Refers to the portion of the near side of the moon which is lit.

As the moon travels around the earth, it keeps the same side toward the earth at all times, and as a result, this side is called the "near side". The other side is referred to as the far side, or sometimes erroneously as the dark side of the moon. The lit portion of the moon changes over time, making a complete revolution in roughly the same time the moon makes a revolution around the earth, or about 29 days.

Since ancient times people have noticed this cycle in the appearance of the moon from earth, and the cycle is divided into several phases.

When the moon is closest to being directly between the sun and the earth, the entire near side of the moon becomes dark, and this phase is called the new moon. (If the moon passes directly between the sun and earth, it leaves a shadow on the earth, called a solar eclipse.)

When the earth is closest to being directly between the sun and moon, the entire near side is lit, and this phase is called a full moon. (Again, if earth passes directly between the sun and moon, it leaves a shadow on the moon, causing the full moon to become dim for a short while. This is called a lunar eclipse.)

There are various other names for the phases in between; see waning, waxing, quarter, gibbous, etc.

phase = P = phase-wrapping

phase of the moon n.

Used humorously as a random parameter on which something is said to depend. Sometimes implies unreliability of whatever is dependent, or that reliability seems to be dependent on conditions nobody has been able to determine. "This feature depends on having the channel open in mumble mode, having the foo switch set, and on the phase of the moon." See also heisenbug.

True story: Once upon a time there was a program bug that really did depend on the phase of the moon. There was a little subroutine that had traditionally been used in various programs at MIT to calculate an approximation to the moon's true phase. GLS incorporated this routine into a LISP program that, when it wrote out a file, would print a timestamp line almost 80 characters long. Very occasionally the first line of the message would be too long and would overflow onto the next line, and when the file was later read back in the program would barf. The length of the first line depended on both the precise date and time and the length of the phase specification when the timestamp was printed, and so the bug literally depended on the phase of the moon!

The first paper edition of the Jargon File (Steele-1983) included an example of one of the timestamp lines that exhibited this bug, but the typesetter `corrected' it. This has since been described as the phase-of-the-moon-bug bug.

However, beware of assumptions. A few years ago, engineers of CERN (European Center for Nuclear Research) were baffled by some errors in experiments conducted with the LEP particle accelerator. As the formidable amount of data generated by such devices is heavily processed by computers before being seen by humans, many people suggested the software was somehow sensitive to the phase of the moon. A few desperate engineers discovered the truth; the error turned out to be the result of a tiny change in the geometry of the 27km circumference ring, physically caused by the deformation of the Earth by the passage of the Moon! This story has entered physics folklore as a Newtonian vengeance on particle physics and as an example of the relevance of the simplest and oldest physical laws to the most modern science.

--The Jargon File version 4.3.1, ed. ESR, autonoded by rescdsk.

In western and eastern astrology, the new moon is significant. It has been a tradition for the Chinese to start or begin things during the new moon phase. Most chinese business or undertakings are done during this phase. The Chinese new year always starts on a new moon.

The same thing goes for western astrology concerning the new moon. It has been said that the full moon would finish or add significance to what was started during the previous new moon phase.

How to tell if the moon is waxing or waning

A waxing moon--that is to say a moon which is between new moon and full moon--will apparently grow larger night by night until it reaches full moon.

A waning moon--between full and new--will seem to decline (or wane) each night until it reaches the new moon.

But how can you tell whether the half or quarter moon is waxing or waning?

Cultural imperialism alert: This only works in the Northern hemisphere. Anyone south of the equator needs to swap hands. But what happens on the equator?

Answer: At the poles the straight bit of the half moon appears vertical, whereas at the equator, the straight edge is horizontal--or parallel with the horizon. The moon kinda seems to tilt over more and more as you travel closer to the equator. The bright bit of the moon is always near the horizon. So on the equator, the waxing and waning moons appear the same, the only way to tell the difference is that the waxing moon is visible in the early part of the night, while the waning moon is visible toward morning. This works in all parts of the world, incidentally.

Form a 'C' shape with the thumb and forefinger of the left hand. Hold it up to the moon. Do the shapes match? Then it's waning. If the moon matches your right hand when making the same shape, it is a waxing moon.

To remember which way around it is, think of a line of moons at different phases, starting at the left with a new moon, progressing through the waxing quarter and half moon to the full moon in the centre, and back to new moon. The waxing moons to the left match your right hand, while the waning moons to the right match the left hand.

Now, hold up your hands and we will try to make the shapes of the moon changing in the sky
Into the curve of the right fits the moon when it's new Make a circle with both and the full moon's in view Then curve to the left, to fit the moon when it's old.
And that is what I was always told

Extracted from What is the Moon? by Caroline Dunant and Liz Loveless

How to calculate the phase of the Moon with only your brain to help you
And maybe some paper and a pen
Or even a calculator

Sometimes, you just want to know what the phase of the Moon is, or will be. Perhaps you need to know in advance for some important reason. And maybe you can't see the Moon because it's cloudy, or you're locked in a cellar. Well, never fear. Just remember these simple instructions and you can be a phase-of-the-moon guru.

1. Count the number of days since January 1, 2001
2. Multiply this by 850
3. Add 5130
4. Divide by 25101 (This bit may be difficult to do in your head)
5. That's it!

So, you've got a weird decimal number. "What does it mean?" I hear you cry. Well, it's the number of lunations since a new moon in December 2000, so the fraction after the point tells you how full the moon is. Dicard the number before the point and look at the other bit, if it's:

• Equal to 0: The moon is new.
• less than 0.5: The moon is waxing
• equal to 0.5: The moon is full
• more than 0.5: The moon is waning

So, just remember this:

850n+5130
---------  (n is the number of days since New Year's Day 2001)
  25101

How about an Example?

Really? Oh, OK. Today (at the time of noding) is May 26, 2005...

1. That's four years from the start of 2001 to the start of this year.
2. That's 365*4=1460 days +1 for the leap year last year --- 1461
3. 1461 * 850 is 1241850
4. 1241850 + 5130 is 1246980.
5. 1246980 / 25101 is 49.6785ish

So we look at the bit after the point and we see that the moon is waning. Which it is. Well, actually it's too cloudy to see at the moment, but... --- /me rummages for his postcard with the moon phases on --- yep! It was full three days ago.