# Relative Size of Moon

Today’s Question: I saw a photo of a full moon within an arch at Arches National Park in an article, and I wondered if it was a composite since the moon appeared so large within the arch. Many of the comments suggest it may be real. Do you think such a photo is indeed possible?

Tim’s Quick Answer: It is indeed possible to have the moon appear at the relative size shown in the photo in question, provided the photographer was positioned about a half mile from the arch. You can view the image referenced in today’s question in the article here:

https://petapixel.com/2020/11/30/full-moon-photo-looks-like-mysterious-giant-eye-through-rock-arch-in-utah/

More Detail: Photographers often refer to “lens compression” to describe a situation where a scene appears compressed, meaning that distant objects appear relatively large and therefore relatively close to a foreground object. This effect is actually achieved by the photographer positioning themselves further from the subject. A long lens is simply used to crop that scene to create the desired photograph.

To have a moon appear large relative to a foreground subject, you need to move away from that foreground subject. In the case of the moon photo that is the subject of today’s question, my calculations indicate that the photographer would need to be about half a mile away from the arch. The number I came up with was actually 2,361.7 feet, which I calculated using the PhotoPills app (https://timgrey.me/pills).

One way to describe the size of the moon in the context of a foreground subject is to define the size as though the moon were painted on that foreground subject. For example, if the width of the arch was twenty feet and the moon fills half the width of the arch, we could refer to the moon as being ten feet wide (even though the actual diameter of the moon is about 2,158.8 miles).

The information I was able to find about the size of the North Window at Arches National Park featured in the photo didn’t make it clear what the inner dimensions might be. So rather than using the arch as a measurement tool, I used one of the people in the photo to estimate the size of the moon. Operating on the assumption that the person I selected was about five feet tall, I calculated the size of the moon to be about twenty feet. I believe this would provide a conservative estimate of the size of the moon, since I suspect the person was more than five feet tall.

So, using the Planner pill in PhotoPills, I made use of the geodetics feature to determine where you would need to stand in order to achieve a relative moon size of twenty feet. With the black pin (for the subject) on the North Window on the map I then tested out various other positions for the red pin (for the photographer). I got an indication of twenty feet for the relative size of the moon when the photographer position was at 2,361.7 feet.

I also used PhotoPills to determine the path of the moon on the date the photographer indicated he captured the photo, and based on my estimates the moon would indeed have appeared behind the arch, so it is possible that the photographer was able to plan for just the right position to feature the moon within the North Window arch.

So again, I obviously can’t state for a fact that the image in question wasn’t a composite, but it was certainly possible to achieve the image with a single capture. But my aim here is not to try to prove that the photo was real, but rather to demonstrate that such a photo is possible, and that you can plan ahead to create such a photo using a tool such as PhotoPills. You can learn more about planning this type of photo using the PhotoPills app, with my comprehensive course that can be found here:

https://timgrey.me/pills