A lunar month takes 29.53 days. This is the amount of time it takes for the Moon to complete a cycle through all the phases, from new moon to full moon and then back to new moon again. This is very close to the length of a month in the Western calendar, which usually have 30 or 31 days. Every month usually has one of each of the phases. So a typical month will have a new moon, first quarter, full moon and last quarter moon. But every now and then, a month will have two of the same phases. When a month has two full moons, the second one is called a “blue moon”.
Blue moons are rare, and that’s where the phrase comes from, “once in a blue moon”. How rare? They always happen once every 2-3 years (2.72 years, to be exact). It’s this second moon in the month that’s considered the blue moon.
Does the blue moon actually turn blue? No. A blue moon is exactly the same color as a regular full moon – yellow.
The Moon can turn blue when there’s a certain amount of dust or pollution in the air. The extra dust scatters blue light, making the Moon appear more blue. For example, the Moon appeared blue across the entire Earth for about 2 years after the eruption of Krakatoa in 1883.
Here is a list of upcoming blue moons:
2018: January 2, January 31st
2018: March 2, March 31st
2020: October 1, October 31st
We have an article here on Universe Today about all the phases of the Moon.
The moon in its waning gibbous phase on Sept. 12, 2014. Photo taken with a Canon 700D attached to a Maksutov 127mm telescope. Credit: Sarah&Simon Fisher
Human beings have been observing the Moon for as long as they have walked the Earth. Throughout recorded and pre-recorded history, they have paid close attention to its phases and accorded them particular significance. This has played a major role in shaping the mythological and astrological traditions of every known culture.
With the birth of astronomy as a scientific discipline, how the Moon appears in the night sky (and sometimes during the day) has also gone long way towards helping us to understand how our Solar System works. It all comes down to the Lunar Cycle, the two key parts of this cycle involve the “waxing and waning” of the Moon. But what exactly does this mean?-day
Lunar Cycle:
First, we need to consider the orbital parameters of the Earth’s only satellite. For starters, since the Moon orbits Earth, and Earth orbits the Sun, the Moon is always half illuminated by the latter. But from our perspective here on Earth, which part of the Moon is illuminated – and the amount to which it is illuminated – changes over time.
When the Sun, the Moon and Earth are perfectly lined up, the angle between the Sun and the Moon is 0-degrees. At this point, the side of the Moon facing the Sun is fully illuminated, and the side facing the Earth is enshrouded in darkness. We call this a New Moon.
After this, the phase of the Moon changes, because the angle between the Moon and the Sun is increasing from our perspective. A week after a New Moon, and the Moon and Sun are separated by 90-degrees, which effects what we will see. And then, when the Moon and Sun are on opposite sides of the Earth, they’re at 180-degrees – which corresponds to a Full Moon.
Waxing vs. Waning:
The period in which a Moon will go from a New Moon to a Full Moon and back again is known as “Lunar Month”. One of these lasts 28 days, and encompasses what are known as “waxing” and “waning” Moons. During the former period, the Moon brightens and its angle relative to the Sun and Earth increases.
Synthetic view of the waxing Moon as viewed from Earth on 2013-10-15 17:00:00 UTC. Credit: NASA/GSFC/Arizona State UniversityWhen the Moon starts to decrease its angle again, going from 180-degrees back down to 0-degrees, astronomers say that it’s a waning moon. In other words, when the Moon is waning, it will have less and less illumination every night until it’s a New Moon.
Waning Phases:
When the Moon is no longer full, but it hasn’t reached a quarter moon – i.e. when it’s half illuminated from our perspective – we say that it’s a Waning Gibbous Moon. This is the exact reverse of a Waxing Gibbous Moon, when the Moon is increasing in brightness from a New Moon to a Full Moon.
This is followed by a Third Quarter (or last quarter) Moon. During this period, 50% of the Moon’s disc will be illuminated (left side in the northern hemisphere, and the right in the southern), which is the opposite of how it would appear during a First Quarter. These phases are often referred to as a “Half Moon”, since half the disc is illuminated at the time.
The moon in its waning gibbous phase on Sept. 12, 2014. Photo taken with a Canon 700D attached to a Maksutov 127mm telescope. Credit: Sarah&Simon Fisher
Finally, a Waning Crescent is when the Moon appears as a sliver in the night sky, where between 49–1% of one side is illuminated after a Full Moon (again, left in the northern hemisphere, right in the southern). This is the opposite of a Waxing Crescent, when 1-49% of the other wide is illuminated before it reaches a Full Moon.
Even today, thousands of years later, human beings still look up at the Moon and are inspired by what they see. Not only have we explored Earth’s only satellite with robotic missions, but even crewed missions have been there and taken samples directly from the surface. And yet, it still possesses enough mystery to keep us inspired and guessing.
![Synthetic view of the waxing Moon as viewed from Earth on 2013-10-15 17:00:00 UTC [NASA/GSFC/Arizona State University].](https://www.universetoday.com/wp-content/uploads/2013/10/tuesday_syntheticview_thumb.png)
