This eponymous theory is that large masses (such as the moon) partially block the gravitational force from more distant objects (such as the sun).

ECONOMIST: Gravitational anomalies

If the ratio of magnesium and iron on the moon conforms with scientists' predictions it will provide evidence that the moon really is a chip off the old block.

ECONOMIST: Astronomy

"For the May eclipse, the moon will be at the furthest distance from Earth that it ever achieves -- meaning that it will block the smallest possible portion of the sun, and leave the largest possible bright ring around the outside, " the agency said on its website.

CNN: Upcoming solar eclipse to project 'ring of fire'