# Does the reflected light from Earth boost Venus' apparent magnitude when it is seen transiting the Sun?

During solar transits of Venus as seen from Earth, exactly 0% of Venus' disk is illuminated by sunlight, as expected. However during these transits, Earth is at opposition and fully illuminated by the Sun as seen from Venus' perspective.

I'm not sure what Earth's apparent magnitude as seen from Venus is, but I am certain it is orders of magnitude brighter than Venus as seen from Earth, due to it being larger (i.e. Having a bigger surface area to reflect light from) and also being fully illuminated rather than a crescent.

So, then. During solar transits of Venus, does the reflected sunlight from Earth boost Venus' apparent magnitude? And if so, what would it theoretically be?

Of course, this is all conceptual, because Venus would be surrounded on all sides by the brightest object in the sky during transits. So no matter how bright it would be due to Earthshine, it would still appear a plain black disk.

Let's collect some data:

(Source Wikipedia Venus and Wikipedia Earth)

Absolute magnitude equals the apparent magnitude when the body is viewed from a distance of 1 AU, directly looking at the illuminated side.

During solar transit of Venus (as seen from Earth), the Venus-Earth distance is 0.28 AU, adding -2.8 to the magnitude for 1 AU.

So, the magnitude of Earth, as seen from Venus in your case should be around -6.8, a lot brighter than the brightest Venus as seen from Earth (magnitude -4.92).

How much does this illuminate Venus, compared to the illumination by the sun?

When viewed from Earth, the Sun has the impressive apparent magnitude of -26.74. With Venus being closer to the sun, -0.28 gets added to the Earth-relative magnitude, meaning that for Venus, the Sun has a -27.02 magnitude.

That means that the Sun illuminates Venus 120 million times brighter than Earth does, or that from Earth's illumination alone, Venus would become at best an object of magnitude 15 or so.

• Thank you! It's insane how an Earthlit Venus has the same apparent magnitude as a sunlit Pluto. Commented Nov 16, 2023 at 14:30