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The Earth likely froze over almost completely about 700 million years ago. This increased its albedo significantly, and I wonder if that would have a considerable effect on its apparent magnitude from other planets. (The Sun was like 4% fainter, though.)

If so, would the Earth be naked-eye visible from farther away 700 mya than it is now? Maybe from Uranus's moons?

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Earth is already pretty bright due to cloud cover, with a typical albedo of .3-.35 -- that is, it reflects about a third of the visible light that hits it.

That means it couldn't get more than about three times as bright even if it were perfectly reflective (albedo 1.0), which means about 1.2 magnitudes brighter.

Spotting Earth from a great distance with the naked eye is less an issue of brightness, more an issue of separation from the Sun. I can't run numbers at the moment to get actual magnitudes or angular separations.

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    $\begingroup$ If the Sun is the problem with visibility, just wait until nighttime (#itsajokedammit) $\endgroup$
    – Carl Witthoft
    Commented Aug 7, 2019 at 18:45
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    $\begingroup$ Max elongation is 3 deg. Stellarium estimates magnitude 3.5 at superior conjunction (full), 4.5 at greatest elongation (quarter phase); Sun magnitude -20. $\endgroup$
    – Mike G
    Commented Aug 7, 2019 at 20:43
  • $\begingroup$ @MikeG, thanks for the numbers! Is that from Uranus, today? $\endgroup$
    – Simon Korneev
    Commented Aug 8, 2019 at 8:05
  • $\begingroup$ @SimonKorneev Those are typical values from Uranus. A snow-covered Earth would be brighter, but you'd still need to hide the Sun with an occulting disk. $\endgroup$
    – Mike G
    Commented Aug 8, 2019 at 19:21

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