Since ancient times, people have observed and praised the moon, its light inspiring countless poems and stories.

However, the moon itself does not emit light. So, how does it shine?

Reflection of Sunlight

The primary reason the moon appears to shine is the reflection of sunlight. The sun is the only celestial body in the solar system to emit light by itself.

The light it emits strikes the moon's surface and is then reflected to Earth, making the moon appear luminous. In reality, the moon itself has no light source, it acts as a giant reflector.

The reflectivity, or albedo, of the moon's surface is about 12%. This means that when sunlight hits the moon, only 12% of the light is reflected in space, while the rest is absorbed by the moon's surface.

Despite this relatively low reflectivity, the sunlight is so intense that the reflected light is sufficient for us to see a bright moon from Earth.

Changes in the Moon's Phases

The moon's light is closely related to its phases. The phases of the moon change due to the relative positions of the Earth, the moon, and the sun as the moon orbits the Earth, causing different portions of the moon to be illuminated by sunlight.

1. New Moon

During the new moon, the moon is positioned between the Earth and the sun. The side of the moon facing the Earth does not receive sunlight, rendering it invisible to us. This phase is often referred to as "no moon."

2. First Quarter Moon

As the moon continues its orbit around Earth, sunlight begins to illuminate part of the moon. This phase is called the first quarter moon, where we see half of the moon's surface illuminated, making it appear semicircular.

3. Full Moon

When the Earth is positioned between the moon and the sun, the entire side of the moon facing Earth is bathed in sunlight. This phase, known as the full moon, is when the moon appears fully illuminated and brightest in the night sky.

4. Last Quarter Moon

After another half month, the moon reaches the last quarter phase, where it appears semicircular again, but with the opposite half illuminated compared to the first quarter.

These phase changes affect not only the moon's brightness but also its position in the sky and the times it rises and sets. This variability is why we sometimes see the moon during the day and at other times only at night.

The Reflectivity of the Moon

The moon's surface is composed of various substances, primarily silicate minerals, which have strong reflective properties. However, the surface is not smooth, it is dotted with craters and mountains. These terrain features influence how the moon reflects sunlight, resulting in the uneven appearance of moonlight.

Certain areas of the moon's surface, known as "lunar maria" or "lunar seas," are relatively flat regions with lower reflectivity. In contrast, the "highlands" on the moon have higher reflectivity, contributing to the moon's spotted appearance.

Influence of the Atmosphere

The Earth's atmosphere also affects the moonlight we perceive. As moonlight passes through the atmosphere, it undergoes scattering and refraction, which can make the moon appear larger and redder when it is near the horizon.

This effect is particularly noticeable during moonrise or moonset when the moonlight travels through a thicker layer of the atmosphere.

Appreciating the Moon's Beauty

Understanding the scientific principles behind the moon's light allows us to appreciate its beauty even more. The interplay of sunlight reflection, phase changes, surface composition, and atmospheric effects creates the mesmerizing spectacle we observe in the night sky.