Planning a Lunar Graze Observation

How to use graze occultation predictions

Example of a Graze Occultation:
Aldebaran (ZC 692)
on 2015 Feb 26th.

Predicted withOccult4

This example uses output from Occult 4 by Dave Herald. The graze files can be produced for your location, and are available to potential observers upon request from the BAA Lunar Section.

The date and approximate graze track locations are published the BAA Handbook. The table (below) summarizes observerability information for each event. For Aldebaran (event 6) we note the time is about 21 min after midnight on Feb 26 (Night of 25/26), the star is bright (V = 0.9), the Moon phase is just after first Quarter (Illumination = 52%), the event is a Southern limit ( i.e. graze at the Southern Cusp), and the event is at the bright limb (B).

In certain situations (such as with a bright star or thin crescent phase) bright limb events can be observed. 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.

Some events listed in the BAAH are:

This Aldebaran event is at the bright limb.

Observers situated near the track will be sent four pieces of information about the graze:

1) The Moon's outline showing where the star is on the limb at the time of the event.
2) A limb profile showing the path of the star against a projection of the Moon's outline (highly expanded).
3) 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.
4) A text file giving the Long / Lat. Time, Altitude and Azimuth of the Moon.

Additional information may be provided by the observing coordinator. In the example above, red lines have been added to the limb profile to indicate the geographical extent of the graze region. The vertical axis is in km (The width of the track on the ground is about 8 km for this part of the Moon), and this can be accurately projected onto GE using a suitable map scaling. The red lines in the example are only approximate. The observer should aim to get down to 100 m accuracy on the ground and record his/her Long/Lat from GE to 0.1" arc (5m accuracy). Suitable observing locations might be seen on GE, but in most cases local knowledge, or a rec'y is in order.

[ It may be advisable to contact the Police with your plans before-hand, especially if a team is to be deployed.]

There is a light blue histogram on the limb profile (software generated) which gives an indication of the number of D-R events 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.

Important regions are:

a) where the maximum number of events may be expected, and compare observation to measurement.
b) at the extreme limit to detect the edge of the Moon - this will need several observers placed in a 50m region (say) - one will get a "miss"
c) to check on small limb details.
d) to enhance the possibility of detecting a double star.
e) to simply to enjoy the event (as a group ?)

Daylight Event:
The 2015 April 21st event (also across Scotland, UK) is a daylight Southern Limit event at a thin crescent:-

Observing and Recording Grazes.

Graze occultations are considered by IOTA to be more valuable scientifically than Total Lunar Occultations. Observers should aim to record the time of D-R events to an accuracy of 0.5 sec or better. Visual observations are typically recorded with dictaphones or other devices, while a continuous time signal is also recorded. I use a camcorder and set the internal clock accurately to UT by synching 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 crytstal displays are effected by ambient temporature and the displayed time is subject to a delay, Eg at 5C, 200ms slow, -5C 400ms This makes the clock unsutable for observing.

Non-visual observers can record the event by video, or planetary web cam. Use a frame rate of 7 to 25 or 30 fps and a Barlow lens to magnify the image. A siderial drive is recommended. To get times for the events, record the start time of the "film" sequence by some 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 can frames.

Be prepared to start recording some minute before the time of central graze. The duration of the graze can be estimated from the limb profile.
The pink maximum line on the profile about 5 minutes.

Observers with low light video cameras can invest in a GPS time text overlay unit (e.g. GPSBOXSPRITE2-U). This puts a time stamp on every video frame. There is more information on my pages describing the timing of asteroid occultations.

Lunar Graze Reporting
Once the observation times have been extracted, they can be reported in the standard IOTA format. Please contact me with your results first.


Tim Haymes
October 2014