Occultations and Lunar Grazing Occultations - What are they?

As the moon orbits the earth, it passes in front of stars. Such an event is called an "occultation". Occultations have provided many valuable discoveries in astronomy, including the discovery of the rings of Uranus, the rings of Neptune, many shapes and diameters of asteroids, the atmospheric structure of Saturn and Titan, and just recently, the first accurate measurement of the size and density of Pluto's moon Charon.

A grazing occultation occurs when the moon just barely grazes a star. The star will be seen to disappear and reappear among the mountains and valleys on the edge of the moon for a period of time lasting from less than a second on up to 3-4 minutes. Recording the exact times of these disappearances and reappearances is scientifically valuable, as it is the most precise way we have of measuring the position of the moon relative to the stars. Perhaps even more important, it provides the most accurate means of measuring the exact topography at the north and south poles of the moon, allowing it to be used a a precision measuring rod for in turn measuring the precise diameter of the sun using total solar eclipses. Understanding changes in the diameter and luminosity of the sun is critical to understanding the influence of the sun on earth's climate; currently a very hot subject of research. The U.S. Naval Observatory supports the gathering of this data, and travel expenses to these events are tax deductable as charitable contributions. The International Occultation Timing Association (IOTA) - an organization of amateur and professional astronomers - was formed 40 or so years ago to help with the observation and reduction of all manner of occultation observations, especially lunar grazing occultations. A good page on the scientific value of occultations is here.

Grazes are visible only along a narrow path a mile or so wide, at the northern and southern limits of the shadow of the moon cast by the star. OCCULT is a computer program freely available which can calculate the circumstances of these events. I use OCCULT to generate the predictions for the events which Cabrillo Astronomy observes.

Besides being valuable, grazes are beautiful to watch. Especially with bright stars. Here is a video recording astronomy club member Jay Friedland made of a graze of a 4.3 magnitude star against the moon, as seen from near the summit of the Santa Cruz Mountains on Sept 17, 2003.

Here are some YouTube videos of lunar grazes...
Graze of 85 Ceti (on 40% waxing cresent moon, 20 events)
Graze of ZC 2268 (close binary, events are in steps)
Graze of Regulus (in daylight on cresent moon cusp, with Limovie analysis)
Graze of Aldebaren (on cresent moon, with excited commentary)
Graze of ZC 2644 (on thin waning cresent dark limb. 4 events)
Graze of ZC 3325 (far on dark limb of waxing ~60% moon. Many events)
Graze of ZC 3108 (~10 events on cusp of waning cresent moon)


Obtaining timings of these events can be done either the low-tech way (cheap), or the high-tech way (expensive). Low tech is the way most students observing stations will likely go. We set up a shortwave radio tuned to 5 or 10 Mhz (National Bureau of Standards WWV time station) and a tape recorder on the ground near you and should "D!" and "R!" each time the star disappears and reappears. Then you play back the tape, which now has your shouts as well as the drone of WWV's tone with clicks every second and announcements of the time every minute. Use a stopwatch (such as most digital watches these days have as part of their repertoire) as you listen to the tape several times, and get the D and R timings precise to 0.1 second. You should comment on tape right after the graze what you think your reaction times were for each event. For most observers who have a solid view of the event, that reaction time is 0.3 to 0.45 second using voice this way, but it may be slower or much slower if the star is faint or there's wind or passing light clouds or any of the myriad of other "gotcha's" that lurk out there for the grazer. The high tech way is to hook up a video camera like the PC164c from Supercircuits (about $150) and a video time inserter/GPS combination with a camcorder as recorder, and then the time is imprinted on each frame of the 30 or 60 frames/second video record. Then it's a simple matter to step through the video later and pull off the times. Here's how that's done. Cabrillo Astronomy owns a KIWI OSD video time inserter from PFD Systems which enables me to get video of our grazes and asteroid occultations which the class can then view later. We also have an older KIWI system which has not performed well, but which some enterprising electro-computer whiz student may want to play with and get de-bugged?

Knowing your reaction time is important if doing grazes with voice and tape recorder. Here's a fun way to try and measure your reaction time.

To decide where to place observers, one begins by plotting the "limb profile". This is a chart showing the silhouette of the moon, with the convention that the star always moves across the page from left to right. Thus, northern limb grazes will be plotted with the moon down and the sky up, and southern limit grazes are plotted with the moon up and the sky down, or upside down. Here's a profile for the graze of 46 Leonis on Oct 10, 2004 from near San Simeon, CA. To make the graze most interesting, and also determine the most contact points against the lunar limb, look for those horizontal tracks which promise the most number of disappearances and reappearances, realizing that the predicted profile is smoother than the actual profile, so smooth but level areas can actually produce many events. The most promising tracks for the 46 Leonis profile above is at 0.50 miles south, and at 1.13 miles south of the predicted limit. This page shows many of the best observed lunar graze profiles.