Earth's position relative to the Sun.

[Khan Wolf]

Would someone please correct my train of thought on this?

I understood that the Earth's position in the Solar System is related to the gravitational attraction between the Earth and the Sun. If this thought is correct, then when the Sun enters its Red Giant phase, the Earth should be pushed back into Mars's orbital path. Thoughts...corrections...help!

 

[tiny-tim]

HI Khan Wolf!

… the Earth's position in the Solar System is related to the gravitational attraction between the Earth and the Sun. If this thought is correct, then when the Sun enters its Red Giant phase, the Earth should be pushed back into Mars's orbital path.

No, the expansion of the Sun will make almost no difference to the Earth's orbit …

once you get into "empty space", the gravitational field of a star depends on its mass, not on its size.

 

[Philosophaie]

When the Sun enters its Red Giant phase, the Earth should be pushed back into Mars's orbital path

The distance between the Earth and Sun will become less and less so the gravitationional attraction will become greater and greater. The Earth would therefore be engulfed by the Sun.

 

[D H]

The distance between the Earth and Sun will become less and less so the gravitationional attraction will become greater and greater. The Earth would therefore be engulfed by the Sun.

Gravitational force is pretty much a function of distance to the Sun's center of mass, not its surface. Assuming the Sun's mass doesn't change, the gravitational attraction between the Sun and the Earth will be pretty much the same whether the Sun is a red giant whose radius is just shy of 1 AU or a tiny white dwarf.

However, the Sun's mass is not constant. It is losing mass due to fusion in the Sun's core and solar wind from the Sun's surface. The Earth will (and is) spiral out a bit from the Sun. The change is rather small, however, and most likely will not save the Earth from being engulfed when the Sun enters its red giant phase.

 

[Nabeshin]

HI Khan Wolf!


No, the expansion of the Sun will make almost no difference to the Earth's orbit …

once you get into "empty space", the gravitational field of a star depends on its mass, not on its size.

While this is of course true, when the Sun enters its red giant phase not just expansion takes place! Realistically, the sun will begin to shed an appreciable portion of its mass, both during the transition and once it becomes a red giant due to increased stellar winds. So the net effect of the sun moving off the main sequence is an increase in orbital radius.