What is a Gibbous Moon? Both Phases Explained

A Gibbous Moon in the night sky.

What is a Gibbous Moon? It is a phase of the Moon in a lunar month when the Sun’s light illuminates more than half (50.1%) of the lunar surface, up to (99.9%) not quite a Full Moon, as seen from Earth. There are two gibbous phases.

  • The waxing gibbous phase comes before the waning phase starting at the end of the first quarter Moon and increasing in brightness over approximately seven days until it becomes a Full Moon.
  • The waning gibbous phase begins at the end of the Full Moon as the light decreases over an approximate seven-day period becoming a third-quarter Moon.

There are many interesting details about the Gibbous Moon, which I will cover further in this article. Please join me!

Before we jump into the waxing and waning Gibbous Moon, lets go over some important details regarding the phases of the Moon to better understand the gibbous phase.

THE PHASES OF THE MOON

The Moon is approximately 237,265 miles (382,500 km) from Earth, but due to their orbits and relative positions, this distance will vary from day to day.

What Causes The Phases Of The Moon?

As the Earth journeys around the Sun, the Moon continues its orbit around the Earth. Their alignment, relative to one another, determines what percentage of the Moon, as seen from Earth, is illuminated by the Sun’s light and how much of the Earth’s shadow is visible or not on the lunar surface. This lunar cycle is known as the phases of the Moon.

How Many Days Does It Take The Moon To Orbit The Earth?

The amount of time it takes the Moon to orbit once around the Earth depends on which approach is used to calculate the answer. One method is the sidereal period.

It calculates the time by using distant stars as fixed points of reference. Once the Moon has returned to its original position from where it began, against a background of stars as seen from Earth, this completes the revolution. When using this method, it takes the Moon 27.3 days to to orbit once around the Earth.

The second approach is using the Moon’s original position relative to the Sun as seen from Earth. This method, known as the synodic period, requires an additional 2.2 days as the Moon must catch up to the Earth as it is also moving, reaching its beginning position relative to the Sun. This complete lunar cycle is the time it takes the Moon to transition through its eight phases of the synodic month, 29.5 days.

An illustration of the four primary phases of the Moon in their order of appearance (New Moon, First Quarter, Full Moon and Third Quarter).

Western culture divides the lunar phases into two categories a primary and intermediate moon phase. The primary phases (New Moon, First Quarter Moon, Third Quarter Moon, Full Moon) are most often what is noted on a lunar calendar, journals, almanacs, and other publications.

The Eight Phases Of The Lunar Month

An illustration of all eight phases of the Moon, primary and intermediate. The waxing and waning gibbous phases are highlighted in yellow text to help illustrate what is a gibbous moon.
It takes 29.5 days to complete the eight phases of the Moon (Synodic Period) – The waning and waxing Gibbous Phases are both highlighted.
  • New Moon: (Primary Phase) The Moon is between the Sun and Earth. The Moon’s bright side faces the Sun, and the dark side faces Earth. The Moon is not visible in the day or night sky during this phase.
  • Waxing Crescent Moon: (Intermediate Phase) As the Earth begins to move out of alignment between the Sun and Moon, the Sun casts a sliver of light onto the lunar surface. Light continues to grow on the Moon’s surface nightly as it approaches the first quarter phase.
  • First Quarter Moon: (Primary Phase) The Moon is at a 90-degree angle of the Sun as observed from Earth and has reached 50% illumination, appearing as a half moon. It will soon move into the Gibbous Moon phase.
  • Waxing Gibbous Moon: (Intermediate Phase) Once the Moon exceeds a 90-degree angle with the Sun, it becomes over half (50.1 %) illuminated. It will continue to get brighter each night until it reaches the Full Moon phase.
  • Full Moon: (Primary Phase) The Moon is fully illuminated in its brightest phase. As it peaks, it will move into the waning gibbous phase.
  • Waning Gibbous Moon: (Intermediate Phase) The full Moon has passed, and its illumination will continue to decrease each night until the last quarter phase.
  • Third Quarter Moon: (Primary Phase) Also known as a Last Quarter Moon, the Moon is now only 50% illuminated and will continue to decrease nightly as it approaches the waning crescent phase.
  • Waning Crescent Moon: (Intermediate Phase) The Earth is almost once again in perfect alignment, allowing just a sliver of the Sun’s light to shine on the lunar surface as it approaches the New Moon phase.

Perfectly Aligned

The same principles that create the lunar phases (the constant changing of the Earth and the Moon’s relative position to the Sun) occasionally produce other magnificent events.

Lunar Eclipse

When the Sun, Earth, and Moon perfectly align so that the Moon passes into the Earth’s Shadow, this creates an event known as a lunar eclipse.

During a total lunar eclipse, the entire disk of the Moon passes through the darkest area of Earth’s shadow, the umbra. As a result, the Moon’s appearance turns into a reddish-brown hue, often called a “Blood Moon.

Interestingly, if you were on the Moon during a lunar eclipse, it would appear on the Moon as a solar eclipse. This is because the Earth would be blocking your view of the Sun.

An image of a total lunar eclipse appearing as a "Blood Moon" with reddish color in the night sky.
Total Lunar Eclipse

Solar Eclipse

A solar eclipse occurs as the New Moon passes between the Earth and the Sun, obstructing the view of the Sun, partially or totally from an area of the Earth.

The annular type of eclipse happens when the lunar disk is not wide enough to block the entirety of the Sun. This is due to the Moon’s distance from the Earth at the time of the eclipse.

Observing The Gibbous Moon

The Gibbous Moon is not as familiar to many as the primary phases of the lunar cycle, but Moon enthusiasts know it’s a perfect time to view the lunar surface with the high magnification of a telescope or binoculars. During the two weeks through the waxing and waning Moon phase, it offers just the right amount of illumination, making this time optimal for observing the Moon. Not too bright and not too dim.

As the light is waxing (growing) or waning (decreasing) over the face of the Gibbous Moon, there is a dividing line between dark and light called the “terminator“. This line of night and day creates an effect of depth similar to 3D, which is quite spectacular when viewing. Large and small craters cast shadows along the terminator, offering the observer a detailed glimpse of the Moon’s surface.

An image of a waning gibbous Moon peaking over trees in a daytime sky.
The waning Gibbous Moon appearing in a morning sky.
A clear blue sky day revealing a waxing gibbous Moon.
The clear day-light view of a waxing Gibbous Moon.

Is it the same phase of the Moon in the Southern vs. Northern Hemisphere?

Yes, the phase of the Moon is the same in both hemispheres at the same time but features appear inverted. For example, the shadow of a waxing Gibbous Moon appears on the left side of the lunar disk when observed in the northern hemisphere, but in the southern, it appears on its opposite side, the right.

The orientation or placement of the Moon while observing it in the sky depends on your location, the time, and the date.

The Gibbous Moon in Astrology

The phases of the Moon have been a integral part of human culture since eyes began to search the night sky. How these phases could impact a harvest or a destiny has long been pondered and developed into astrology, bridging people’s psyche from an ancient past into the modern minds of today.

In astrology, it is thought people born during a Gibbous Moon have a strong desire to seek knowledge, learn and understand—a journey through life to be complete or as good as they can be.

A waxing phase of the Gibbous Moon is said to be a period of growth, a transitional time, while the waning phase can represent the consequences of actions taken from decisions made.

Does the Gibbous Moon Affect Nature?

The Moon does affect nature through light, tides, and time. There is a lunar clock, a biorhythm called circalunar rhythm which affects certain organisms according to the changing levels of Moonlight and ocean tides, much like the circadian rhythms of the day and night.

An example would be how coral of the great barrier reef times its spawning soon after a full Moon during the waning gibbous phase between October and December. The circalunar rhythm works in tandem with other environmental triggers and cycles.

Frequently Asked Questions

Why is a Moon called Gibbous?

The term “gibbous” is derived from the Latin noun “gibbus“, meaning “hump.” It has been adapted into English as an adjective when describing the exterior curved or rounded edges of a sphere.

When used to describe the Moon, it implies the Moon is more than half illuminated by the Sun’s light but has yet to be a full Moon.

How Does a Waxing Gibbous Moon affect Us?

Our natural environment is affected by the light and gravitational pull of the Moon. As an example, during the waxing phase of a Gibbous Moon, its light increases each night, allowing us to see better whether walking or driving. It is the phase leading to the Moon’s brightest period, a full Moon. Its gravitational pull also impacts nature and, most evidently, our oceans.

The Sun works against the pull of the Moon during quarter phases resulting in Neap Tides, which equalizes the effect of a low or high tide. So after the first quarter, entering a waxing gibbous, the tides will increase in strength to lower lows and higher highs as the full Moon approaches.

Some also believe that the Moon’s phases affect us spiritually through energy. A few spiritual attributes associated with the waxing phase are reevaluating one’s direction and trying new actions, redirecting to achieve one’s goals.

How Does a Waning Gibbous Moon affect Us?

The waning gibbous phase also affects our natural environment, like the waxing but with different results. As the full Moon has passed, entering into the waning gibbous, the light of the Moon is now decreasing each night. The night sky becomes darker, affecting how well we can see if out walking or driving.

The gravitational pull has peaked with the full Moon, and tidal strength continues to decrease during the waning gibbous as the last quarter phase approaches.

For some people, the Moon’s phases are taken into account while considering spiritual and self-evaluation. The waning phase is a new time to reflect with spiritual attributes of enthusiasm, gratitude, and generosity as the benefits of earlier work are harvested. A feeling of giving back is said to enter one’s being.

A Cityastronomy logo of a cityscape under a starry night with the Cityastronomy name worded across the circular picture.
Written by Darin Anthony

Astronomy has peaked my curiosity and imagination from an early age. I am always thrilled to read about the latest galactic discovery or planning my next celestial observation. More about me [..]

You May Also Like

The article you just read is part of the City Astronomy “Astronomy” collection. If you enjoyed it we think you'll love these other recommendations:

A view of Las Vegas from afar as stars are visible in the night sky.

Discover spots only minutes from the Strip to Dark Sky Destinations with thousands of shimmering stars.

A shooting star racing across the sky at night over a forest canopy

In this article we explain what a Shooting star is and where they come from.

A picture of six images of the Moon in different colors.

Find out what color the Moon REALLY is and why it can look different from night to night.