Is the Moon seen as a crescent (and not a "boat") all over the world? Is the same phase of the moon visible from the Northern and Southern hemispheres? (Advanced)

Recently a friend of mine visited the country of Bali in Africa. She claims that because that country is south of the equator, the Moon, instead of having a crescent shape during certain phases, will actually have a "boat" shape. Is she pulling my leg ?? Isn't the crescent shape seen the whole world over??

Kristine: Your friend is right; the orientation of the crescent moon depends on the latitude of the person observing it (the size of the crescent, however, is the same wherever you are). Think about a crescent moon as seen from the Northern hemisphere, and the same Moon as seen from the Southern hemisphere; if we take the people in the Northern hemisphere as "right-side up", then those in the southern hemisphere are "upside-down", since the Earth is spherical. Since the existence of the crescent Moon depends only on the relative locations of the Moon, Earth and Sun (and not on one's location on the Earth), the Moon seen in the Southern hemisphere is upside down when compared to that seen in the Northern hemisphere. This means that if the concave part of the crescent points "left" in North, it will point "right" in the South. Since the transition from a "left" pointing crescent to a "right" pointing one must be smooth, we require that the Moon be a "boat" instead of a crescent at the equator.

Sabrina: The appearance of the crescent moon will also change depending on the season for an observer staying at a single location on the Earth. We know the Earth does not sit right-side-up in its orbit - instead the Earth's axis is tilted and this tilt is what causes the seasons. Just as the Sun's path is different across the sky depending on the season (the path is longer during the summer giving us more direct sunlight and hotter days), the Moon's path will be different as well. What part of the Moon gets illuminated (i.e. whether it looks like a crescent or a boat) depends on how high the Moon is in the sky. During summer in the northern hemisphere, we are tipped away from the Moon's orbit, putting the Moon lower in the sky and creating more of a crescent. During winter in the north, we are tipped toward the Moon's orbit, putting the Moon higher in the sky and creating more of a boat.

The Moon’s path (and thus appearance in the sky) will depend on the season. Credit: Goddard Space Flight Center

The Moon's path (and thus appearance in the sky) will depend on the season.

Suniti: The lunar orbital plane is only inclined by about 5 degrees relative to Earth's orbital plane (as illustrated by Britannica), so the same phase of the moon would be simultaneously visible to two people at the same longitude but different hemispheres of Earth, as long as the skies are sufficiently dark.

However, as described by Kristine and Sabrina, given the near-ecliptic orbit of the moon, people viewing the moon from Earth's northern hemisphere will generally look southward while those in the southern hemisphere will look north. This causes the lunar surface to be viewed in roughly opposite orientations. Consequently, while the same phase of the moon will be visible from both hemispheres, the appearance of the lunar surface and the orientation of the phase as viewed from the northern hemisphere will be inverted relative to those as viewed from the southern hemisphere. This also means that the moon appears to wax from its right limb when viewed from the northern hemisphere and from the left limb when viewed from the southern hemisphere. More details are available at Access Science, Wikipedia, and Britannica. You could simulate the view from Earth's southern hemisphere by inverting the Wikipedia lunar phase animation with a convex lens (or by inverting a tablet PC monitor).

About the Author

Kristine Spekkens

Kristine Spekkens

Kristine studies the dynamics of galaxies and what they can teach us about dark matter in the universe. She got her Ph.D from Cornell in August 2005, was a Jansky post-doctoral fellow at Rutgers University from 2005-2008, and is now a faculty member at the Royal Military College of Canada and at Queen's University.

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