show were the observer should be positioned with respect to the mean limb shown on GE to see those contacts. The green line is the same as the white line but ive offset it slightly! This is the path of the star at a tangent to the mean (smooth) moon limb.

3) The lines are measured in Km either North or South of the mean limb. Fig-4 is a report displayed in the Occult-4 reporting page.

4) The date and approximate graze track locations are published the BAA Handbook which summarises observability information for each event. In some situations, as with a bright star or thin crescent phase, a bright limb event maybe predicted. The majority of events in the BAA Hand Book are at dark limb. The Cusp Angle (CA) is an indication of how far the star is from the illuminated cusp. For dark limb events, the larger the CA, the better from the observers' point of view.

Planning Details
Observers situated near a graze track will be using three pieces of information about the graze:

5.1 ) A limb profile showing the path of the star against a projection of the Moon's outline (highly expanded). Fig-3
5.2 ) A Google Earth KMZ file. This displays the graze track in GE. The single green line is for a star grazing the mean limb at Sea Level. Fig-1
- although not apparent from diagrams, the observers' height above MSL can make a considerable difference to what is seen. This is not obvious at the time of observation. Additional software is needed to explore this effect - beyond the scope of these notes.
5.3 ) A text file giving the Long / Lat. Time, Altitude and Azimuth of the Moon.

6) It should be noted that the graze limit is a green line on Google Earth and this corresponds with the white line on the Limb Profile in Fig-3. If the white line doesn't intersect any peak in the limb profile, the observer should find a location that does, and hopefully maximise the number of contacts.

7) There is a light blue histogram on the limb profile (software generated) which gives an indication of the number of D-R contact that might be seen if the observer was in that region of the limb cross-section. Using the limb profile, observer(s) can align themselves at places within the track to observe events of interest, and time them.

8) Observing positions are usually selected to:

8.1) maximise the number of D-R timings, and to compare Observation to Calculation. The O-C value called "residual" is the result of interest to the amateur. This is returned to the observers when the report is submitted and analysed. [ Observers using Occult-4 will be able to produce preliminary O-C values ]
8.2 ) find the extreme limit and to detect the edge of the Moon - this will need several observers placed in a 50m region (say) - one will get a "miss"
8.3) check on small limb details.
8.4 ) enhance the possibility of detecting a double star.
8.5 ) simply to enjoy the event (as a group ?)

9) The vertical axis on the limb profile is in km. and this can be projected in the minds-eye onto GE using a suitable map scaling. Planning a location is then a question of finding an area that can accommodate several mobile observers with telescopes at strategic positions based on the number of contacts expected. The team leader for an expedition should seek permission for use of non-public locations. Its best to do this face-t- face or by a telephone call. This writer has found that email generally doesn't always produce a reply.

Observing and Recording Grazes

10) Graze occultations are considered by IOTA to be more valuable scientifically than Total Lunar Occultations. Observers should aim to visually record the time of D-R contacts to an accuracy of 0.3 sec or better. Visual observations are typically recorded with dictaphones or other devices, while a continuous UT time signal is also recorded. I can use a camcorder and set the internal clock accurately to UT by sync'ing on a Telephone time pip, or I can point the camera at a radio controlled clock and record that. There are many possibilities. I also have a home made MSF (60KHz) receiver with a pizo sounder.

Note: Radio clocks with liquid crystal displays are effected by ambient temperature and the displayed time is subject to a delay, Eg at 5C, 200ms slow, -5C 400ms This makes the LCD clocks unsuitable for observing in cold conditions.

11) Mobile phones have clock or stop-watch functions that will record multiple lap times - practice and care is needed - perhaps with a second phone recording audio. I have found the action of "pressing" a soft-key non-intuative, as there is no tactile feedback.

12) Another possibility to time events by audio is using the mobile app.:"TimeThe Sat" from This has an audio clock setting providing a continuous speaking clock 1 sec tone sync'ed to UT. The settings give a choice of NTP time servers. I tested this app. (once) and found the pips in step with the BT land-line (123) speaking clock with no appreciable correction. This App can be recorded along with audio observations. Its original purpose is irrelevant.

Time The Sat gives an audio time announcement with pips that may be recorded.

13) Accurate wrist watches: An "atomic watch" or "radio watch" can be be used as a relaible secondary standard. I have a Cassio "wave ceptor" that uses a build in radio received (60 or 77 KHz) which is sychronised at midnight or on demand. When the alarm function is set the alarm pulses are coincident with the BT land line speaking clock. This could be applied to timeing occultations or creating time markrs for audio recordings.

14) CCD observers can record the event with planetary web cam. Use a frame rate of 10 to 25 or 30 fps and a Barlow lens to magnify the image. A sidereal telescope drive is recommended. To get times for the events, record the frame times by a reliable method. If this requires a computer internal clock, ensure that the clock is synchronised with UT using an internet time protocol. Dimension 4 software (free windows app by Thinking Man ) will do this. EZ planetary software and FireCapture have the ability to add a time stamp to the web-cam frames.

Be prepared to start recording/observing some minutes before the time of central graze. The duration of the graze can be estimated from the limb profile.

15) Observers with low light analogue video cameras can invest in a GPS time text overlay unit (e.g. GPSBOXSPRITE or the IOTA-VTI). This puts a time stamp on every video frame. This is the most reliable method, and the method of choice. The WAT-910HX will record at field level (50 fps)

There is more information on my pages describing the timing of asteroid occultations.

Lunar Graze Reporting
15) Once the observation times have been deduced, they can be reported in the standard IOTA format (Fig-4). This is available through Occult4, LOW, and Lunareport.exe software applications. There is also a spreadsheet option. Written reports are no longer accepted, it is all electronic.

See here for further information:

16) Keep all your original records ( images, video) and notes so that you can refer to them at a later time in order that observations may be checked or re-analysed.

In retrospect
17) Before accurate surveys of the Moon by orbiters, the limb profile was observed and measured from Earth by C.B Watts.

"Watts: a 1950's photographic survey conducted by C.B. Watts (U.S. Naval Observatory) in which he measured the heights of lunar features along the edge of the Moon as recorded in thousands of photographs taken at different phases and librations The Watts profile was the best available in the last century"

Watts is no longer used. In fact I have graze data from the 1980s reduced with Watts giving an O-C of 0.53" arc. but when analysed with LRO data the O-C is 0.05". So the observation was pretty good with tape-recorder and clock and careful use of the Personal Equation (reaction time).

18) The BIG advantage with high frame rate video is that fade events and double star features will be time resolved and recorded.

Tim Haymes
October 2014
Updated Aug 2017