A New Moon is an astronomical phenomenon which occurs about every 29.5 days (every 29.53059... days – that’s 29 days, 12 hours, and 44 minutes, and 2.8 seconds – to be more precise), or, that is to say, (usually) once every calendar month. (Occasionally there are two new moons in the same calendar month. When this does happen, the second one is sometimes referred to as a Black Moon, though there are different definitions for that term, as explained here.)
The New Moon marks the stage of the lunar cycle when the moon is passing between the earth and sun, during which time it is essentially invisible from earth, lost in the glare of the sun and with the side facing us obscured in shadow. Actually, in modern astronomical terms, the New Moon is defined as the precise moment when the center of the moon, while traveling in the portion of its orbit on the near side of the earth and closest to the sun, passes through an imaginary plane extending through the center of the earth and the center of the sun. In ancient times, the New Moon was regarded as having occurred a day or two (on average) after that event, on the day when the moon could first be sighted once more as a very thin crescent shape on the sun’s eastern side (most easily spotted shortly after sunset). This was usually regarded as the beginning of each new month, and in most ancient cultures it was one of the central duties of the priestly class to make this determination.
Religious feasts associated with the New Moon include the New Moon festivals that the ancient Israelites observed, as record in the Bible, the Hindu festival of Diwali, and the Muslim observance of Ramadan. (Also, since Islam follows a strictly lunar calendar, Ramadan can fall during any month of the Gregorian calendar year. In fact, its observance cycles slowly backwards through the western calendar over a period of about 33 years.)
The days immediately following the New Moon are also a great time to observe the phenomenon known as Earthshine (also known as Da Vinci glow, or by the poetic phrase “the Old Moon in the New Moon’s arms”), when the portion of the moon unillumined by the sun’s direct light is nevertheless faintly visible, having a dusky blue appearance, due to indirect sunlight reflected back on it from the earth’s surface. As is commonly known, the moon’s gravitational pull is also what causes tides in coastal areas upon the earth. The days surrounding the New Moon (and the Full Moon as well) see the greatest differences between high and low tides, a phenomenon known as Spring Tides (think “spring” in the sense of jumping or leaping, not in connection with the season.)
As most people are aware, a solar eclipse is caused when the New Moon passes directly between the sun and the earth. This fact, combined with the above explanation, is liable to raise the following question: why isn’t there a solar eclipse with every New Moon? The answer lies in the fact that the imaginary planes represented the earth’s orbit around the sun (also known as the ecliptic) and the moon’s orbit around the earth are not in perfect alignment (although they are close). Only when a New Moon happens to occur at the same time when the moon is also at or near one of two points where its orbital plane intersects the ecliptic will there be an eclipse. (These two points are known as the Ascending and Descending Nodes). While this doesn’t happen with every New Moon, it does happen fairly often. But even so, not every eclipse event is a total one – some are only partial and some may be partial or total, depending on your precise vantage point on earth. And what is more, every eclipse is only visible for a portion of the inhabitants of the earth anyway. So, while total solar eclipses aren’t all that rare overall, it is a very rare event for any given location on earth’s surface to witness.
