The Moon has been a fascination since Ancient Times. Showing moon phases on Calendars has been a tradition for many years. Recording the moon and its phases has been performed with greater and greater precision up to the present times. I will not explain “why” the moon appears to change during the month since there are many science classes, web pages and sources devoted to this phenomenon. I will try to briefly explain why different web sites give different answers to the timing of the phases.
The moon has a cycle that has been measured to be 29.53058867 days from the start of the darkest “New Moon” to the next “New Moon”. This cycle is called the synodic cycle. The moon’s synodic cycle is 29.53058867 days and was observed by Greek astronomers and Babylonian skywatchers with a bit less precision but with quite useful results. This time is pretty much a constant and is used to calculate the phases of the moon.
In the case of the moon phase chart displayed here the time period is divided by four to calculate the four principle phases (“Last Quarter, New Moon, First Quarter, Full Moon”). This makes an assumption that the Earth is a perfect sphere, and the moon makes its transit of the heavens at a precise constant angular rate. This gives the first reason for a source of variations.
Some calculations do a more precise movement of the moon around a slightly flattened Earth (called an Oblate Spheroid). This will slightly change the apparent times of the phases.
To do a calculation of the cyclic nature of the Moon Phase events you need to have some starting point. Some arbitrary time is used to choose one time of a "New Moon" to use as a starting point. This becomes the reference for the calculator. From this point the next "New Moon", and its fractional elements, is used to determine the next phase. If this is continued into the future or the past without resetting the reference, the times will become less accurate. Thus, another reason that the times reported by different Moon Phase calculators will vary. This calculator uses the New Moon as a reference at: Jan 6, 2000 at 1814UTC.
An another variation is where the calculation is performed. The clocks are set to different time zones and the place on the Earth where the moon is seen will cause the apparent times of the moon phases to vary as seen by an observer. Time zones are not regularly spaced around the globe and they are marked by borders that are locally convenient for the people near the borders. This calculator uses your local computer time to perform the calculations. You can display the time of the phase in UTC (Universal Time Coordinated) if you like.
In the big scheme of things, how the moon appears in the sky is not always needed to be a precise time of an event. There are exceptions, such as timing of religious events by some cultures where great care is used to observe the important phase. For this Moon Phase Calendar take it as an interesting tool, and be aware that it is not a high precision measurement. Times shown can be from a few minutes off or more than an hour different from other published charts or tables for reasons mentioned above.
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