Planning a Lunar Graze Observation
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).
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.
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.
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.
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.
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