Earth’s Moon has quite the attitude, but don’t worry! It’s just going through a phase. Teenage astronomical objects can be unpredictable and very moody. The Moon is at least 4.53 billion years old, but that doesn’t stop it from having 8 unique phases, which means the Moon probably has a personality disorder. Try not to judge. We all go through phases, right? Some of us dye our hair unusual colors, while other people wear leopard skin pants.
- New Moon
- Waxing Crescent
- First Quarter
- Waxing Gibbous
- Full Moon
- Waning Gibbous
- Last Quarter
- Waning Crescent
Everyone is familiar with the gaudy crescent Moon phases, or the full Moon phase—that special time of the month when a special kind of person transforms into a werewolf.
…did you notice something strange?
There are two weird phases which occur after the first quarter: waxing gibbous and waning gibbous. Hmmm…what the hell is a “gibbous”? Let’s see what Webster-Merriam has to say:
1a : marked by convexity or swelling
b of the moon or a planet : seen with more than half but not all of the apparent disk illuminated
2 : having a hump
We don’t have to worry about swelling or humps (unfortunately). Let’s go ahead and concentrate on the middle definition, but we’ll modify Webster-Merriam’s complex choice of words.
3: more than half but less than full
If the Moon looks somewhat full, but it’s not totally full, then it’s most likely in a gibbous phase. Simple as that. Well…not really. There are two distinct gibbous phases: waxing and waning. The waxing gibbous phase occur before the full Moon phase, and the waning gibbous phase occurs after the full Moon phase. Simple as that.
So…how do we figure out if the Moon is in a waxing or waning gibbous phase? Let the terminator tell you. Calm down! We don’t have to recruit John Connor, or run away from a T-1000.
The terminator is the border between the night and days regions on the lunar surface (or any astronomical surface). If the dark region is located on the left side of the Moon’s disc, then it’s in a waxing gibbous phase. If the dark region is located toward the right side of the Moon’s disc, then it’s in a waning gibbous phase. See the Moon above this paragraph? Look really closely toward the Moon’s right limb —that slight dark shadow is the terminator, and it marks the beginning of a waning gibbous phase.
This principle also applies to both quarter phases, but the day and night side are equal.
The moon phase map shows the phases in context. You can easily see how both gibbous phases flank the full Moon phase. Both gibbous phases can exhibit varying degrees of visible lunar terrain.
The gibbous comparison photograph depicts how much the visible lunar terrain can vary. Both images represent a gibbous phase, but the photograph on the left has much less terrain illuminated by the Sun. The image on the right can potentially be confused with a full Moon phase, but the entire disc is technically not illuminated 100%.
The Moon goes through 8 distinct phases every 29 days! Imagine if you wore leopard skin pants one day, and then decided to dye your hair green a few weeks later. Yeah. Imagine all that, and then imagine repeating those lovely phases every month! Now you know what it feels like to be the Moon.
Wanna see a gibbous Moon? Click here and schedule a meeting!