# FAQ overflow

## Great Answers to Questions About Everything

#### QUESTION

M = mass of the sun

m = mass of the earth

r = distance between the earth and the sun

The sun is converting mass into energy by nuclear fusion.

$F = \frac{GMm}{r^2} = \frac{mv^2}{r} \rightarrow r = \frac{GM}{v^2}$

$\Delta E = \Delta M c^2 = (M_{t} - M_{t+\Delta t}) c^2 \rightarrow \Delta M = \Delta E / c^2$

$\rightarrow \frac{\Delta r}{\Delta t} = \frac{G}{v^2 c^2}.\frac{\Delta E}{\Delta t}$

Sun radiates $3.9 × 10^{26} W = \Delta E/\Delta t$

Velocity of the earth $v = 29.8k m/s$

There is nothing that is stopping the earth from moving with the same velocity so for centripetal force to balance gravitational force $r$ must change.

Is $r$ increasing? ($\Delta r/ \Delta t = 3.26070717 × 10^{-10} m/s$)

#### ANSWER

I think the reasoning has an error. It assumes $v$ is constant, but instead we ought to assume the angular momentum is constant.

By dimensional analysis that leads to

$r \propto \frac{L^2}{GM}$

so as $M$ decreases, $r$ increases (the original post had $r \propto M$, not $r \propto 1/M$.

On the other hand, assuming a circular orbit seems dubious.

As the other commenters said, this effect is minute. A significant effect on the orbit of the moon around the earth is tidal evolution, which does actually push the moon further away. See http://en.wikipedia.org/wiki/Orbit_of_the_Moon#Tidal_evolution