Months and phases of lunar system

Lunar month is referred to as a time period. During which a moon completes the full movement. But there are several ways to calculate days in lunar month - that is why their length may vary, everything depends on what system is used. Lunar calendar may have different uses: some religions establish their calendars based on lunar months, pregnancy time and the best days to conceive a baby are also determined by this calendar. One of the commonly accepted methods used to determine lunar month is to check when both the moon and the sun are standing in one line, making a full or a new moon – this period is also known as 'syzyrgy'. If this method is used, synodic month (that is how lunar month is also called) is lasting from one full moon to the beginning of next full moon, or from one new moon to the next new moon. Synodic month lasts for exactly 29 days, 12 hrs, and 44 min.

Other technique of lunar month calculation uses the orbital period, here one month is called a 'sideral month'. Another way of calculation is tracking the lunar movement, while it goes to perigee from perigee – it is when moon comes closest to the Earth, or its orbit crosses the Earth's orbit (these are just 2 nodes on the orbit). As you can see – ways of lunar month calculation differ, which yield one month lasting something between 27 and 29 days in each.

The big problem in this difference is that when lunar calendar is being used for a long time, calendars will start differing and being off. Eventually, the moon calendar will not match the moon phases. To avoid this, lunar calendars must be adjusted from time to time, to match the moon movements and cycles as much accurate as possible. New lunar calendars are issued early.

In western Gregorian calendar system years also vary one from another, to solve this leap years are inserted to balance the deviations. Leap seconds are also added periodically. Why does it happen? Because the orbits cannot be predicted precisely – there are always small variations, which will transform into huge miscalculations over time. In those nations, where lunar calendar is used by many people, all dates are often presented in both Lunar and Gregorian calendar systems for citizens' convenience, converters are also available.

Now let us describe what the lunar phases are – these are the different stages of the moon cycle, while it goes by its orbit around the Earth. While it moves – people see the moon illuminated at a different levels: from a new moon, where all the surface is covered by the shadow, to a full moon, where all moon surface can be clearly visible. The whole cycle takes a bit less than regular Gregorian month (30 days) to be completed, this shorter cycle is referred to as a moon cycle.

People have paid much attention and tracked lunar phases for many centuries, and even did it in prehistoric times. Waning and waxing of the moon could be clearly observed by every human society in every age, months’ measurements and some other calculations were based on the moon cycles in many cultures. Hundreds of beliefs existed, events were also associated with particular phases of the moon. One of the strong belief, which survived until nowadays is: waning moons are associated with new endeavors and bad outcomes, it is not recommended to start something new and important during this time.

The science of astronomy can easily explain the lunar phases and their change. Every time half of the Earth and fully half of the moon are both being illuminated by the Sun. When the moon moves around the Earth by its orbit, different levels of illuminated moon surface may be observed by people from the Earth. In case the moon is located between the Earth and the Sun, major part of its surface cannot be seen (as it is almost fully covered by the shadow) – this creates a new moon. On the contrary, full moon can be seen when the Earth is located right between the Sun and the moon.

Constant and cyclical solar and lunar eclipses may be expected in correspondence to certain phases of the cycle, but this is impossible because orbits of both moon and Earth are angled, and not the straight line – that is why full eclipses are so rare.

We have already mentioned, that 'waxing' moon is a growing one, and 'waning' is the shrinking moon. Direction of moon's cycles moving depends on the Hemisphere where the observer lives. In the Southern Hemisphere, moon phases move from left to right: if the left quarter is illuminated, this means the moon is waxing towards a full moon, if the left side is darkening – the moon is moving to the wane. In the Northern Hemisphere the opposite is true, but on the equator moon phases appear horizontally.

Such variations, described above, happen due to the angles of the Earth and moon orbits and their relationships with the Sun angle. Even small changes in these angles will have a strong influence on how we see the moon, depending upon our location on earth. These variations also serve as an explanation to different times of sunset and sunrise at different seasons.