Where it sits
Sun to Earth to Moon to you. Two reflections, and still visible from a garden.
Tell it apart
| Aspect | Earthshine | Planetshine |
|---|---|---|
| Scope | Earth and Moon | Any planet-moon pair |
| Visible to us | Yes | With a telescope |
| Brightness varies with | Earth's albedo | The planet's albedo |
| Category | An instance | The general case |
Earthshine is one case of planetshine — the general phenomenon of a moon's dark side being lit by its parent planet. The same effect operates on moons throughout the solar system; ours is simply the one we can see from underneath.
What it doesn't mean
It's Earth's own reflected light. The path is Sun to Earth to Moon to your eye — two reflections, off Earth and then off the Moon. From the Moon's surface, our planet is a brilliant full 'Earth' lighting the lunar night.
Its brightness tracks Earth's albedo, which varies seasonally. It peaks from April to June because snow and ice reflect far more than vegetation or water — which makes earthshine a crude but real planetary reflectivity gauge.
What it really means
A thin crescent moon, and behind the bright sliver you can make out the rest of the disc — dim, grey, unmistakably there. English has a lovely old name for it: the old Moon in the new Moon's arms.
The light making that visible has been reflected twice. Sun to Earth, Earth to Moon, Moon back to your eye. Which means that when you look at earthshine, you are looking at Earth's own reflected light, returned to you from a quarter of a million miles away.
Stand on the Moon during our new moon and the reason is obvious: Earth would be full, and enormous, and very bright — roughly fifty times brighter than a full moon is to us. Lunar night isn't dark. It's lit by the planet.
Leonardo da Vinci worked this out around 1510, describing it as sunlight reflected by Earth's oceans, which is why it's sometimes called the da Vinci glow. He got there four centuries before anyone could check.
And there's a genuinely useful consequence. Earthshine brightness tracks Earth's albedo — how reflective the planet currently is — so the glow on the dark part of the Moon is a crude measurement of our own reflectivity, taken from outside. It peaks between April and June, when Northern Hemisphere snow and ice are still extensive and Earth is at its most mirror-like.
Where it comes from
English — direct compound
The poetic English name is 'the old Moon in the new Moon's arms'. The general physical category is planetshine — earthshine is simply our local instance of it.
- c. 1510
Leonardo da Vinci explains the effect as sunlight reflected by Earth illuminating the Moon's dark surface — hence the nickname 'the da Vinci glow'.
- 2001
Goode and colleagues turn the glow into an albedo record with the Big Bear Solar Observatory earthshine photometer (Geophysical Research Letters 28, 1671). NASA Earth Observatory puts the peak between April and June, when Northern Hemisphere snow and ice persist at high latitudes — though Goode's own data note that the spring maximum is also the cloudiest part of the year.
In real life
- A thin crescent Moon hangs low in the west after sunset, and behind it the rest of the disc glows a faint ash-grey — the old Moon in the new Moon's arms.
- Leonardo da Vinci explained the glow around 1510 as sunlight reflected from Earth back onto the Moon.
- Stand on the Moon during its night and Earth hangs nearly full and very bright, lighting the grey ground — the same light we see returned as earthshine.
Questions
Q01What causes earthshine?
Sunlight reflects off Earth, travels to the Moon, and bounces back — faintly lighting the part of the lunar disc that direct sunlight doesn't reach. From the Moon's surface, Earth is a brilliant object lighting the lunar night.
Q02Who first explained it?
Leonardo da Vinci, over 500 years ago, describing it as sunlight reflected by Earth's oceans. It's sometimes called 'the da Vinci glow' for that reason.
Q03When is it easiest to see?
Around new moon, when the crescent is thin and the unlit portion is large — and brightest in the April to June window, when Earth's albedo peaks because of Northern Hemisphere snow and ice.
Sources
- Earthshine — NASA Earth Observatory.
- The Da Vinci Glow — NASA Astronomy Picture of the Day.
- earthshine (etymology) — Wiktionary.