The Deep Space Climate Observatory (DSCOVR) is a NOAA-operated satellite that orbits the Earth-Sun Lagrange-1 (L₁) point. This location, at approximately 1.5 million kilometers away from the Earth, offers a unique view of our planet where the disk is almost fully illuminated. The DSCOVR spacecraft serves as a host to both Sun- and Earth-observing instruments.

The Earth Polychromatic Imaging Camera (EPIC), a NASA instrument onboard DSCOVR, utilizes a 30 cm Cassegrain telescope with a 0.62° field of view to photograph the Earth from this vantage point. Equipped with a 2048 × 2048 charge-coupled device (CCD) sensor, it takes 13 or 22 image sets daily. Each set consists of 10 narrowband images covering wavelengths between 317 and 780 nm, with the more frequent sets occurring during the Northern Hemisphere summer (Marshak et al., 2018). This wavelength range permits a unique view of the Earth in ultraviolet, visible, and near infrared, which enables detection of ozone, sulfur dioxide, aerosols, vegetation, ocean, cloud properties, and other science applications (Marshak et al., 2018).

This brief report documents an observational phenomenon in EPIC imagery and demonstrates a first-order, physically plausible mechanism. We do not attempt a full radiative-transfer or global occurrence analysis. Our aim is identification and plausibility, leaving comprehensive modeling for follow-up work.

The EPIC camera nominally observes just the sunlit Earth, but its 0.62° field of view allows it to occasionally capture the Moon as well, as seen in the left portion of Figure 1. The instrument observes these events roughly four times a year - two events with the Moon passing in front of the Earth and the remainder with it passing behind.

In 2015, a special imaging was performed where the EPIC camera was trained upon the Moon, following it as it transited behind the Earth. Upon more detailed inspection of the images, an interesting effect was seen as the Moon passed behind the Earth. In this sequence, as seen in the right, zoomed-in portion of Figure 1, a small “flange” is seen, appearing as almost a mount for the Moon against the planet’s surface. It was not known what this phenomenon was or how it originated. This paper will discuss the origin of the phenomenon and the underlying optics.

A review of the original, unprocessed level 0 data, which contains the raw counts as received from the instrument, also revealed the feature, eliminating possible level 1a processing calibration artifacts as the cause. As part of this investigation, we determined that pixels in the feature are not saturated, indicating that the cause is not CCD bloom, nor does it mimic any known optical artifact of the telescope including stray light (Cede et al., 2021). This was true for all images investigated as part of this paper.

Because the sequence was part of a public outreach imaging to acquire color photos of the Moon passing behind the Earth’s disk, the instrument recorded only the red (680 nm), green (551 nm), and blue (443 nm) channels. The phenomenon was seen in all 3 wavelengths.

A review of the EPIC archive was conducted and all images where the Moon passed behind the Earth were obtained. Out of the 16 image sets obtained where the Moon intersected the Earth’s horizon, only two additional sets contained the phenomenon. In the set of data collected over 10 years, it occurred only 16.7% of the time. As seen in Table 1, the only common elements between times it is seen is that it occurred when the limb was dark, and the Moon was halfway below the horizon. The orientation of the Moon on the CCD, or the physical location the lunar limb intersects with the Earth appear to be independent. The phenomenon is not seen when the limb is illuminated where the Moon intersects.

In an additional dataset, all 10 bands were obtained (Figure 2), enabling a view of the limb phenomenon in different wavelengths. The phenomenon is mostly invisible in ultraviolet (UV) bands 317, 325, 340, and 388 nm and becomes increasingly clear as the bands progress through 443, 551, 680, 688, 764, and 780 nm. This indicates that the phenomenon is not likely due to atmospheric scattering, which is smaller in the visible and near-IR bands (Figure 2).

Initial inspection of the Moon passing behind the Earth from other spacecraft, such as GOES and the ISS, did not yield the same phenomenon (as seen in Figure 3). Furthermore, crew members on board the ISS who had viewed comparable Earth-Moon transits reported that no flange-like feature was visible to the naked eye, corroborating the absence of the phenomenon at low-Earth viewing geometry (Pettit and Hague, 2025). Both images from the International Space Station (ISS), at an altitude of ∼400 km, and Geostationary Operational Environmental Satellite (GOES) Advanced Baseline Imager (ABI), at ∼35,800 km, display expected optical compression from refraction through the atmosphere. While this image is not included in this paper, the optical compression can also be seen in images from Himawari-8 (Universe Space Tech, 2023). However, while this paper was in the review process, another, newly taken picture from Himawari-8 revealed a similar phenomenon to the flange. The origin of this and the situations in which it appears is discussed in Section 7.

Based on the known mirage in Figure 4, a further review of atmospheric phenomenon was conducted, and from a catalogue of possible features (Naylor, 2002), the following observation was made: that the appearance was very similar to a feature known as an “omega mirage.”

Mirages are an optical phenomenon where light is bent by different, contrasting combinations of air temperatures and humidity layers. This bending, produced by different refractive indices of the layers, causes displacement and distortion of distant objects in the sky.

Mirages are separated into roughly three classes: inferior, superior, and “complex.”

In inferior mirages, as seen in Figure 5, an inversion of the light rays causes a mirror reflection of an object to appear below it. This is caused by scenarios where the air has high temperatures near the ground followed by layers of cooler temperatures as the height increases. A common example of this is the “highway mirage” where a hot road appears as if it has a reflective puddle, but instead it is caused by the temperature differences above the road (Hecht, 2002).

In superior mirages, as seen in Figure 5, light bends in such a way to cause an object to appear to float above its original location. In this situation, layers of cool air followed by warm cause an expansion effect, which makes the objects appear higher. Superior mirages often cause objects that are visually below the line of sight to appear on the horizon when they would not in normal conditions (Lynch and Livingston, 1995).

The final mirage, a “complex mirage,” occurs when there are alternating layers between hot and cold air. In these mirages, known as fata morgana, the optical effects consist of inverted images, compressed and stretched layers, as well as objects “floating” on the horizon (van der Werf, 2022).

Terrestrial mirages occur when different temperature layers cause changes in the air density, which leads to changes in the refractive index. Near the ground, these thermal layers form due to local conditions: for example, heat rising from the ocean or the sequence of cool air overlaid by warm air creates distinctive optical distortions.

It was considered that the phenomenon was perhaps based on temporary atmospheric conditions. To investigate, we used the Goddard Earth Observing System Forward Processing (GEOS-FP) data set, a product that consists of near-real-time, global weather maps. GEOS-FP merges millions of satellite and ground observations with a numerical weather model every 6 h; this data-assimilation step produces hourly fields of temperature, humidity, winds, and other variables on a grid of roughly 25 km resolution. (Global Modeling Assimilation Office, 2017). An example of GEOS data can be seen in Table 2.

Atmospheric parameters from the GEOS product, including temperature and humidity, were obtained and reprojected into the EPIC projection and field of view at the times of EPIC partial lunar occultation events. However, the results revealed no evidence of a relationship between temporal atmospheric conditions and whether the phenomenon appeared.

Further review of the data was conducted, and it was observed that all three times the phenomenon was observed when the Moon was halfway below the horizon. Although this is not statistically significant, but because no other explanation fits the observation as well, we adopt a working hypothesis: that this phenomenon was not fleeting but occurred every time the Moon passed behind the Earth and occurred while the Moon was at least halfway below the horizon. This is not as unlikely as it seems, as for the majority of the sets the imagery cadence was planned for normal, Earth-imaging operations and not intended for capturing the Moon or this event. Two out of the three datasets where the phenomenon was observed were fast-cadence imaging sequences specifically targeted for imaging the Moon and therefore more likely to catch it in the right physical position. Also, importantly, the imagery serves only as an initial clue; the hypothesis is tested chiefly by a physics-based ray tracing model, rather than imagery alone.

Under the assumption that this is a consistent event, we propose a new working hypothesis. The atmospheric layers, while generally characterized by a decrease in temperatures with an increase of altitude, do not follow a linear gradient. Instead, there is a switching back and forth of the temperature as altitude increases. The troposphere, stratosphere, mesosphere, and thermosphere are characterized by alternating cooling and rising temperatures. These different layers create an environment ripe for mirage-type optical distortion by providing contrasting refractive indices.

There are many ways to model the effect of atmospheric distortion on light. Common astronomical methods use heuristics as a function of viewing angle. Many, including the EPIC geolocation algorithm (Blank, 2019), use layer-based ray tracing models that treat the atmosphere as a linear, 2-dimensional pathway. However, these common methods had not revealed any type of optical phenomenon, other than compression, at distance. If the phenomenon is a mirage, resolving it requires a fully three-dimensional model rather than the simpler 2D approximations.

The ray tracer was run at 2 km resolution, resulting in Figures 7, 8. As seen in the raytraced image compared to the actual image from EPIC, the same phenomenon is seen. A vertical sampling of the rays reveals a maximum 0.45° distortion that decreases with distance away from the Earth. This is not unexpected, as the refractive indices decrease in relation to the air density, but it is interesting to note that the descent has several “bumps” where it increases instead of decreases.

An inspection of the rays as they traverse through the atmosphere (Figure 9) shows that the rays are primarily traveling through the stratosphere (∼20–50 km altitude) and lower mesosphere (∼50–85 km).

This would account for the apparent lack of the phenomenon in UV light, as the light passes almost horizontally through the lower stratosphere (15–30 km) and it would be filtered out by the ozone layer. Although computing the atmospheric loss for scattering and absorption is outside of intent of this paper, for reference the loss at 41.8° above the horizon is approximately 99.8% at 300 nm and 47.8% at 350 nm (Spectra-AM1.5, 2025). The rays that hit the troposphere approach the critical angle and are reflected.

To ensure that the model’s result is not affected by the snipping of reflected rays, a review of a number of calculations that results in reflected rays was conducted. Of the 245,592,557 refraction calculations that were conducted, only 776 resulted in a reflected ray, or 0.0003%. This indicates that reflection does not play a statistically significant role in the construction of the final image.

The model was run on a very small segment at 1 km resolution; the smaller voxel size did not alter the image morphology.

Viewing the full ray paths (Figure 10) shows the degree of distortion and where the pixels of the phenomenon originate. The small slice of atmosphere and the distortion caused over the 384,400 km distance between the Earth and the Moon generates over a 3,000 km bending of the rays.

Zooming in on the rays provides more detail in Figure 11. Here, several inversions of the light rays can be seen, showing that this phenomenon is not just optical distortion, but a complex mirage. It is interesting to note that while the appearance of the phenomenon is similar to an “omega mirage,” the effect is obtained through different means.

Inspection of the rays along the Z-axis yields a different picture. Here, the magnitude of distortion is less, but with much greater variance along the slice.

Viewing the rays shows a much more complex distortion in the horizontal vs. vertical axis. A zoomed-in view shows much more mirage-like ray inversions.

While the analysis shows that the phenomenon has aspects of a complex mirage, it does not demonstrate the differences that occur between the views seen by EPIC versus geostationary imagery.

In order to determine how the GOES scene differs from EPIC, it is necessary to run the ray tracer from the GOES point of view. The GOES ABI has a 17.76° field of view, and by initializing the first vectors with corresponding offsets, it is possible to create a GOES model of the scene.

Running the ray tracer for GOES produced the matching optical compressive effect, as seen in Figure 12, top left. When viewing the magnitudes of distortion (Figure 13), the degree of distortion is in correlation to the density of the atmosphere according to altitude. Unlike that seen in the EPIC data, there are no “bumps” or aberrations that cause the ray inversions. This is true of both the vertical and horizontal slices. Unlike the EPIC phenomenon, this is only a straightforward, atmospheric lensing effect, and has no evidence of being a complex mirage.

However, when looking at the magnitude of the rays for geostationary, as seen in Figure 13, there is a substantial difference between what is seen for GOES, Himawari-8, and EPIC. The magnitude of distortion is far less, plus there is no evidence of the magnitude changes which would indicate the ray inversions as seen in Figure 11. (This Figure 11 plot is not duplicated for GOES/Himawari-8 because the angular distortion due to these instruments’ wide field of view renders the relatively small effect of atmospheric disturbance invisible. The addition of this plot would exceed the number of figures permitted by the publication.)

It was initially believed that this limb effect was unique to EPIC. However, in an image taken by Himawari-8 on 11 August 2025, after an initial draft of this paper had been submitted for publication, showed otherwise, as seen in Figure 12, bottom left. We were able to replicate this, without modification to the ray tracer, after setting the Earth and Moon configuration to represent the Himawari-8 scene. The reason it was originally missed was that the time interval at which the geostationary version of the phenomenon occurs is very short. EPIC’s phenomenon occurs for over a course of ∼30 min. Empirical analysis of Himawari-8’s phenomenon using the ray tracer indicates that it is likely visible for less than 2 min. The shorter time span is partially due to the lesser amount of distortion seen at geostationary versus L₁, approximately 2.5 times less. Additionally, the Moon’s relative motion is faster from a geostationary platform. Locked to the Earth’s rotation, the spacecraft is moving at a rate of 15.04° per hour compared to L₁’s 0.041° per hour.

What is happening in the EPIC image that is different from the geostationary view? How can two pictures of the same apparent scene cause a different optical phenomenon? The answer lies in the fact that although the images appear similar, that similarity is itself an illusion.

DSCOVR orbits the Earth-Sun Lagrange-1 point at over 1.5 million kilometers away from the Earth. Taking the pictures requires a telescope with a 0.62° field of view. GOES, on the other hand, is in geostationary orbit, a relatively cozy 35,000 km away from the Earth. Capturing the entire Earth in a single frame requires the instrument to image at a wide angle of 17.76°. (Himawari-8 is very similar at 17.8° field of view) (Japan Meteorological Agency, 2015). This influences the scene; when the Moon is visible to EPIC, it is obscured by the Earth for GOES.

When considering the angles and slice of the atmosphere through which the two instruments view the scene, EPIC’s view is across a much wider swath of the atmosphere, while GOES’s is a narrow section near the center (Figure 14 bottom left and right). This changes the entry angles of the rays; the more extreme angle for EPIC causes a corresponding increase in magnitude of refraction, which causes a more extreme reaction when the ray collides with the different refractive indices. This is why the ray inversions occur with EPIC, but not in GOES or other platforms closer to Earth, such as ISS or other lower orbit satellites.

When looking at the GOES results in Figure 13 vs. EPIC’s in Figure 8 it can be seen that the magnitude of distortion has much more consistent delineation. There is no evidence of ray inversions, only optical distortion from the atmosphere. Ray slice figures, such as seen for EPIC in Figure 10, do not appear for GOES because the ray paths are so heavily dominated by the wide-angle view of the instrument that it hides the effect of refraction.

In the GOES and Himawari-8 versions of distortion, the vertical distortion is ∼0.2°, but the horizontal distortion is comparatively weak at ∼0.02°. The vertical distortion dominates the resulting images. In EPIC, the vertical distortion reaches a maximum of ∼0.45°, and the horizontal distortion, while at ∼0.11° still has a substantial impact to the image. Although the general shapes of the distortion look similar for both spacecraft, on close inspection of the Himawari-8 image, the craters on the Moon can be clearly made out, although warped. Unfortunately, the resolution for the EPIC data is far less, however at a resolution similar to Himawari-8, the images would likely look very different due to the additional horizontal distortion seen from EPIC. The mirage-like components caused by the ray inversions, seen only in the EPIC results, would create additional distortion in the instrument’s images, possibly creating a more fata morgana-like appearance. The end result is two phenomena that are structurally very different from each other.

The above results were presented to the DSCOVR/EPIC operations group. They agreed to conduct a “special imaging” activity with the instrument in order to capture the phenomenon. The operations group advised imaging a single wavelength band, as that would permit the shortest interval between exposures. For the activity, it was decided to image the Moon as it passed behind the Earth at 3-min intervals in the 780 nm wavelength. The short interval between exposures would permit capturing the phenomenon in different stages; 780 nm, as seen in Figure 2, would provide the clearest view. In this configuration, along with the limits of onboard storage and downlink, would permit 40 images to be collected over a period of 2 h.

Based on the orbital dynamics, a favorable configuration of the Earth and Moon, permitting a transit, was found for 11 May 2025, from 21:30 to 23:30 UTC.

The ray tracer, as seen in Figure 15, was able to correctly predict the shape the flange-like shape as it begins broad and then shrinks as the Moon rises, until it eventually disappears once it is more than halfway above the limb. This demonstrates that the working hypothesis, that the phenomenon disappears when the Moon is halfway above the limb, was correct. The changes in shape are due to the movement of the Moon from the path of the highly bent rays that pass through the lower atmospheric layers, into lesser bent rays of the upper atmospheric layers.

Table 3 contains the differences in length between the phenomenon as predicted in the simulation and the measured. While the level of difference may seem to be large, it is not unexpected as predicted rays become more spread out at the edges and the phenomenon becomes very dim at the edges in the original level 1a data, averaging about 400 counts/second. For contrast, the middle of the phenomenon is ∼3,000 counts/second, the mean count value of the Earth is ∼14,000 counts/second, and the mean count for the dark space around Earth is ∼80 counts/second. The quantum efficiency of EPIC at 780 nm is 58%, which would attenuate the signal more (Epic.gsfc.nasa.gov, 2024). Atmospheric loss from absorption and scattering would contribute as well, although not as significantly. Although computing the atmospheric loss from scattering and absorption is outside of the intent of this paper, for reference it is ∼4.65% at 780 nm at 41.8° elevation (Spectra-AM1.5, 2025).

The full set of images are available on the EPIC Gallery (https://epic.gsfc.nasa.gov/galleries). Many thanks to NOAA and NASA for conducting operations and permitting use of the instrument to investigate atmospheric optical properties.

The findings demonstrate that the phenomenon seen in the EPIC image when the Moon is at the horizon is a combination of unique atmospheric distortion effects including refraction and ray inversions, making it similar to a complex mirage caused by regular differences in temperature in the atmospheric layers. Although a visually similar effect is seen at geostationary, caused solely by atmospheric refraction, it lacks the ray inversions and distortions common with mirages. EPIC’s phenomenon, “Gaia’s Crown”, is an optical effect between the Earth and the Moon when the two visually intersect and is seen only in deep space due to the unique viewing geometry causing greater distortion from atmospheric refraction. The demonstrated sensitivity to refraction and high-altitude temperature inversions hints that L₁ observations could provide a coarse probe of atmospheric structure. The technique could be applied to other planets with significant atmosphere to help understand their composition.