1 Overview
Earthshine is the faint illumination of the Moon's dark side, caused by sunlight reflected from Earth back onto the lunar surface. When the Moon is a thin crescent, the majority of its visible disk is in shadow, yet that shadowed portion often glows with a soft, ashen light. This phenomenon is sometimes called the old moon in the new moon's arms, a poetic description of the crescent cradling the dimly lit disk. The effect is most striking when the crescent is thin, typically one to three days after new moon in the evening sky, or one to three days before new moon in the morning sky. At these times, the Earth as seen from the Moon is nearly full, so our planet reflects a great deal of sunlight onto the lunar night side. The result is a ghostly, grayish glow that reveals the Moon's familiar dark plains and bright highlands even where direct sunlight does not reach. Observing earthshine is a rewarding pursuit for anyone with a clear western horizon and a bit of patience. It connects the observer to the geometry of the Earth-Moon-Sun system and offers a glimpse of our own planet's influence on the lunar landscape. Unlike many deep-sky objects, earthshine is a naked-eye phenomenon, though binoculars and telescopes can enhance the view. The challenge lies not in finding the Moon but in catching it at the right phase and under sufficiently dark skies. With practice, you can learn to anticipate when earthshine will be most prominent and make the most of those brief windows each month.
2 What the eyepiece actually shows
Through binoculars or a small telescope, the thin crescent Moon reveals a wealth of detail along the terminator, the line dividing day and night on the lunar surface. Craters near the terminator cast long shadows, creating a dramatic, three-dimensional appearance. The earthshine-lit portion, by contrast, appears flat and muted, but it is not featureless. You can trace the dark maria, the large basaltic plains that appear as gray patches, and even make out the brighter highlands and ray systems around prominent craters like Tycho or Copernicus. The contrast between the brilliant crescent and the dim earthshine is extreme, so your eye may need a moment to adapt. At low power, the entire Moon fits in the field of view, and the earthshine gives the disk a complete, spherical look rather than a simple crescent. Higher magnifications can reveal finer details in the earthshine, but they also spread the light over a larger area, making the faint glow harder to see. Many observers find that a medium power, around 50 to 100 times, offers a good balance. The color of earthshine is typically a soft gray or pale blue, influenced by Earth's atmosphere and cloud cover. If Earth is largely covered by clouds, the earthshine may appear slightly brighter or have a different tint. The view is dynamic: as the Moon's phase changes night by night, the extent of earthshine shifts, and the terminator sweeps across different features. Keeping a sketch or log of what you see each evening can help you track these changes and improve your observational skills.
3 Finding a target: charts, star hopping, and timing
The Moon is the easiest celestial object to find, but catching it as a thin crescent with good earthshine requires timing. The best evenings are those immediately after new moon, when the crescent is young and the earthshine is bright. New moon occurs when the Moon is between Earth and the Sun, so it is not visible. One or two days later, a slender crescent appears low in the western sky shortly after sunset. Each evening, the crescent grows thicker and moves higher and farther east. The earthshine fades as the crescent brightens, so the window for good earthshine is narrow, typically three to four days after new moon. To plan your observing session, consult a star chart or a lunar phase calendar. Many general astronomy apps and websites provide the Moon's phase and position, but you can also use a simple star chart to see where the Moon will be relative to the constellations. The Moon travels along the ecliptic, the same path the Sun and planets follow, so it passes through the zodiac constellations. A nearby bright star or planet can serve as a landmark. For example, if Venus or Jupiter is near the crescent, it can guide you to the Moon before it becomes obvious. Star hopping to the Moon is not necessary, but understanding its motion helps you anticipate its appearance. The Moon sets roughly 50 minutes later each night, so if you miss it one evening, try again the next. Also, the angle of the ecliptic relative to the horizon affects how high the crescent appears. In spring evenings, the ecliptic is steeply inclined, so the crescent can be surprisingly high; in autumn, it hugs the horizon. These seasonal differences influence how easy it is to spot the thin crescent and how long it remains visible before setting.
4 Gear that changes the view
You do not need expensive equipment to enjoy earthshine. The unaided eye, under dark skies, can show the crescent and the ashen glow beautifully. In fact, some observers prefer the naked-eye view because it captures the whole scene, including the twilight sky and any nearby bright planets. Binoculars, especially those with wide fields and moderate magnification, like 7x50 or 10x50, offer a comfortable step up. They brighten the earthshine and reveal the major lunar maria without overwhelming the view. A small telescope, even a simple refractor or reflector with a modest aperture, can provide more detail, but it also magnifies the brightness difference between the crescent and the earthshine. To manage this, use a low-power eyepiece and consider a neutral density filter or a variable polarizing filter to reduce the glare from the crescent. However, such filters also dim the earthshine, so use them sparingly. A moon filter is not always necessary; many observers simply let their eyes adapt. The type of mount matters less for the Moon, but a tracking mount can be convenient at high power. For casual observing, a simple alt-azimuth mount is fine. If you wear glasses, look for eyepieces with long eye relief. Remember that the Moon is bright, so a large aperture is not required; in fact, too much aperture can make the earthshine harder to see because the crescent becomes blindingly bright. A small, high-quality instrument often outperforms a large one for this subtle phenomenon. Finally, a comfortable chair and a warm layer of clothing will keep you at the eyepiece longer, which is often the key to catching the best moments of steady seeing.
5 Reading brightness, size, and contrast
The magnitude scale is a measure of brightness, with lower numbers indicating brighter objects. The crescent Moon can reach magnitude -10 or brighter, making it one of the brightest objects in the night sky. Earthshine, however, is spread over the dark portion of the lunar disk, so its surface brightness is much lower, roughly comparable to a faint star or a dim nebula. This means that when you look at the Moon, your eye must cope with a huge dynamic range. The bright crescent can cause glare and reduce your ability to see the faint earthshine. To improve contrast, observe before the sky is fully dark, during twilight, when the crescent is less overwhelming. The size of the Moon is about half a degree across, the same apparent size as the Sun. This is small enough that it fits easily in the field of view of most binoculars and telescopes, but large enough that details are visible. The earthshine covers the entire dark side, which is roughly the same area as the crescent, so you are seeing a large, dim region. Contrast is the key to observing earthshine. A dark sky background helps, but the Moon itself is bright, so you need to position yourself away from streetlights and other sources of glare. Using a hood or cupping your hands around the eyepiece can block stray light. Also, allow your eyes to dark-adapt; it takes about 20 minutes for your eyes to become fully sensitive. Even then, the bright crescent will affect your night vision, so try looking slightly away from the crescent, using averted vision to detect the earthshine. With practice, you can learn to see more detail in the faint glow. Remember that earthshine is a low-contrast phenomenon, so patience and careful observing techniques matter more than equipment.
6 Common mistakes at the telescope
One common mistake is trying to observe earthshine when the Moon is too thick. After the first few days, the crescent becomes so bright that the earthshine is washed out. Aim for a crescent that is less than 25 percent illuminated. Another mistake is using too much magnification. While high power can reveal fine details on the crescent, it spreads the earthshine over a larger area, making it dimmer and harder to see. Start with low power and only increase if the view remains comfortable. Many observers forget to wait for the sky to darken sufficiently. Earthshine is best seen when the sky is dark enough to provide contrast, but not so dark that the Moon sets. The window between sunset and moonset is often only an hour or two, so plan accordingly. A third mistake is ignoring atmospheric conditions. Haze, thin clouds, and even high humidity can scatter light and reduce contrast. If the sky looks milky, earthshine will be poor. Also, observing from a location with bright lights in the direction of the Moon will ruin your dark adaptation. Another error is not using a hood or shield to block stray light from entering the telescope or your eyes. Even a nearby porch light can make a difference. Finally, some observers expect earthshine to look like a fully illuminated Moon, but it is inherently faint and gray. Adjust your expectations and enjoy the subtle beauty. Keeping a log of your observations, noting the phase, sky conditions, and what you saw, will help you improve. Over time, you will learn to recognize the best conditions and make the most of each opportunity.
7 Frequently asked questions
What causes earthshine? Earthshine is sunlight that reflects off Earth, travels to the Moon, and then reflects back to our eyes. It is brightest when Earth is nearly full as seen from the Moon, which happens when the Moon is a thin crescent as seen from Earth. Can I see earthshine with the naked eye? Yes, under dark skies, the earthshine is visible to the unaided eye as a faint glow on the dark side of the crescent Moon. It is often easier to see than many people expect. How often can I observe earthshine? You can see it during every lunar cycle, but only for a few days around new moon. In the evening sky, look one to three days after new moon; in the morning sky, look one to three days before new moon. Do I need a telescope? No, binoculars or even just your eyes are sufficient. A telescope can enhance the view but is not necessary. Why does earthshine vary in brightness? The brightness depends on how much of Earth is covered by clouds and how reflective those clouds are. Also, the phase of the Moon and the angle of reflection change from night to night. Is earthshine visible from light-polluted cities? It is challenging because the sky background is bright, but you can still see it if you block stray light and observe during twilight when the Moon is higher. Can earthshine help me see lunar features? Yes, the earthshine illuminates the dark side just enough to reveal the major maria and some craters, though not in as much detail as the sunlit side. What is the best phase for earthshine? The thinnest crescent, ideally less than 20 percent illuminated, gives the best contrast. After that, the crescent becomes too bright. How long does earthshine last? It fades as the Moon's phase increases, typically over a period of about four to five days after new moon. Each evening, the crescent grows and the earthshine becomes less prominent. Can earthshine be seen during a total lunar eclipse? No, earthshine is a phenomenon of the crescent phase. During a lunar eclipse, the Moon is full and Earth's shadow covers it, but the illumination comes from refracted sunlight, not earthshine.