Keeping Track of the Moon's Cycle

For amateur astrologers like myself (note that I wrote 'astrologer' not 'astronomer'!), it is good to keep track of the Moon's cycle. The easiest way to do this is to look into the sky to observe the shape of the Moon. In all regions north of the Tropic of Cancer, including Britain, when the light is to the right of the disc, it indicates that the Moon is waxing (the area of light growing), and when it is to the left, waning. In the evening, you are more likely to see the waxing Moon, in the early hours of the morning, the waning Moon. This is because the Moon's light clusters in the direction of the Sun.

However, in Britain, the sky is cloudy as often as not, but there is another way to keep track: this is to create a lunar calendar for yourself, which is fun for astro-geeks like me! (For non-geeks, it is definitely not fun; you should stop reading now and get back to whatever job or lockdown activity you were engaged in!) The lunar calendar allows you to know where the Moon is in its cycle, based on the date.

This is how it works. Our twelve months per year are based on the lunar cycle, but not accurately so. The lunar cycle is approximately 29½ days, which is the time it takes to go from New Moon to New Moon or Full Moon to Full Moon. After twelve such cycles, you are left with an extra period of 11 days per year (or 12 in leap years), as 29½ x 12 = 354. Twelve lunar cycles are 11 days short of a full year.

So you need to choose a block of time of 11 days each year which will become your crossover period from one year to the next. The obvious one for most people is the 11 day period 21 - 31 December. Then your crossover period coincides with Christmas and New Year's Eve.

Now count out twelve lunar 'months'. They need to alternate between 29 and 30 days (to give you an approximate 29½), making sure that 28 February is included in one of the shorter months, as that one will increase by a day in leap years. From 1 January, these 'months' would be:

Month 1 begins on January 1 and is 30 days

Month 2: begins on January 31 and is 29 days (30 in a leap year)

Month 3: March 1, 30 days

Month 4: March 31, 29 days, etc.

Month 5: April 29

Month 6: May 29

Month 7: June 27

Month 8: July 27

Month 9: August 25

Month 10: September 24

Month 11: October 23

Month 12: November 22

Crossover (Month 13): begins on December 21 and is 11 days

These dates you need to note down and remember. They stay the same from year to year. If you decide to start your lunar calendar on a date that is not January 1, this will give you a different list of dates which you will need to calculate for yourself.

Each year, you need to find out when the first New and Full Moons are that occur after your start date. You can do this using the internet, or a diary, or an ephemeris. This year (2021), the first New Moon after 1 January was on 13 January and the first Full Moon on 28 January. So during 2021, you can assume that in each of your lunar months, the New Moon will be on the 13th day and the Full Moon will be on the 28th. In month 2, starting with January 31, this would be February 12 and February 27. The actual dates for the New and Full Moon are 11 and 27 February, so you can be one day out with this system, but never more. You will always have a good sense as to where the lunar cycle has reached as long as you remember the timing of the New and Full Moons in your lunar months for that particular year.

Now you can enjoy naming your lunar months. For example, Christians could use feast days which occur within the lunar month so the first one could be Epiphany; Pagans and Wiccans will prefer to use names associated with the old agricultural year, so the second one could be Imbolc (people in many other world faiths already have lunar calendars!). Or you might have more personal choices for the names of the lunar months.

Along with the names, the choice of start or end date of your lunar calendar could make it highly personal: your birthday or wedding anniversary, or the birth of your first child. January 1 as a start date is obvious, but it is not very personal. I use 16 August, so that my 11 day crossover period culminates in the Feast of the Assumption of Mary on the 15th; as I was in education for many years, a mid-August date made more sense as the beginning of a new year. Then I got married on August 15, so now my lunar calendar comes to a grand finale with my wedding anniversary! I can't really forget it, can I?