Moon Phases and Libration

The Sun's Illumination

• The Sun is the only object in our solar system that emits its own light.
• This light illuminates both the Earth and the Moon.
• The illumination causes the phases of the Moon.

Moon Phases

• Moon phases are the different ways the Moon appears to us from Earth.
• These phases are a result of the changing angles at which we view the Moon's illuminated surface.
• There are eight lunar phases:
  • New Moon
  • Waxing Crescent
  • First Quarter
  • Waxing Gibbous
  • Full Moon
  • Waning Gibbous
  • Third Quarter
  • Waning Crescent
• The cycle repeats approximately every 29.5 days.

Individual Phases Explained

New Moon

• The Moon is invisible as the illuminated side faces the Sun, and the night side faces Earth.
• The Moon is in the same part of the sky as the Sun, rising and setting with it.
• The Moon doesn't usually pass directly between the Earth and the Sun due to the inclination of its orbit; it only passes near the Sun from our perspective.

Waxing Crescent

• A small sliver of the Moon becomes visible.
• The illuminated half of the Moon faces mostly away from Earth.
• The visible portion grows daily as the Moon's orbit progresses.
• The Moon rises a little later each day.

First Quarter

• The Moon is a quarter of the way through its cycle.
• Half of the illuminated side is visible.
• The Moon rises around noon and sets around midnight.
• It is high in the sky in the evening.

Waxing Gibbous

• Most of the Moon’s dayside is visible, making it appear brighter.

Full Moon

• The entire day side of the Moon is illuminated.
• The Moon is opposite the Sun from the Earth's perspective.
• The Moon rises around sunset and sets around sunrise.

Waning Gibbous

• The illuminated side begins to shrink as the Moon moves back toward the Sun.
• The Moon rises later each night.

Last Quarter

• Also known as the Third Quarter Moon.
• Half of the illuminated half of the Moon is visible.
• The Moon rises around midnight and sets around noon.

Waning Crescent

• Only a thin curve of light is visible as the Moon approaches the point where its dayside faces the Sun.

Lunar Libration

• Libration refers to the slight twisting, nodding, and rolling motions of the Moon.
• This allows us to see slightly more of the Moon's surface than we would otherwise.
• These motions include libration in longitude and libration in latitude.

Libration in Longitude

• The Moon's orbit is not perfectly circular, causing its distance from Earth and speed to vary.
• The Moon's rotation rate remains constant.
• When the Moon is closest to Earth and moving fastest, its rotation lags slightly, allowing us to see more of its eastern side.
• When the Moon is farthest from Earth and moving slowest, its rotation is slightly ahead, showing us more of its western side.

Libration in Latitude

• The 5-degree tilt of the Moon's orbit causes it to appear to nod.
• The tilt sometimes brings the Moon above Earth’s northern hemisphere and sometimes below Earth’s southern hemisphere.
• This allows us to see slightly more of the northern or southern hemispheres of the Moon.

Effects of Earth's Tilt

• The Moon appears to tilt back and forth due to the 23.5-degree tilt of Earth's axis.
• This tilt creates a changing perspective as we observe the Moon from different points in Earth's orbit. The observer's perspective changes every two weeks, causing the ground beneath their feet to tilt in the opposite direction.

Earthshine

• Earthshine is the dim light visible on the darkened portion of the Moon during its crescent phases.
• It is caused by sunlight reflecting off Earth's surface onto the Moon.
• Because Earth is nearly full from the moon's perspective, the reflected light is bright enough to illuminate the surface.

Daytime Moons

• The Moon is often visible during the day.
• The best times to see it are during the first and last quarter phases when it is about 90 degrees from the sun.
• The moon can be seen during the day at any phase, except for the new moon and the full moon.

Overview from Space

• From space, the Moon and Earth are seen locked together in a gravitational embrace.
• The Moon orbits Earth at an average distance of 238,855 miles (384,399 km). The distance is about 30 Earths.
  238,855 \text{ miles} = 384,399 \text{ km}
• The Moon orbits Earth every 27.322 days in an elliptical orbit, but the time from new moon to new moon is 29.5 days.
• The Moon is tidally locked with Earth, meaning it rotates on its axis once each time it orbits the planet.
• As a result, we only ever see one side of the Moon. The moon has synchronous rotation.