Grazing occultation predictions for Australia and New Zealand are supplied courtesy of the International Occultation Timing Association (I.O.T.A.) in the U.S. They are computed and despatched by Alfred Kruijshoop in Melbourne.
INTERPRETING THE PREDICTIONS
This section describes the format of the grazing occultation predictions available for download from this site. Predictions are generated by one of two sources:
The explanation below describes the IOTA-generated predictions. However the OCCULT predictions are very similar in format.
The predictions consist of two parts - the Predicted Path and the Lunar profile:
It also gives a series of latitudes and longitudes which define a line on the Earth's surface. This line, called the "predicted limit" is one where the star would just appear to skim along the limb of a smooth-limb moon. For each latitude/longitude point the following information is given:
For a more complete description of the quantities on the first page click here.
Note that the zero kilometre line always corresponds to the "predicted limit" line defined by the co-ordinates on the first page of the predictions. North is always to the top, and south always to the bottom.
The shadow of the mountains shown in the profile sweeps across the Earth as the graze takes place. By placing observers an appropriate distance north or south of the predicted limit, each will see a different "slice" of mountain. By combining their differing timings you should later be able to build up a complete picture of the shape of the lunar mountains at the position of the graze.
The information in the lunar limb profile is based on data compiled by C.B. Watts of the U.S. Naval Observatory in the 1950s. It was Watts' life work to micrometrically measure the height of mountains on the edge of the Moon from thousands of photographs taken at different position angles, and at different librations. This was a mammoth task, as the irregularities on the moon's limb as seen from Earth are exceedingly small. So it is not surprising that the Watts profiles often contain errors. The purpose of grazing occultation observations is to try to correct these limb profiles.
USING THE PREDICTIONS AND PROFILE INFORMATION
To obtain best coverage you can for example place several observers reasonably close together (although generally at not less than about 100 metre intervals) at places where many disappearances and reappearances might be expected. Conversely, at fairly uninteresting places on the profile observers might be more widely separated.
Observing stations for grazing occultations should be set up at easily identifiable points (e.g. road intersections), because if the graze is successful you will need to determine the latitude and longitude of those points to an accuracy of 1" and height to within 30 metres. (See the appendices about measuring from maps). This means that in practice you invariably have to find a reasonable compromise between a position whose co-ordinates can be determined fairly easily, and an interesting region on the profile.
When planning grazes try to decide on the sites you will use beforehand - don't leave it until the night of the graze as you will usually run out of time. If you can, check out the site the night before, or the weekend before in daylight. (A tip - avoid main roads if possible; many graze observations have been ruined by car lights or heavy trucks going past right on graze time. Also avoid sites close to railway lines - most long goods trains seem to travel during the night).
ERRORS ON NZMS260 MAPS
Some NZMS 260 1:50,000 maps issued by the New Zealand Department of Survey and Land Information (DOSLI) to replace the old 1:63,360 (1 inch to 1 mile) maps, contain errors in the placement of the tick marks for latitude and longitude along the sides of the map. This means that you will need to take very special care when you are plotting positions on, or measuring latitude and longitude from these maps.
The error consists of an occasional random displacement of a latitude or longitude tick by up to 2 mm from its correct location. This can be noticed by examining the distance between a number of the ticks along the side of a map. For example, latitude ticks (at 1 minute of arc intervals) are usually about 37 mm apart. However, you will sometimes find one tick which is 35 mm from the one above and 39 mm from the one below, or vice versa. A similar situation pertains with the longitude marks. Unless the distance between all the tick marks on a map are checked beforehand, errors in plotting of up to 3 seconds of arc are possible.
In addition, DOSLI state that because the parallels of latitude on these maps are very slightly curved, for a site near the centre of a map the difference between the curved parallel of true latitude and the straight line joining the tick marks may be up to 1 mm, corresponding to about 1.5 arcseconds.
DOSLI recommend that all determinations of position from their NZMS 260 maps be done using the map grid, rather than the latitude and longitude marks. (Brian Loader has some comments about this - see section below). We therefore suggest that all New Zealand graze positions be reported to the Section in terms of their grid co-ordinates, from which we will determine latitude and longitude via software we have available. If you want co-ordinates for a position for another purpose (e.g. a new total occultation observing site) we will also be happy to compute these from the grid co-ordinates.
GRID REFERENCES FOR SITE POSITIONS AT GRAZING OCCULTATIONS
If grid references are to be given for site positions at grazing occultations, then to be useful they must be accurate enough and give complete information.
It should be noted that 1 arcsecond of latitude is approximately 30 metres, or about 100 feet on the ground. Normally grid references are given to the nearest 100 metres or 100 yards depending on whether the New Zealand 1:50,000 or older 1:63360 maps are being used. This is NOT accurate enough when the location of an observing site must be known to an accuracy of at least 1 arcsecond. Consequently at least one additional figure is needed to the conventional three figure Easting and three figure Northing. That is, an attempt has to be made to give the reference accurate to the nearest 10 metres.
The obvious way to get this is by measuring the position from the side of the printed grid square using a FINELY and ACCURATELY marked scale. Many plastic rulers are not accurate enough; often the variation in size of the millimetres can be seen by eye.
If the metric 1:50,000 maps are being used measurement is relatively easy because the grid squares representing 1000 metres should be 20 mm along each side. Hence it is necessary to measure the distance of the site from the side and bottom of the grid box to 0.2 mm, although do try estimating to 0.1 mm.
A method often used is to mark a small spot on the map representing the site and make independent measurements from each side of the box, as well as checking the dimensions of the box itself. (Paper can expand and contract, particularly as humidity changes). By measuring from both sides of the box for the Eastings, and from the top and bottom for the Northings, you have a check on the accuracy of your measurements. Only make sure the actual measurement you finally use is from the left side and the bottom of the box.
As an example, my home site measures out to be 12.4 mm from the 85 East grid line and 14.0 mm up from the 47 North grid line. So to the nearest 10 metres the reference is 8562 East and 4770 North. It is best to give this in metres so that it becomes 85620 East and 47700 North. However to get a conversion to longitude and latitude it is necessary to give a FULL grid reference including the small numbers which are given at the bottom left corner of the map. Then my position becomes 2485620 East and 5747700 North.
If you are using the older 1:63360 maps the process is exactly the same but the calculation is slightly more complicated as the side of the grid box representing 1000 yards is about 14.4 mm. (Check this!)
Finally, a couple of points about the maps which the then Lands and Survey Department issued some years back. Roads and railways (which are of course exaggerated in width on the maps) are normally marked so that the CENTRE line on the maps coincides with the position for the centre of the road, unless two features, e.g. a road and railway, are so close that this is impossible. So measure your position beside a road from its centre line. Secondly, the maps are printed to an accuracy of 0.5 mm (or better), which only just provides our required accuracy of 1 arcsecond for graze purposes.
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