eli5: If I’m in the Southern hemisphere and I see the full moon, will someone in the Northern hemisphere see it too?

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eli5: If I’m in the Southern hemisphere and I see the full moon, will someone in the Northern hemisphere see it too?

In: Earth Science
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They could; it really depends how far you are. You could be in Ecuador, standing in the southern hemisphere, and someone three feet away from you could be in the northern hemisphere because you’re both right next to the equator. 🙂 In that case the two of you would be seeing 99.999% the same sky and you’d probably both see the moon.

The further apart you get, the less sky you’d have “in common”.

My intuition says that somewhere between 90 and 180 degrees of angular separation you’d no longer have any sky in common, but I’m not sure I have the geometry skill to figure out exactly where.

Yes, the phases of the moon depend only on the relative positions of the sun, moon and earth.

The moon does appear “upside down” to people on different hemispheres though.

Yes, or at least mostly yes.

The moon’s orbit is relatively aligned with the Earth’s so it’s typically visible from anywhere on Earth each day for the same reason that daylight is. Since the phase of the moon is determined by the sun-moon-earth angle – which doesn’t really depend meaningfully on the observer – you will generally speaking both be able to see the full moon.

Now, if you are very close to the poles, it is *possible* that the moon will be above the horizon for one of you and not the other, even if it’s nighttime for you both. But if the moon is south of the sun in the sky, it’s possible for (a) it to be nighttime close to both poles, (b) the moon to be visible for one pole, and (c) the moon to be below the horizon for the other. But this can only happen very close to the poles, where no one really lives.