All About The Moon

Why is the moon visible at some times but not at others? Why is it always changing? We have explanations for these and other questions you might have asked about our nearest neighbor in the Solar System.

Full Moon with Background of Stars

The Basics

For all of recorded history, people have kept track of time based on the cycles of the Sun and the Moon, for the obvious reason that they are the two biggest and brightest objects in the sky and they both exhibit regular patterns that correspond with and even determine cycles in human behavior. (For an agricultural society to be successfully established, for instance, there is a fundamental need to able to reliably predict when crops should be planted and harvested.) Each of the fundamental units of time observed and shared in common throughout most societies – days, weeks, months, and years – has its basis either in some aspect of the solar cycle or the lunar cycle, or a combination of both. (Our English word “month,” is related to our word “moon” – think “moonth.” “Solar” and “lunar” are taken from the Latin words for Sun and Moon, respectively – more on the ancient Romans’ contributions to our calendar below.)

A difficulty arises, however, from the fact that the solar and lunar cycles are not in phase with one another. The monthly lunar cycle is not neatly divisible by a whole number of days. The solar year is likewise not divisible by a whole number of days (hence the need for a Leap Year every four years), but it is also not neatly divisible by a whole number of lunar cycles. The number of complete lunar cycles that occur within each solar year is between 12 and 13 (12.36826623..., to be more precise). Therefore, since precise uniformity between the two isn’t possible, this means that societies wind up having to choose which cycle, solar or lunar, they will ultimately give preference to in their reckoning of time. Most very ancient societies followed a lunar calendar. The Egyptians, being Sun worshippers, were fairly unique in that they followed a solar calendar. The ancient Romans at first observed a lunar calendar, but they eventually saw advantages to a solar calendar (after adding Egypt to their Empire) and made the switch in the first century B.C. That set a precedent for the entire western world thereafter: we still follow to this day, with slight adjustments that have been subsequently imposed, essentially the same calendar that Julius Caesar decreed for observance throughout the Roman Empire in 45 B.C.

"Each of the fundamental units of time observed and shared in common throughout most societies – days, weeks, months, and years – has its basis either in some aspect of the solar cycle or the lunar cycle, or a combination of both."

With a solar calendar, the concept of a month becomes more of an arbitrary thing: the solar year is divided neatly into 12 months of around 30 days each (some slightly more or less, to make things fit), but the Moon then has to be “set free” to keep doing its thing on its own timetable. Certain key events in the Moon’s more-or-less monthly cycle (New Moon, First Quarter, Full Moon, and Last Quarter) do not fall at regularly predictable times within each calendar month under a solar calendar, and have to be kept track of independently.

The roughly 29.5 day lunar cycle corresponds to the amount of time it takes for the Moon to accomplish one complete orbit around the Earth and return to the same position relative to the Earth and the Sun. (And, it should be remembered, this happens while the Earth and Moon are, together as one unit, completing their year-long orbit around the Sun.) Since the Moon is not lit by its own light but merely reflects light from the Sun, this means that, from Earth’s vantage point, as the Moon travels around us throughout its monthly journey, we sometimes see more, sometimes less of its surface illuminated by the Sun’s light. (Also, the Moon is locked in its orbit such that the same side is always facing the Earth. The far side of the Moon isn’t “dark” in the literal sense; it receives as much light throughout the course of a month as the side nearer the Earth does. But it is “dark” in the sense of being unknown to us, at least until spacecraft were first sent to view and map its far side, beginning in the 1960s.)

A 7 day week corresponds to approximately one quarter of the lunar cycle. Thus New Moons and Full Moons are always about 2 weeks apart. The following table accounts for the entire cycle:

New Moon – The Moon is effectively invisible from our position on Earth, being positioned at that time between Earth and Sun, lost in the Sun’s glare and with its shadowy side turned fully toward the Earth. Solar Eclipses, when they occur, happen during the New Moon, as explained more fully HERE.
Waxing Crescent* – Less than half of the Moon’s surface appears lit from our earthly vantage point, with the illumined portion continuing to grow larger day-by-day.
First Quarter (about 1 week after New Moon) – The Moon has a Half-Moon appearance (with lit side facing west, in the middle of the sky around sunset).
Waxing GibbousMore than half of Moon’s surface appears lit from our earthly vantage point, with the illumined portion continuing to grow larger day-by-day.
Full Moon (about 2 weeks after New Moon) – The Moon’s surface appears as fully lit, fully circular in appearance. Lunar Eclipses, when they occur, happen during the Full Moon, as explained more fully HERE.
Waning GibbousMore than half of Moon’s surface appears lit from our earthly vantage point, with the illumined portion continuing to shrink day-by-day.
Last Quarter (about 3 weeks after New Moon) – The Moon has a Half-Moon appearance (with lit side facing east, in the middle of the sky around sunrise).
Waning Crescent* – Less than half of the Moon’s surface appears lit from our earthly vantage point, with the illumined portion continuing to shrink day-by-day.

19514941-g.jpg

Blue Moons, Blood Moons, and Supermoons

A Blue Moon really has nothing to do with the moon’s apparent color: a Blue Moon looks no more (and no less) blue than it does at any other time (which is to say, not very much). A Blue Moon refers (in modern usage) to the occurrence of two full moons within the same calendar month, which happens, on average about once every 2.75 years (every 33 months). When a full moon falls very early in a given calendar month, then it is possible that the next full moon will occur (about 29.5 days later) before that same calendar month ends. When this does happen, the second full moon in that calendar month is called a Blue Moon. Obviously, months with 31 days have a better chance of hosting a Blue Moon, but they can occur in any month except February (which is only 29 days long at its longest, and that only every four years).

"A Supermoon is simply a full moon that appears slightly larger than a typical Full Moon and occurs when the Full Moon also happens to be at or near its perigee, which is the point of its elliptical orbit when it is closest to the Earth."

A Blood Moon is simply another name for a Total Lunar Eclipse (explained more fully HERE), which has a distinctly reddish appearance due to the fact that the only sunlight striking it is filtered through Earth’s atmosphere, like a ring of sunrises and sunsets.

So-called Supermoons have received increasing attention in recent years. A Supermoon is simply a Full Moon that appears slightly larger than a typical full moon and occurs when the full moon also happens to be at or near its perigee, which is the point of its elliptical orbit when it is closest to the Earth.

Full Moons by the Month

Various cultures have assigned names to specific lunar cycles throughout the year, associated most particularly with the Full Moon that occurs within that given cycle. Among these, a somewhat standardized list has been developed within the last 100 years or so, which seems to incorporate names handed down from both New World (Native American) and Old World (German and Anglo-Saxon) sources. The loosely-accepted official naming scheme runs as follows:

January – Wolf Moon
February – Snow Moon
March – Worm Moon
April – Pink Moon
May – Flower Moon
June – Strawberry Moon
July – Buck Moon
August – Sturgeon Moon
September/October – Harvest Moon (The Full Moon occurring closest to the Autumnal Equinox, around September 22, is designated as the Harvest Moon, whether it falls in September or October)
September (if not designated as Harvest Moon) – Corn Moon
October (if not designated as Harvest Moon) – Hunter’s Moon
November – Beaver Moon (but it is the Hunter’s Moon if the Harvest Moon falls in October)
December – Cold Moon

Other Assorted Moon Facts

  • The Moon’s average diameter is 2,159 miles. That’s about the same as the distance from New York City to Phoenix, Arizona. If the earth were the size of a basketball, the moon would be about the size of a baseball.
  • Most other planets in the solar system which have moons dwarf their satellite companions. The Earth and Moon, by comparison, are fairly close in size. For this reason, some scientists like to think of theEarth and Moon as a double planetary system.
  • The color of the Moon’s surface is, on average, about the same as a not-too-recently paved asphalt parking lot. And, believe it or not, the Earth’s surface actually reflects considerably more sunlight than does the Moon’s. The brilliant silvery-white appearance which the Moon has is simply the result of its contrast the the surrounding blackness of space.
  • The pull of gravity on the surface of the Moon is only about 1/6th of what is felt on Earth.
  • The average distance from the Earth to the Moon is almost 240,000 miles. Using the above analogy, the baseball-sized Moon would be about about 28 feet away from the basketball-sized Earth.
  • As the Moon orbits the Earth, it travels at a rate of about .64 miles a second!

* Although in modern English the word “crescent” refers strictly to the shape, the Latin verb crescere, which is its source, actually means “to increase” or “to grow larger.” This makes the term “waxing crescent” something of a redundancy and the term “waning crescent” something of an oxymoron.

Related Stories

LATEST STORIES BY THIS AUTHOR:

In celebration of Hank’s 100th birthday, we’ve attempted to distill his mammoth legacy down to the ten most influential songs.
On this date in 1287, one of the largest floods and greatest natural disasters on record permanently altered the landscape of the Netherlands and changed the course of history.
One hundred and fifty years ago on this date (December 5, 1872) a derelict ship was found adrift in the Atlantic Ocean, launching one of the most enduring – and still unsolved – maritime mysteries.