The changing phases of the moon
View of the full moon from North Hill at 2:55 a.m. on Sept. 3.
(Photo courtesy DON BARTLING)
| September 10, 2020 1:00 AM
“When the moon is not full, the stars shine more brightly." (Ancient proverb)
Most people have a passing familiarity with the phases of the moon. Some nights you look up into the sky and see a bright Full Moon like last Wednesday night. On the other nights you can only see a tiny crescent of light and on other occasions the Moon seems to be half there or three quarters there. The Moon phases are the different ways in which we see the Moon from here on earth and they follow a regular cycle every 29.5 days.
Have you ever looked up at the sky and noticed how the Moon seems to change shape each night? Well, it doesn’t ‘change shape’ at all. The Moon doesn’t emit light itself, the ‘moonlight’ we actually see is the Sun’s light reflected off the lunar surface. As the Moon orbits the Earth, the Sun lights up different parts of it, making it seem as if the Moon is changing shape, but it just our view of the moon that is altering.
Just like the Earth, the Moon is a sphere, and it is always half-illuminated by the Sun. However, from here on Earth, we see it as not illuminated at all, fully illuminated or partly illuminated, because the Moon is orbiting the earth. This means that the proportion of the half illuminated lunar surface we can see varies from day to day throughout the lunar month. The apparent changes in illumination, seen from earth, are what constitute the Moon phases.
There are eight lunar phases in total. In order, they proceed as follows: New Moon, Crescent Moon (also called Waxing Crescent Moon), First Quarter Moon, Gibbous Moon (also called Waxing Gibbous Moon), Full Moon, Waning Gibbous Moon, Third Quarter Moon (also called Last Quarter Moon), and the eighth phase of the moon is the Waning Crescent Moon.
A New Moon occurs when the Moon lies directly between the earth and the Sun, so no direct sunlight reaches the part of the Moon that we can see.
During the Crescent Moon, we see a crescent sliver of the Moon illuminated, on the right-hand side of the Moon as you view it.
The First Quarter Moon actually looks like a half-Moon, as half of the side we can see is illuminated.
The Gibbous Moon looks as though the right-hand three quarters of the Moon is illuminated, as seen from earth.
The Full Moon is when the Moon’s disc appears to be fully illuminated.
In the Waning Gibbous Moon, the left hand three-quarters of the Moon’s visible disc in the Northern hemisphere (and the opposite in the Southern hemisphere), is illuminated.
During the Third Quarter Moon, half of the Moon’s disc is illuminated.
In the phase of the Waning Crescent Moon, we can see a thin crescent illuminated on the Moon, on the left-hand side as we view it. And then finally the New Moon phase beginning the cycle all over again.
Here is a quick and easy chart provided by National Geographic to the 8 different Moon phases. Each Moon phase last for approximately 3.5 days.