# The effect of gravitational lensing during the lunar eclipse

During the lunar eclipse, the Moon turns into bloody colour while the shadow of the Earth is casting over the lunar surface. The red hue can be explained by means of the refraction of light and Thomson scattering. Besides these two, will the gravitational lensing effect take a role in the eclipse? That is, the Sun light is bent due to the Earth and shone onto the lunar surface, and then the light is reflected towards the Earth. If so, how significant will this effect be?

• The moon's red hue can be explained by scattering, but not by refraction. Shorter wavelengths are bent more than long, so any color shift from refraction will be towards blue, not red. This is apparent in the "green flash" sometimes seen seconds after sunset. If you observed the eclipse from the Moon (it would be a Solar eclipse, not a Lunar eclipse) the Earth's disk would be rimmed in red since you would be observing every sunset and sunrise at the same time. Nov 22, 2021 at 0:17

We can use the following equation to calculate the angle of deflection for light that passes within a distance $$b$$ of some mass $$M$$ (source) $$\begin{equation} \alpha = \frac{4 G M}{c^2 b} \end{equation}$$ Taking $$M$$ to be the mass of the Earth and $$b$$ to be the radius of the Earth, we find $$\begin{equation} \alpha = \frac{4 G M_\oplus}{c^2 R_\oplus} = 3\times 10^{-9}\ {\rm rad} = 0.6\ {\rm milliarcseconds\ (mas)} \end{equation}$$
Earth's gravity is far to weak to make much difference. But the Sun's gravity makes a solar lens that could be used as a telescope, if we devoted enough resources and ingenuity to it. We could literally map the surface of an exoplanet $$100$$ light years away at $$25$$ km/pixel. See The Solar Gravitational Lens will Map Exoplanets. Seriously.