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:

• IOTA (International Occultation Timing Association);
• OCCULT (Dave Herald's occultation software)

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:
   
   
   • The date and time
   • The closest the graze line comes to the location listed
   • Whether the star grazes the Northern or Southern limb of the Moon
   • Star name, magnitude, and whether it is a double star
   • Moon phase data

   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:

   • The expected Universal Time of central graze at that point
   • Moon altitude
   • Moon azimuth (angle of the moon measured around the horizon from north towards the east)
   • Tan (Z), the tangent of the zenith angle of the Moon
   • Sun altitude
   • Position Angle of the graze (angle measured around the Moon's limb from the north point of the Moon towards the east; positive numbers are measured into the dark limb; negative numbers into the bright limb)
   • Cusp Angle of the graze (angle measured around the Moon's limb from the cusp to the point of central graze)

   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:
   
   
   • The Dark lunar limb is defined by the D's
   • The Bright (sunlit) lunar limb is defined by the B's
   • The "Worst-case" scenario of a three-kilometre high lunar peak being sunlit is defined by the W's
   • The Terminator is defined by the T's
   • The expected profile of the edge of the Moon is defined by the x's, o's or *'s. The o's define data from C.B. Watts' original charts (see below). The x's and *'s define points which have actually been observed at previous grazes - so they will be more accurate.
   • Across the top of the profile are indicated minutes before (to the left) and after (to the right) of central graze for the location for which the profile is plotted. (The latitude and longitude of this position are listed at the bottom of the sheet).
   • Down the left side of the profile are distances in arcseconds, while down the right side are distances in kilometres, from the "predicted limit".

   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

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.

OTHER POINTS

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