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Today The Full ‘Harvest Moon’ Will Begin To Rust: Why It’s Our Fault And What We Can Do About It

Jamie Carter

Is Earth making the Moon rusty? As our natural satellite in space reaches the point in its orbit where Earth is between it and the Sun, the side of it that’s tidally locked to Earth is fully illuminated.

It’s a beautiful celestial moment that’s happening right now, with the full “Harvest Moon” having just turned full—that moment being on Monday, September 20 at 23:55 Universal Time.

This monthly moment is also when the Moon begins to rust. Why? Because it enters Earth’s tail.

Wait. What? Earth has a tail? Yes—and it’s sending oxygen to the Moon while simultaneously blocking space weather from the Sun.

Here’s everything you need to know about Earth, its tail and why we need a space observatory to figure-out exactly what’s going on.

What is ‘space weather?’

The surface of the Moon is being continuously hit by solar particles from the Sun. So is Earth. The Sun’s corona releases energy in flares containing hot ionised gas, which travel towards Earth. All that light, energy and solar material from the Sun can spell trouble for computers, communication networks, satellites, power distribution networks, aviation, astronauts and the Internet.

However, for a short time each month Earth blocks space weather from reaching the Moon—and instead it receives oxygen from our upper atmosphere.

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Yes, Earth has a ‘magnetotail’

Earth has a “magnetotail”—a bubble of magnetism caused by the molten dynamo in our planet’s core—that trails in its wake. From three days before Full Moon to three days after, the Moon moves through it, with Earth’s magnetotail blocking 99% of the solar wind.

During that time oxygen from Earth’s upper atmosphere is transported to the Moon, causing dust storms, electrostatic discharges ... and rust.

Rust on the Moon

The rust comes from iron on the lunar surface oxidating into lunar hematite. Already known to be common on Earth, the “red planet” Mars and some asteroids, it was found in 2008 by the Indian Space Research Organization’s (ISRO) Chandrayaan-1 orbiter (which also discovered water ice). It’s common at high latitudes of the Moon.

Why we need a space observatory to study ‘Moon-rust’

The authors of a pre-print paper propose a space weather observatory to study to monitor the effect on the Moon of Earth’s magnetic field, which contains trapped charged particles with oxygen and other gasses.

It will teach astronomers exactly how Earth’s magnetotail causes the Moon’s surface to rust.

What is the Earth-Moon L1 Lagrange Point?

The Earth-Moon L1 Lagrange point is between the Earth and the Moon. It’s a point in space where, thanks to an equal gravitational pull from the Earth and the Moon, a space observatory would stay where it is.

A spacecraft near this point would get a nearly unobstructed view of the Moon and the upper atmosphere of the Earth, putting it in alignment of Sun-Earth-Moon precisely when the oxygen is being transported to the Moon.

You’ll never look at a full Moon the same way again.

Wishing you clear skies and wide eyes.


