# Is ISS ever visible at all on the earth, unaided, by full moon moonlight alone?

Assuming ideal conditions of moonlight and phase angle, is the ISS ever even slightly 'visible' by full moon moonlight alone, that is to say, when the station is not in direct sunlight at all, but only in moonlight from the full moon, perhaps eclipsed by the earth from any direct sunlight?

Although I am researching the amount of light coming off the full moon, compared to direct sunlight, and some other related issues such as phase angle, would I need to have an owl's eyes for even the least hope of seeing the ISS illuminated by moon light alone, well after the normal orbital position required for a pass visible due to directly reflected sunlight?

• Do you mean something like this: reddit.com/r/astrophotography/comments/53zs93/iss_moon_transit – Paul Oct 25 '18 at 4:52
• @Paul - I did not read that yet, but by the title, no. I'm almost finished with a pyephem project now for finding sun, moon, or other body ISS transits, for planning observations. During a lunar transit, if the ISS is eclipsed from sunlight by the earth, the sun moon ISS phase angle is basically zero and the eye would be trying to see the 'dark side' of the station, What I am asking here is not concerning a lunar transit, but spotting the ISS 'in the clear', lit by only moonlight reflected off of the ISS. Thank you for the question. And I will read that link later. – always_learning Oct 25 '18 at 5:19
• You can't have both night time and a full moon illuminating the station at the same time. The moon needs to be below the horizon to reflect light onto the station but then you can't get a full moon without the sun being up the opposite side. – Vincent B Oct 25 '18 at 11:27
• @Vincent- thank you for your interest. Consider: 2am localtime, the full moon is climbing high in the otherwise very sunless night sky. And there is a local ISS pass too. The sun is long down and its light will not reach the ISS on this pass, but the moon is bright enough to ruin my night vision. The moon is to my south, ISS pass to my north. Consider: I stand looking down a road at night. A deer is 50 yards ahead on the side of the road. A car is 150 yards ahead and its lights illuminate the deer 'slightly' because the deer is not 'directly' between me and the car's headlights ... – always_learning Oct 25 '18 at 14:37
• @Vincent ...(phase angle near but not zero). If instead, the deer is down the road in the other direction from me than the car, now the deer is 'fully' lit to my eye by the headlights behind me, shining full on the deer (phase near max). Me, ISS, full moon. Think of the moon as the headlights. Yes, it is lit by the sun, but the sun is in a position where its direct light can not reach the ISS, but it still reaches the full moon easily. The earth is not in the way of sunlight reaching the moon, as it would be during an eclipse. Is the moonlight alone now enough to light the ISS to my eye? – always_learning Oct 25 '18 at 14:40

This short answer interprets "visible" as you can see it with your eye.

As seen in the vicinity of the Earth, the visual magnitude (brightness) of the Sun and Moon are -27 and -13, respectively. That's a difference of 14 magnitudes, or $$100^{14/5} \approx$$ 400,000 times different.

Neglecting geometrical effects and unusual reflected "flares" off the flat solar panels, if the ISS illuminated by the Moon were at the limit of visual detectability at magnitude +6, the sunlit ISS would be magnitude -8.

Answers to this Quora question mention that Wikipedia's Apparent magnitude article mentions that Heavens above puts the ISS' maximum apparent brightness at about -6, which is less than a factor of 10 smaller than our simplistic model would require.

The Heavens above creator also has experience with calculating Iridium flares which can max out near -10, and the ISS has way, way bigger (though much less reflective) flat surfaces that make flares possible.

Answer: So it's really unlikely this could be done regularly with the unaided eye, though a good sky camera with a substantial aperture could certainly capture the trail if the Moon were not above the horizon in the location of the camera causing sky brightness issues.

below "View of one of the Main Mission Antenna. The hinged base is on the right side." Cropped. From here.

below: "Scott Kelly fixing a cooling pump during a spacewalk." from Gizomodo's Astronaut Scott Kelly on Liquid Salt, a Stinky Station, and Sleeping in Freefall. Image credit: NASA/Kjell Lindgren