What is a Gibbous Moon? When is the Best Time to Observe It?

Space, Science
What is a Gibbous Moon? When is the Best Time to Observe It?
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Waxing and Waning

Have you ever wondered what a gibbous moon is? The word itself comes from a root word that means ‘hump-backed’ and the term refers to one of the lunar phases, when the illuminated region is more than half but less than a full moon.

When it passes from the first quarter to a full phase, we have a waxing gibbous moon since the illuminated portion in constantly increasing (right side is 51–99% visible from the northern hemisphere). When it passes from a full phase to the last quarter, we have a waning gibbous moon; the illuminated part is constantly decreasing (left side is 51–99% visible from the northern hemisphere). The situation of left-hand and right-hand side is reversed for the southern hemisphere_._

Waxing or Increasing Gibbous Moon

Relative Positions of the Celestial Bodies

All lunar phases are the result of the relative positions of the Sun, the Earth and the Moon. When the three celestial objects are lined up perfectly and the Moon is in the middle, the angle between the latter and the Sun is 0-degrees. This is the New Moon phase. During the orbit the angle increases and as soon as it reaches a value between 90 and 180-degrees, we have a waxing gibbous moon. When this value is between 180 and 270-degrees_,_ we have a waning gibbous moon.

Explaining the Lunar Phase Sequence in a Few Words

The Eight Lunar Phases

During the orbit of the Moon around the Earth, we see various degrees of its surface illuminated. The whole phase cycle lasts 29.53 days and it is called a lunar month. The image shows the eight phases of the moon during this month from two different perspectives, a distant observer’s view and the Earth.

1. New Moon: Appears fully shadowed. During this phase, solar eclipses are likely to take place.

2. Waxing Crescent: Only a small edge of the Moon is illuminated (first sliver).

3. First Quarter: The Moon is half-illuminated, forming a 90-degree angle with the Sun.

4. Waxing Gibbous: Appears less than full but more than half lighted. The illuminated region is increasing day by day.

5. Full Moon: Appears fully illuminated. During this phase, lunar eclipses are likely to occur.

6. Waning Gibbous: Appears less than full but more than half lighted. The illuminated region is decreasing day by day.

7. Last Quarter: Appears half illuminated.

8. Waning Crescent: Only a small edge is illuminated (final sliver), before the Moon goes into darkness again.

Click here to see an animation of the lunar phase sequence, as seen from the Northern Hemisphere.

When Do We Observe Them?

So, when can we see the two phases? A waning gibbous moon can be spotted near the eastern horizon, as it rises some time after sunset and before midnight. When it is close to the horizon it has a glowing red color and it resembles a full moon. Since it rises late at night, it has to set some time after sunrise, making it visible in daytime, only early in the morning. The standard time of culmination at mid-phase is 3 am.

A waxing gibbous moon rises some time between noon and sunset and sets after midnight. It can be seen in the afternoon high in the east sky before the sun sets. The standard time of culmination at mid-phase is 9 pm.

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