For the moment, the sun is in a stable phase, but in around 5 billion years when it will enter the red giant phase, things will turn tougher in the Earth-moon system.

At that moment, the sun will swell so much that its atmosphere will reach out the Earth and moon, which will both be affected by gas drag. As the moon is slowly coming off from Earth, by that time it will be situated in an orbit about 40 % larger than the current one, of 239,000 miles (385,000 km). "The Moon's actual path is a wiggly line around the sun, with it moving faster when it is slightly farther out (at full moon) and more slowly when it is slightly closer (at new moon)," said Lee Anne Willson of Iowa State University.

"So the gas drag is more effective at the farther part of the orbit and this will put the Moon into an orbit where the new moon is closer to Earth than the full moon."

The Moon emerged around 4.5 billion years ago after an impact between Earth and a Mars-sized planet. The debris expelled into the Earth's orbit formed the moon.

The Moon's gravity provokes tides in the Earth's oceans and the fast spinning Earth attempts to drag ahead of the slowly satellite, pushing away the Moon from Earth by 1.6 inches (4 centimeters) annually and our planet's rotation is getting slower.

When the Moon will make a rotation around the Earth in about 47 days to orbit the Earth and the Earth's spin will take 47 days, then both Earth and Moon would maintain the same faces permanently turned towards one another.

The Sun's turning into a red giant will be an obstacle to the moon's movement away, turning it into a belt of debris around the Earth. "The density and temperature both increase rapidly near the apparent surface (photosphere) of the future giant sun," Willson explained.

The moon will be pushed closer to the Earth up to 11,470 miles (18,470 kilometers) above our planet, the Roche limit. "Reaching the Roche limit means that the gravity holding [the moon] together is weaker than the tidal forces acting to pull it apart," Willson said.

The moon will be torn to pieces forming a 23,000-mile-diameter (37,000-kilometer) short-lived ring of debris above Earth's equator that will fall down onto the Earth. "Particles of different masses will have different survival times; the smaller particles will be removed first, and the biggest ones last. Most of the ring particles would be gone by the time the Earth reaches the stellar photosphere," Willson said.

When the Sun's photosphere reaches the Earth, the planet also will be burnt into the Sun. But there could be also some alternatives: if the sun as a red giant stops its swelling before reaching Earth, our planet could survive, however, without a moon.

In this stage, the Earth will accompany the sun, until it eventually turns into a white dwarf, fading over the following trillions of years. Or, if the swelling sun expels 20 % of its mass before getting close to the Earth and the moon, both corpses could be saved, facing each other for eternity.

All this remains theoretical as no red giant star has been watched during this crucial phase.

Photo credit: NASA