Understanding the Plus Signs in Moon Landing Photographs: What Are They and Why Are They There?

The plus signs observed in some of the famous moon landing photographs, particularly those taken during the Apollo missions, are not merely artifacts of the photographic process but serve a vital purpose. These are known as fiducial markers, reticles, or registration marks. This article will delve into the purpose and use of these marks in photographic and scientific endeavors during the Apollo moon landings.

What Are Fiducial Markers?

Fiducial markers, also known as fiducial points, are small, visible dots or crosses that are placed on a photograph to serve as reference points for measuring, calibration, and analysis. They are commonly used in various fields such as photogrammetry, cartography, and scientific photography. Specifically in the context of the Apollo missions, these markers are crucial for several reasons:

Navigational and Landing Aids

The plus signs in the photographs were etched onto a glass plate called a Reseau plate. This plate was positioned between the film and the camera lens. The markers were designed to help astronauts and mission control personnel verify their position and orientation during the landing process. They served as reference points for the Lunar Modules' descent, ensuring precise landings and reducing the risk of malfunction.

Photographic Calibration and Documentation

These fiducial marks were also used to ensure accurate calibration of cameras and exposure settings. By using the markers, the photographic exposure could be standardized for each frame, enabling consistent and precise documentation of the lunar surface. This consistency is vital for scientific analysis and contextualization of the historical images.

Scientific Analysis for Measurement and Scale

The presence of these fiducial markers in the photographs allowed for precise measurement of distances and scales. This was particularly useful in scientific analysis of the lunar terrain. Scientists could use the markers to measure the sizes and distances of various objects and features on the moon's surface, providing crucial data for various scientific studies and research.

How Were They Achieved?

The fiducial markers were typically created by etching them onto a small piece of glass, known as a Reseau plate. This glass plate was then inserted between the film and the camera lens. This ensured that every photograph taken would have these markers recorded on it. The process was straightforward and reliable, and it has been used in scientific and photographic applications since World War I. In the context of the Apollo missions, each Hasselblad 500 EL camera was equipped with a removable metal slide called a dark slide, allowing for the insertion of the Reseau plate.

Capturing the Images

During the Apollo missions, the Hasselblad 500 EL cameras were used, which featured a 120 size film about 2.25 inches wide. The astronauts had to advance the film a few frames after removing the dark slide to avoid creating spoiled images. Despite this occasional imperfection, the use of fiducial markers ensured that the images captured were consistent and accurate.

Conclusion

The plus signs in the moon landing photographs, often misunderstood as mere artifacts, are in fact crucial fiducial markers that served multiple purposes during the Apollo missions. From aiding in navigation and landing to providing photographic calibration and scientific analysis, these markers were an integral part of the mission's success and have contributed to our understanding of the moon's surface for decades.

Additional Resources

For further reading and detailed explanations, you can refer to the Réseau plate - Wikipedia article.