Occultation is a generic astronomical term for a situation where one body "covers" or "passes in front of" another body. Eclipses and transits are common examples of occultations. In a solar eclipse, the moon occults (covers) the sun for observers on the earth. In a lunar eclipse, the earth occults the sun for observers on the moon. We use the term asteroid occultation for the situation where an asteroid occults a star for observers on the earth. During an asteroid occultation "event" the asteroid momentarily hides the star. We use the term "asteroid's shadow" to refer to the "shadow" thrown by the asteroid with respect to the light from the occulted star. And the term "shadow path" refers to the path of the asteroid's shadow as it passes across the earth. Depending on the size of the asteroid, its distance from the earth, and its speed in its orbit, the asteroid may cover the star for a fraction of a second or close to a minute. If the star is relatively bright compared to the asteroid, an observer will see a noticeable drop in magnitude as when the asteroid covers the star. As asteroids move through the sky they often pass in front of stars from the perspective of an observer on the earth. However, the vast majority of these occultations involve small asteroids and relatively dim stars, and these events would be very hard to detect with amateur equipment.
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Occultations defined
An asteroid occultation prediction predicts the path of the asteroid's shadow on the earth. Even the largest asteroids subtend small angles on the sky. Although new instrumentation has provided significant advances in recent years, the positional uncertainty for both asteroids and stars is still relatively large compared to the size of an asteroid. As a result, we can only make a statistical prediction of the path of an asteroid occultation. The uncertainty in the path's location depends on the uncertainty in the position of the star and the position of the asteroid. Therefore, the likelihood of seeing an asteroid occultation from a location in the predicted path is different for each occultation event. For most events, the uncertainty is large enough that we must spread multiple observers across the predicted path to ensure a good chance of observing the event.
To observe an asteroid occultation you find the star which will be occulted and monitor the star during the predicted time of an occultation for your location. An observation can yield three types of useful data. First, the observer can report a negative or positive event. If the star dropped in magnitude during the predicted time, the observer reports a positive event. If there was no change in the star's brightness during the predicted time, the observer reports a negative event. Both negative and positive observations are valuable data. Secondly, if the observer sees a positive event, they can report the duration of the occultation event. Third, the observer can report the exact times of the beginning and end of the event from their location (UT). There are several methods for determining the duration and exact times of an event. I have listed a few common methods in the following page: Timing Asteroid Occultations . Also, you should note the latitude, longitude, and elevation of your observing location.
A couple of additional suggestions... The stars are often dim (mag 10 or dimmer) so you should allow plenty of time to find the "target" star (practicing on a previous evening is a good idea). Start observing a few minutes before the event and observe for a few minutes after the event. The predicted times are predictions and may be in error by tens of seconds. The detailed info for each event provides an estimate of the uncertainty in the time of an event. The detailed info for each event provides accurate times for several locations along the path - these times are more accurate than the times plotted on the path maps. And the detailed info provides the star's altitude and sun's altitude for each point along the path. Pay attention to the predicted magnitude drop - magnitude drops of less than 1.0 magnitude can be hard to see visually and a drop of 0.5 or less probably requires special photometric equipment for reliable results.
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IOTA (International Occultation Timing Association)
IOTA Yahoo Group (public bulletin board with lots of advice & info)
Fill out an observation report. If you are in North America, you will find more instructions at this webpage: North American Report Forms. If you are in another part of the world, this webpage should lead you to information for your region: Links to Observations Pages for various regions of the world )
Since these maps are just predictions there is a chance that you could see any event where the star is visible from your location at the time of the event. Obviously your chances are higher if you are near the predicted path. The probability of seeing an event is greatest if you can "go mobile" and locate your telescope in the center of the predicted path. And, the probability increases when the event has smaller uncertainty in the prediction.
I encourage observers to try any event they can. With more observers looking there is a much greater chance of collecting useful data. We gain much more information about the asteroid when we have multiple observations so we always needs more observers. Ideally, we would have many observers located perpendicularly across the predicted path and far enough out from the predicted path to catch a significant shift from the predicted path.
- Event Date/Time
This column give the date and time (UT) for the midpoint of the event's path as it crosses the earth's globe. To find the time for your location, review the maps and the detailed info provided in the Event Details page. - Rank
The event Rank is a measure of the likelihood of observing an event. Currently, the Rank equals the probability of at least one successful observation by a team of two observers where the two observers are positioned 3/4 path width apart symmetrically about the center of the path. This probability is a function of the size of the asteroid in the sky (in arc seconds), the uncertainty in the position of the asteroid, and the uncertainty in the position of the star. Note that due to rounding, the event rank can be 100%, but in reality there is always a chance of no success. Eventually, I will evolve the Rank to also consider the brightness of the star, the magnitude drop, the duration of the event. - Asteroid
The asteroid which is occulting a star and the predicted magnitude of the asteroid. - Star
The star to be occulted and the visual magnitude of the star. - Visibility
The general region of the predicted path, shadow path, where the occultation should be visible. - dM D A
This column gives the magnitude drop (deltaM), the duration of the event (in seconds), and the maximum altitude of the star during the event. Note that this is the maximum altitude of the star and the star's altitude will probably be lower at your location. - Details
This column includes a link to the Event Details page. The link is labeled with the date of the most recent prediction update.
The map above is an example of the basic path prediction plot from Occult. The Occult plots have three parts: header, star chart, and path plot. The header provides details of the event. The star chart is a 2 degree by 2 degree star chart centered on the star that will be occulted by the asteroid. The path plot shows the path of the asteroid's "shadow" across the Earth. The approximate time (UT) for the occultation along the path is marked with cross lines and numbers. The plot also shows which part of the Earth is illuminated by the sun (daylight) during the time of the event. The portion of the globe with multiple parallel lines is in sunlight. Twilight regions are indicated with a dashed lines. In the example above the occultation occurs during nighttime for observers in New Zealand and Australia.
Header description:
The first line of the header gives the designation of the asteroid, the name of the star, and the range of times for the event. The range of times for the event is NOT the expected duration of an occultation but the approximate time during which the asteroid's shadow will pass across the earth. The leftmost column gives the visual magnitude and position of the star at the time of the event. The middle column gives the expected maximum duration of the occultation, the approximate drop in magnitude when the asteroid covers the star, and the relationship of the moon and sun to the star. The rightmost column gives the asteroid's magnitude, diameter in km and arc seconds, the parallax of the asteroid, and the asteroid's rate of motion across the sky. The last line of the header gives the longitude and latitude of the center of this plot and specifies the uncertainty ellipse for the path prediction. The uncertainty ellipse is specified via the major axis, minor axis, and position angle of the major axis.
Shadow path description:
Occult plots the predicted occultation path with solid parallel lines showing the edges of the asteroid's "shadow" as it travels across the earth. When Occult draws the path is assumes that the asteroid's shape is a circle. Furthermore, if the occulted star is not on the zenith, the width of the shadow path is greater than the width of the asteroid. The width of the asteroid's shadow is determined by the altitude of the star at the time of the event. The width of the shadow = (diameter of asteroid) / SIN (maximum altitude of star along the path).
The dashed lines on either side of the path lines indicate the effect of a 1-sigma shift in the edge of the path. In theory, there is a 68% probability the actual path of the asteroid's shadow will fall somewhere between the 1-sigma lines. The uncertainty ellipse is also plotted somewhere on the path plot and shows the full orientation of the 1-sigma uncertainty in the path prediction. As of 2005, I am not plotting 2-sigma lines on the plots. But you can estimate the location of two sigma lines. Given the distance from a path edge to the nearest 1-sigma line as a distance of 1-sigma, the associated 2-sigma line would be located at a distance of 2 x 1-sigma from the same path edge. In theory, the actual path of the occultation should be located within the 2-sigma lines with a probability of 95%.
Occult shows the time of the occultation along the path by plotting cross lines and labeling them with the time in UT (Universal Time). UT is the standard "time zone" for astronomers and roughly corresponds to standard time in London, England. For more information on UT see the NIST website. The time marked on the path is the predicted central time for an event. If the predicted duration of the event is 20 seconds, the event should start 10 seconds before the marked time and end 10 seconds after the marked time.
Also, Occult plots show the location of cities and towns via small circles.
For some events, I have posted maps of the occultation's path (shadow path) which show a more detailed view (more cities and sometimes roads). I generate these map using Microsoft Mappoint. Eventually, I will incorporate a legend on the map. In the meantime, I have been using the following scheme for the plots: the blue lines mark the path of the asteroid's shadow and the red lines show the one sigma limit for a shift in the edge of the path. In other words, the actual path is unlikely to shift far enough to push the edge of the path to either of the red lines..
The chart is 2 degrees by 2 degrees. Also, North is Up and East is to the Left.
Unless labeled otherwise, uncertainty is an estimate of 1-sigma for the error in the predicted path (actual path vs predicted path). In theory, there is a 68% probability that the shift from the predicted path to the actual path will be within the estimated uncertainty. On the plots, uncertainty is plotted as an ellipse and the uncertainty in the movement of the path edge is show with dashed lines. The detailed info for an events lists uncertainty in terms of an ellipse on the sky, in terms of RA,DE, in terms of time (seconds), and in terms of the path width.
The are very few sources of highly accurate astrometric data. I only generate an update when I receive recent astrometric observations from one of these sources. Although FASTT and others produce a tremendous amount of data for these updates, sometimes there are no recent observations for an asteroid and I can't improve the predicted path.
One other possibility is that I haven't found the time to do the update. If you are interested in a particular event and my web page doesn't say "no recent observations", feel free to email me and ask.
Although the updates usually include plots of the asteroid's shadow path on the earth and include the latitude and longitude for several points along the center of the path, you may wish to generate your own, more detailed, path plots. In this case I recommend that you install the latest version of Occult and use the information posted on my site to generate your own plot. Occult is freeware and you can download the latest version from IOTA. On the main page of asteroidoccultations.com I have posted two version of the Occult file containing the path predictions posted on the website: Future.xml and FutureAll.xml. Future.dat contains all the path predictions for future events posted on the Current Events (main) page and FutureAll contains all of the events posted to the site. With Occult, you can download either of these files, open the file in Occult, then display the path via the "List & Display occultations" dialog under Asteroid Predictions.
In reality there are probably hundreds of asteroid occultations every night. However many cannot be observed for practical reasons (e.g. star too dim, event duration too short, etc.). For the event that could be observed, IOTA prioritizes the events to emphasize the events mostly likely to be observed. Then we compute updates for these "better events". In the following document I have written a brief description of the overall process: UpdateProcess (as of summer 2007).
As of 2019, the majority of predictions on this site use the DR2 release of Gaia for the star's position. DR2 is an early release of Gaia data and does not yet provide full treatment (or identification) of double stars and other "complications" with star positions. Therefore some of the star positions may not be as reliable as indicated by the formal statistics in DR2 catalog. RUWE is a statistic developed by a group at Heidelberg University. When RUWE is high, the star's position might be compromised. In addition, the basic DR2 dataset includes a flag called "Duplicated Source" which is an indication that the star might have a companion star nearby. So, again, the DR2 positional data might have issues.