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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.


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:

  1. The predicted path page gives general information about the graze. This includes:

    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.

  2. The profile page gives a diagram of the expected shape (profile) of the Moon's limb at the point of occultation. On this diagram:

    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.


  1. First, print out the predictions and profile using a non-proportional font such as Courier 10 point. (This will ensure that the proportions on the lunar limb diagram on the second sheet will remain correct).

  2. Now, very carefully, plot the latitude and longitude given on the first sheet on a map of about 1:50,000 scale or better. (N.B: When plotting on New Zealand maps pay careful attention to the notes in the appendix below). Remember that the plotted line gives the "predicted limit" - i.e. the line on the Earth along which the star would seem to graze the edge of a perfectly smooth Moon.

  3. On the second sheet, join the x's and *'s to give an idea of what mountains are expected at this point on the edge of the Moon. Note that the profile you draw is exaggerated by about 20 times in the vertical direction. You should now have a "feel" for the shape of the Moon's edge at the position of the graze, and this will help you when it comes to placing observers on the ground.

  4. Recall that down the right hand side of the profile sheet are given distances in kilometres. These are perpendicular to the predicted limit - i.e. perpendicular to the line you have plotted on your map; north is towards the top and south towards the bottom. Using the shape of the profile, the kilometre marks, and the limit line on your map, you can now choose positions for your observers at intervals either perpendicularly north or south of the limit line. (Your observers do not need to be all exactly on the same line perpendicular to the limit line - a degree of spreading in the east/west direction is quite permissable. But what is important is to remember not to put two observers the same perpendicular distance from the limit line - because then they will see the same events!).

    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).

  5. In deciding on your observing stations it is most important not to take the predicted profile as "gospel" - it is after all the point of your observations to try to correct the errors in shape of the profiles. Remember too that the star's position might be in error by some small amount, leading to differences in the location of the profile on the ground. (An indication of potential errors is often given on the first prediction page under "Probable error of star's declination"). There is nothing worse than fine-tuning the placement of your observers to just get a lunar peak, and then finding that the whole profile has shifted north or south by a few hundred metres. For this reason don't be too pedantic about the placement of your observers and, if you have people to spare, it is a good idea to spread them a little beyond the northern and southern bounds of the interesting parts of the profile.

  6. If you were now to draw a horizontal line across the profile at the position of one of your observers, you would be able to get an approximate idea of what that observer will see. From his or her perspective the star will enter from the left side of the diagram, and you will be able to tell where it might disappear and reappear along the limb. Do however remember that you can often determine much more fine structure from your observations than is plotted on the profile.

  7. Now look at the top of the profile. Across the top is listed "Minutes from C.G. (Central Graze)"; negative before mid-graze, and positive afterwards. You can obtain the approximate time of central graze from the appropriate entry corresponding to your longitude in the time column on the first page. Using the time of C.G. from the first sheet and scale along the top of the profile, you can estimate how many minutes before and after C.G. events will occur at each of your sites. E.g: you can deduce that one observer might expect events starting about 2 minutes before central graze and ending one and a half minutes after. However, it is highly advisable to be observing continuously at least five minutes before the first expected events.


  1. Allow plenty of time for observers to get to their sites and to have their equipment set up, with at least half an hour to find and lock on to the star. One of the major causes of graze expedition failures is that not enough time is allocated for people to get to their sites and then find the star. A good rule of thumb is to think of how long you will need to do this - and then triple it! (Yes, seriously!).

  2. The last column of the predictions gives the "Cusp Angle", or angle measured from the northern or southern cusp of the moon around the dark limb to the star. (Negative cusp angles are measured into the bright limb; The cusp angle is an easier quantity to use than the Position Angle, because the cusp is much more obvious than the north or south point of the Moon). The larger the cusp angle, the greater the distance of the star from the cusp at mid-graze. Don't forget that you will not in general be able to see the dark limb of the moon. Also remember that in the southern hemisphere the southern cusp of the moon is the one which is highest in the sky, and the northern cusp is the one nearest the northern horizon when the moon is viewed with the naked eye. (In a telescope of course the image may be inverted).

  3. Make sure you have located the star at least 15 minutes before the time the graze starts (i.e. time of central graze minus a few minutes). Remember that the Moon moves about its own diameter each hour, so even 15 minutes before the graze the star will be quite some distance from the Moon.

  4. Because the Moon is moving eastwards in the sky, before Full Moon the star will appear to move along the dark limb towards the bright part; after Full Moon it will move in across the bright limb (which of course may make the star more difficult to find).

  5. Your observers must be aware that graze events will generally occur suddenly, and without any warning, so critical concentration is essential for about five minutes centred on central graze time if personal "reaction times" are to be kept to a minimum. Once the graze is complete, individual observers should work out and report their occultation timings, and the graze co-ordinator should measure their geographical positions accurately.



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.


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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