An easy trick to remember what moon phase the moon is in on any given night is the mnemonic DOC. DOC, (or COD if you’re in the southern hemisphere), reflects the amount of the moon illuminated, which you can use to discern the current phase the moon is in.
DOC represents the half moon, the full moon, and the crescent moon, and whether or not the moon is waxing or waning.
Defining The Phases Of The Moon
Phases Of The Moon:
- New Moon
- Waxing Crescent
- First Quarter
- Waxing Gibbous
- Full Moon
- Waning Gibbous
- Last Quarter
- Waning Crescent
Though it’s often thought that the phases of the moon are caused by the Earth’s shadow being cast on the surface of the moon, this actually doesn’t happen very often and is known as lunar eclipse when it does. The phases of the moon are actually caused by shifts in the moon’s position relative to the sun. When part of the moon appears dark, this is because this portion of the moon is turned away from the sun. At least half of the moon is always in shadow, and half is illuminated. Yet because the moon always faces the Earth the same way (this is called being tidally locked), we perceive the shift in illumination as changing phases.
A New Moon occurs when the side of the moon that isn’t illuminated faces the Earth. None of the moon is light up, and as such it isn’t visible at all, unless a solar eclipse is happening.
The moon is said to be “waxing” when more of its surface is being illuminated. As such, a waxing crescent moon is when the moon is only partially illuminated by the sun. A crescent moon means that less then half the moon illuminated, so it’s between 1% to 49% illuminated.
A First Quarter moon is when one half of the moon’s visible surface is illuminated. The First Quarter happens during the waxing period when the percentage of the moon’s surface that is illuminated is increasing.
A Waxing Gibbous moon is when the visible surface of the moon is more than half illuminated, but not fully illuminated. When the portion of the moon illuminated is between 51% to 99%, and the percentage of the illuminated moon in increasing, it’s a Waxing Gibbous moon.
The Full Moon happens when the illuminated portion of the moon is directly facing the Earth, and as such the entire disc of the moon appears to be lit up.
A Waning Gibbous Moon occurs when more than one half, yet less than the whole moon’s visible surface, is illuminated by the sun. The percentage of the moon’s illuminated surface is decreasing during this time and is between 99% to 51%.
The Last Quarter Moon occurs when only one half of the moon is illuminated by the sun, and when the amount of illuminated surface is decreasing.
A Waning Crescent moon occurs when there’s less than one-half of the moon illuminated, and when the percentage of the moon illuminated is decreasing. The percentage of the moon illuminated during a waning crescent is between 49% to 0%.
The cycle then resets with the New Moon. The cycle takes about 29.5 days to complete or to reach the same visual phase. Meanwhile, it takes the moon takes around 27.4 days to revolve around the Earth. These two different lunar months are referred to as the synodic month and the sidereal month respectively. A lunation is a complete cycle of lunar phases.
Due to the fact that the amount of time it takes the moon to complete a cycle is shorter than most months of the year, the phase the moon is in at the beginning of the month often repeats itself at the month’s end. If there are two full moons in a month, it’s called a Blue Moon.
Using The DOC Mnemonic
The DOC mnemonic is a handy tool for remembering which part of the moon is light up, and hence which part of the cycle the moon is currently in.
The “D” manifests itself on the moon as a lit up arch on the right side of the moon, with a straight line down the moon’s center, where it transitions into darkness. If the illuminated portion of the moon makes a D, then the lunar cycle is just starting (D is the first letter in DOC). While half the moon will be illuminated twice during a full cycle of moon phases, only when the moon is waxing will it resemble a letter D, with the illuminated portion on the right-hand side.
The letter “O” in DOC is representative of the full moon, naturally. You may be able to guess that the “C” in DOC represents the crescent moon. Remember that although the moon adopts a crescent shape at two points in the cycle, only the waning crescent moon (coming at the end of DOC) will resemble a letter C. If the crescent moon resembles a C, a new moon will soon arrive.
Remember that if you’re in the southern hemisphere, you’ll want to reverse the mnemonic device and use COD instead.
Moon Phases And The Tides
You should also know that the phases of the moon are somewhat correlated with the intensity of the tides. Tides are at their most extreme during new moons and full moons. During these two time periods, the normal tides will become more severe, creating very high tides and very low tides. This is because when the moon is full, the moon and sun are pretty much in straight lines on either side of the Earth. The combined gravitational forces of the Sun and Moon create slightly larger tidal bulges than normal. The Sun and Moon are in a straight line on the same side of the planet at the New Moon, also cresting larger tidal bulges.
By contrast, the tides of the Earth are at their least extreme when the moon is at one of the two-quarter phases. This is because it is essentially creating a 90-degree angle with the Sun, and the gravitational forces from the Sun and Moon are diminished since they are acting at perpendicular angles.