Short answer: we speak of a full moon when, in order, the Moon, Earth and Sun are aligned. The face presented by the Moon, visible from the Earth, is then completely illuminated by the Sun’s rays.
Moon, Earth and Sun aligned…but not perfectly
The order of appearance of the stars is indeed important to observe a full Moon but their alignment must not be exact. Otherwise, we will have to deal with a lunar eclipse, i.e. the Moon will be in the shadow of the Earth and will therefore not be illuminated by the Earth. If, as shown in the image below, the Earth, the Moon and the Sun are aligned in this order, then we will have to deal with a new Moon, i.e. a Moon whose side illuminated by the Sun is not visible from the Earth.
Seen from Earth, it is during the full moons that our natural satellite appears the brightest. But beware, contrary to what the common man might think, a full Moon lasts only a few minutes because the 3 stars involved in staging this event are in constant motion: the Moon is in orbit around the Earth while the Earth rotates around the Sun. Today, we can announce very precisely, and for a whole calendar year, the exact time of appearance of a full Moon by geographical area. (see this article for the lunar phases of the year 2020). Note that the interval between two moons is precisely 29.53 days.
Every full moon has a name
Many cultures felt the need to humanize the Moon by giving it a distinct name per month/season. Whether New England tribes, European settlers, Native Americans, Chinese or Celts, there is generally a common point in the choice of names given to the full Moons over a calendar year: the seasonal marker which is different depending on our location. The seasons are effectively reversed if we are located in the northern hemisphere or the southern hemisphere (but you already know that :)). In June, the name “Cold Moon” is used in the southern hemisphere and in December in the northern hemisphere.
Example of names given to the full moon per month in the northern hemisphere (if you are interested, here is a complete list by culture here) :
|Wolf Moon, Old Moon, Moon After Yule
|Snow Moon, Hunger Moon, Wolf Moon
|Sap Moon, Crow Moon, Lenten Moon
|Grass Moon, Egg Moon, Pink Moon
|Flower Moon, Planting Moon, Milk Moon
|Rose Moon, Flower Moon, Strawberry Moon
|Thunder Moon, Hay Moon
|Green Corn Moon, Grain Moon
|Fruit Moon, Harvest Moon
|Harvest Moon, Hunter’s Moon
|Hunter’s Moon, Frosty Moon, Beaver Moon
|Cold Moon, Moon Before Yule, Long Night Moon