Sunrise and Sunset Rays

by Jeff Medkeff (1968-2008)

Sunrise and sunset rays are shafts of light which shine through clefts in crater walls and mountains to illuminate a streak of the lunar surface. They occur only at very low angles of illumination, hence during sunrise or sunset. They are not to be confused with the bright crater rays which are best seen at full moon. Many sunrise and sunset rays are triangular-shaped illuminations.

Sunrise and sunset rays are not rigorously defined phenomena. In my opinion, it is most reasonable to call a lighting effect the length of which is ten times its average width, or one which crosses 3° of longitude while being less on average than 1/4° in latitude wide, as a "true" sunrise or sunset ray. In rough terms, these criteria describe an event that is pretty unusual-looking and one that justifies making a deliberate effort to see.

These unusual illuminations are mentioned in the published books and papers of many of the classic lunar observers, such as Neison, Kreiger, Elgar, Birt, Goodacre, and Wilkins, among others. Therefore the vast majority of them have been "discovered" more than half a century ago. In recent years there has been a renaissance in observing sunrise and sunset rays, with seemingly a new report of a ray being made by an observer every few months or so from 1999 to 2001. In general, I have tried to research whether or not the modern report constitutes the first publication about the ray, and if so, I give credit to a modern observer below (if no names are mentioned, it is documented in the classic literature). A list of "true" sunrise and sunset rays follows, with the approximate colongitudes of the occurrence given in parentheses at the end of the explanation.

Sunrise and Sunset Rays:

Barrow Ray
A thin but long shaft of light that crosses the floor of Barrow at sunrise. Donald Qualls provided the first image. (348)

Burnham Ray
This ray has been rather thick when the author has observed it but it seems likely it is sensitive to changes in solar latitude, and so is being included here. It is caused by a low area or break in the western crater wall, and thus the ray extends across the small plains to the west of Burnham, rather than across its floor. (355)

Curtius Ray
A ray mentioned by other observers but apparently first imaged by Donald Qualls, this ray appears at a somewhat higher illumination than the average ray. At sunrise, light shines through a cleft in the western wall, making a triangular patch of illumination across the crater floor and eastern rim.

Gemma Frisius Ray
This ray is the result of a break in the eastern wall of this southern highlands crater, which allows a shaft of light to shine across the crater floor and illuminate the three smaller craters on Gemma Fisius' western rim. Some very small ridges - only several feet in height! - cause this ray to have a few dark bands in it roughly perpendicular to the ray. (345)

Halley Ray
First reported by Akkana Peck, the Halley ray occurs at sunrise and sends a spike of light across the crater floor. (348)

Hesiodus/Pitatus Double Ray:
Popularized by an article in Sky & Telescope, July 1996 about the observation of Phillip Koch of Las Vegas, Nevada; this article may have jump-started the surge in interest in sunrise and sunset rays that we've seen in recent years. The wall between these two craters has a deep cleft. At sunrise, a ray is seen crossing Hesiodus' floor. At sunset, a ray crosses Pitatus. This latter event seems to be very sensitive to solar latitude - I've only seen it once in five tries. The Hesiodus illumination was mentioned by Birt. Jay Reynolds Freeman apparently predicted the appearance of a Pitatus ray before its being observed. (Hesiodus 18, Pitatus 196)

Hypatia Double Ray
Discovered by expert Lunar observer Akkana Peck, this double ray is formed by a cleft in Hypatia's eastern wall, which creates a ray shining across its floor. A second ray shines through a cleft in the western wall and falls across the terrain to the west of the crater. (337)

Maginus Ray
A wedge-shaped ray that crosses the floor of Maginus. (2)

Maurolycus Ray
At sunset, a ray of light projects from the eastern wall to the central peak of the crater. (162)

Orontius Ray
A thin shaft of light that crosses Orontius' floor, along the north rim of the crater in the morning. (3)

Ptolemaeus Triple Ray
The Ptolemaeus Triple Ray was first called to the author's attention by John Cassella of Sierra Vista, Arizona. During sunrise, three shafts of illumination extend across the floor of Ptolemaeus, which otherwise remains in darkness. These rays illuminate the gently undulating terrain of the Cayley Formation and provide visibility of extremely fine relief. Goodacre mentions the clefts without calling attention to the illumination pattern.

Stofler Ray
A ray caused by a gap in the crater's eastern wall. (350)

Vogel Ray
A sunrise ray that crosses the floor of Vogel, starting as a thin triangle of light that progressively thickens. (354)

Walter Ray
Discovered by Dave Mitsky of Lewisberry, Pennsylvania, the Walter ray is a fairly dramatic shaft of light that crosses the floor of Walter at sunset. (178)

Other Interesting Illuminations:

Toricelli
This is really just a closely-spaced double shadow that occurs at high illuminations compared to most rays. Still, this illumination and the other complex shadow effects around Toricelli are interesting, so it is worth listing.