Question

Recognize Cause and Effect During a new phase of the Moon, we cannot see it because no sunlight reaches the side facing Earth. Yet sometimes when there is a thin crescent visible, we do see a faint image of the rest of the Moon. Explain what might cause this to happen.

Short answer

The faint image is caused by 'Earthshine,' light reflected from Earth onto the Moon.

Step-by-step solution

Identify the Observable Phenomenon

When there is a thin crescent moon visible, we sometimes observe a faint image of the rest of the Moon. This phenomenon is important for us to explain and understand.

Understand the Cause of a New Moon Phase

During a new moon, the side of the Moon that faces the Earth is not illuminated by the Sun. This is why the Moon is not visible. The alignment of Sun, Earth, and Moon during this phase means that the Moon is positioned between the Earth and the Sun.

Observe the Thin Crescent

A thin crescent of the Moon becomes visible when a small portion of the Moon starts reflecting sunlight again, making it observable from Earth. This usually occurs as the Moon progresses past the new phase.

Explain the Faint Image of the Rest of the Moon

The faint image of the rest of the Moon that we sometimes see is due to 'Earthshine'. This is a phenomenon in which sunlight reflecting off the Earth's surface and atmosphere is cast back onto the Moon, illuminating the side not directly lit by the Sun. This light, although faint, helps in illuminating the dark portion of the Moon.

Key concepts

New Moon

The new moon is an interesting phase in the lunar cycle. During this time, the Moon is positioned between the Earth and the Sun. This alignment results in the Moon's entire illuminated half being turned away from us.
As a result, the side facing Earth does not receive direct sunlight. This is why the moon appears invisible to us during the new moon phase. It is like trying to see the dark side of a ball that is only lit from the opposite side.
The new moon marks the beginning of the lunar cycle and lasts for a brief period before moving on to the next phase. This phase is crucial for setting the timing of the month's lunar calendar.

Crescent Moon

After the new moon, the Moon moves along its orbit, and a sliver of sunlight begins to illuminate the visible side. This creates what we call a 'crescent moon.'
It is a phase where only a small portion of the Moon is lit up by sunlight, forming a curved shape. The crescent moon continues to grow, or 'wax,' each day as more of the lunar surface becomes visible.
Watching the crescent moon rise up in the sky is a captivating part of the lunar phases because it represents the Moon's journey of becoming fully illuminated. This waxing crescent phase is observed just after the new moon and leads to the first quarter.

Earthshine

Earthshine is a magical phenomenon during the crescent phase of the Moon. When you observe a crescent moon, you might notice a ghostly glow illuminating the rest of the moon's surface.
This subtle glow is called earthshine and occurs because sunlight reflects off Earth's oceans and clouds. The light then bounces back to the Moon, softly illuminating the part not directly lit by the Sun.
Earthshine provides a unique opportunity to observe the entire outline of the moon. It has been a source of enchantment for stargazers and plays a crucial role in our understanding of lunar observation.

Sunlight Reflection

Sunlight reflection is the primary driver of lunar phases and visual perception of the Moon from Earth. The Moon does not emit any light of its own. Instead, it reflects sunlight that hits its surface.
The varying angles at which sunlight strikes the Moon result in different lunar phases, including the new and crescent moon phases. As the Moon orbits Earth, the changing positions cause different portions of its surface to be illuminated, creating the familiar phases we know.
Understanding sunlight's role in lunar visibility helps explain why we sometimes see only parts of the Moon and why phenomena like earthshine occur.