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Why the Moon is more lemon than sixpence

New Delhi, July 31: Those who still believe the Earth is flat can now cast a longing look at the Moon. Scientists have determined that the Moon was sculpted into an orb with flattened poles, an equatorial bulge and some mystery features.

The conclusion has been made nearly five centuries after the Portuguese explorer Ferdinand Magellan circumnavigated the Earth and established its spherical shape.

Scientists in the US have shown how swirling molten material could explain deviations from a simple figure of equilibrium that could be expected from the history of the Moon, which was born when a Mars-like planet slammed into the Earth in an embryonic solar system.

As this newly formed Moon — a chunk thrown up by that collision — cooled and solidified about four billion years ago, it acquired what researchers call a “tidal-rotational shape” — flattened poles and a bulge at the equator.

The US scientists said their study of the Moon explained its shape, linked the geology of its near and far sides and resolved longstanding questions about its shape and its gravity.

“Imagine a spinning water balloon; it will start to flatten at the poles and bulge at the equator,” Ian Garrick-Bethell, assistant professor of Earth and planetary sciences at the University of California, said in a media release issued by the university.

“You also have tides due to the gravitational pull of the Earth, and that creates a sort of a lemon shape with the long axis of the lemon pointing to the Earth,” said Garrick-Bethell, the first author of a research paper on how the moon was sculpted that was published today in the journal Nature.

But these forces cannot fully explain the current overall shape of the Moon, which is complicated by its large basins — vast low-lying zones — and the craters formed when giant rocks from space crashed onto its surface.

Garrick-Bethell and his colleagues from the Massachusetts Institute of Technology and the Arizona State University analysed large-scale lunar topography and the Moon’s gravity field to determine how it might have acquired its crust and its current shape.

Their study suggests that tidal heating and tidal-rotational forces gave the Moon a “slight lemon shape” with a bulge on the side facing the Earth and another bulge on the opposite side.

The researchers have also found that the Moon’s overall gravity field is no longer aligned with its topography as it would have been when the tidal bulges were frozen into the Moon’s shape, the media release said.

The principal axis of the Moon’s overall shape, or the long axis of the lemon, is now separated from the axis of gravity by about 34 degrees.

“The Moon that faced us a long time ago has shifted, so we’re no longer looking at the primordial face of the Moon,” Garrick-Bethell said. “Changes in the mass distribution of the Moon shifted the orientation of the Moon.”

Magellan’s voyage around the world between 1519 and 1522 was intended to establish the spherical shape of the Earth. Although Magellan himself was killed, his iconic expedition is widely viewed as providing hard evidence for a concept circulating for nearly 2,000 years.