The Old Moon in the New Moon's Arms
When the Moon is a thin crescent, you can often see the rest of it glowing faintly, like a ghost of the full Moon. Leonardo da Vinci worked out why.
What was reported
A few days after a new Moon, the bright crescent is often joined by a faint, greyish glow over the rest of the Moon's face. In England, sailors and country people called it 'the old Moon in the new Moon's arms', and some saw it as a sign of stormy weather.
Around 1510, Leonardo da Vinci wrote in one of his notebooks, now called the Codex Leicester, that the glow was sunlight reflected off the Earth onto the Moon, and back to our eyes.
Explanations offered
Leonardo was right. The glow is earthshine: sunlight reflected by the Earth's clouds, oceans and land onto the dark part of the Moon. Scientists measure it today to study how much sunlight the Earth reflects, which matters for understanding climate.
- When
- Explained by Leonardo da Vinci around 1510
- Where
- The dark part of the crescent Moon
- Witnesses
- Leonardo da Vinci and sky-watchers everywhere
- Status
- Explained
Famous cases are real, widely reported events. Each file sets out what witnesses said and the explanations that have been offered, without taking a side.