By

Rob Robinson

The first thing that comes to mind when someone says "lunar rays", are the beautiful ray systems that are best seen during full moon. The ray systems are fascinating to study, and at the present time, the Association of Lunar and Planetary Observers is beginning a new joint project to catalog, map and study the bright lunar ray systems. This article though, is not about the lunar ray systems, but rather "lunar light rays".

A lunar light ray, is that phenomena which occurs when the sun is at a low altitude angle in relation to a lunar feature (typically a crater), and the sunlight shines through a cleft in a crater wall, thus exhibiting a spike of light across the dark crater floor. Jeff Medkeff further defines the "true" light ray as one which crosses 3 degrees of longitude while being less, on average, than 1/4 degree in latitude width. These can occur at either sunrise or sunset. Because of the geometry of the event, they are usually short-lived and most occur within only a 2-3 hour "window of opportunity".

It is not known when the first lunar light ray was first observed. Observers such as Johann Kreiger (1865-1902), William Birt (1804-1881), Walter Goodacre (1856-1938) and H. P. Wilkins (1896-1960) noted them, so they have been observed for at least the last 150 years or so.

As early as 1999, I belonged to an Internet maillist called "shallowsky". It was primarily for observers of solar system objects, but the concentration was more on lunar studies and observations. It was from this mail list that I saw the first report of a "lunar light ray". I hit on the idea of creating some web pages where these reports could be archived, along with pertinent information, maps, and predictions for future events.

Because of the criteria for low sun angle on a feature, I used a program called "Lunar Toolkit" which is written and distributed by Harry Jamieson, to predict future events. Harry is a member of A.L.P.O. and designed the program primarily for observing and cataloging lunar domes. Since the software offered a module to predict future events by sun angle (also used in computing predictions for dome studies), it was easy to compute the next dates for similar sun angle over a lunar crater. Predictions were generated for four standard stations across the United States so those observers could be alerted the next time a light ray would be visible. The predictions have proven accurate within a half-hour of the event.

For those that are using the Lunar Toolkit, one can generate predictions for these rays, using their own coordinates for more accurate determination of when the light ray will be visible. For example, a report of the Mercator sunrise ray, was reported by Jane Houston Jones on April 21, 2001 at 6:00 UT. Her coordinates were given as 122.30 degrees west longitude, and 37.57 degrees north latitude. First one needs to compute the observation using her coordinates to find the solar altitude at the time of the observation. By using the "Schedule for a Specific Feature", input her longitude, latitude and feature name, and for the starting date, you can enter any date BEFORE her time of observation. Check the radio button for "By Date". Enter the Date and Time of the observation. This will then generate data for this instant, along with numerous other dates. The information that is important is the "desired solar altitude", which shows the sun was rising, 2.006 degrees above the horizon over Mercator. Now it is a simple matter of going back, and entering YOUR site longitude, latitude and then click the radio button to generate predictions using "By Solar Altitude". This will then produce the predictions specific to your site coordinates.

Although this is not an official observing program for any one organization, anyone can "discover" or observe a lunar light ray with a telescope. They are best detected by scanning the lunar terminator, and if you are lucky, you just might spot one. If you suspect you have seen a lunar light ray, or submitting an observation on a previously seen ray, you need to send me a report so that predictions can be generated for follow-up. The report needs to have date and time of observation; your location; type of telescope; crater name; and description of what you saw.

I look forward to hearing from anyone that is interested in observing these elusive lunar events. One can easily see numerous geological features on the moon. One can easily observe occultations and grazes. But it takes a really dedicated lunar observer to catch a lunar light ray!

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