When the Moon first formed, how bright was it in earth's sky?

When the Moon formed, according to the Giant Impact hypothesis, it was located much closer to the Earth, perhaps by an order of magnitude, with the resulting tidal effects increased by a factor of 1000. The Moon is also believed to have been molten for a short time after its formation, because it coalesced from material which was itself molten initially, as well as the energy of the collisions and the gravitational potential energy released from the Moon’s differentiation.

Would the Moon, emitting its own visible light in addition to reflecting it from the Sun, as it does now and would have done then, at a temperature of around 3000 K (at the distance of either 1/10 of its present one, or simply at the present distance) be bright enough to cause climate change on Earth, if it occurred today? Would this climate change be catastrophic?

Climate change effects? If the Moon was still that hot and it formed by a giant impact with Earth, the Earth would also still be a molten blob so that is kind of a moot point, same if a similar impact occurred today, which would be vastly larger than the asteroid that wiped out the dinosaurs. In any case, the reflected light alone wouldn’t be enough to have any noticeable effect; think of how bright a full Moon is today; even 1,000 times that isn’t anywhere close to the Sun, which is about 400,000 times brighter than a full Moon according to Wikipedia). As for a Moon that was at 3000K for whatever reason today, if it was at the same distance I’m guessing it would look like a dim Sun (they appear to be the same size as seen from Earth); not sure how much heat it would contribute relative to the Sun, but it would be much dimmer (radiation changes with the fourth power of temperature; the Sun is about 5800K, so about 14 times more radiation per unit area); if it was 10 times closer, then it probably would have a significant effect.

Interesting. Since the Moon and the Sun are roughly the same angular size, would that mean that in the case of the Moon being at the same distance and heated to 3000 K, the total radiation intensity on Earth would be as if the Sun had become brighter by a factor of 15/14 (the sun plus the 1/14 extra radiation)?

Italo Calvino wrote a great short story about when the Moon was much closer to the Earth; so close that when the tide was high you could row out and climb up to the moon with a ladder! :slight_smile:
The Distance of the Moon (pdf)