Brightness of ISS in Earth's shadow

[Count Iblis]

I observed the ISS when it was almost overhead and well inside Earth's shadow with binoculars. The brightness was perhaps magnitude 8 or so, I'm not sure. I observed the ISS enter the Earth's shadow when it was at 60 degrees over the horizon, the brightness declined rapidly as it entered the half shadow. But it remained visible as it passed overhead and moved toward the horizon in the East. The brightness gradually declined until I couldn't see it anymore; it didn't seem that there was ever an abrupt change in brightness after it had entered the shadow.


So, how bright is the ISS when it is overhead in the Earth's shadow?

 

[Integral]

I think you lost a minus sign. Venus can be as bright as -4.4. From my observations the ISS is at least that bright. I would guess that it is somewhere between -3 and -5, but that is just a SWAG.

 

[Count Iblis]

I think you lost a minus sign. Venus can be as bright as -4.4. From my observations the ISS is at least that bright. I would guess that it is somewhere between -3 and -5, but that is just a SWAG.

Yes, but that's when the Sun is shining on the ISS. Then it can be very bright. In this case it was a little less bight (about magnitude -2.5 I think) because it entered the shadow at 60 degrees.

But when the ISS is in the shadow, then it cannot be seen with the naked eye. I still saw it with bnoculars and would estimate the brightness to be plus 8, so very dim but still visible. The brightness then furhter declined as the ISS was moving toward the horizon until I couldn't see it anymore.

 

[Integral]

Oh, my bad! I now understand what you are saying. Since the ISS is not a source of light it must be reflecting light from somewhere. I would guess the amount of light it receives depends on the portion of the lit surface of the Earth it sees. So its apparent magnitude will change as it orbits the earth.

Of course as its magnitude will decrease as it approaches the horizon since you are observing it though a increasing thickness of atmosphere.

 

[russ_watters]

You are also observing it through an increased distance. At 30 degrees elevation, it is about twice as far away as when it is directly overhead (lower than that and the geometry gets harder because you have to take into account the curvature of the earth).

As for the source of the illumination, I can think of only two possibilities: lights from Earth and cabin lights in the ISS. I would have no idea the strength of either. Right now, the moon is new, so it wouldn't have been contributing.