Lunar calendar

Key takeaways

Lunar calendar belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Lunar calendar to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Lunar calendar from memory before moving on to harder problems.

Reference excerpt

A lunar calendar is a calendar whose months record the cycles of the Moon's phases (synodic months, lunations). This in contrast to solar calendars, whose annual cycles are based on the solar year (which is about 11 to 12 days longer than twelve lunar months). It is also to be contrasted with lunisolar calendars, which also count lunar months but recover the accumulated differences by adding ("intercalating") a thirteenth leap month every few years. The most widely observed lunar calendar is the Islamic calendar. Since each lunation is approximately 29+1⁄2 days, it is common for the months of a lunar calendar to alternate between 29 and 30 days. Since the period of 12 such lunations, a lunar year, is 354 days, 8 hours, 48 minutes, 34 seconds (354.36707 days), lunar calendars are 11 to 12 days shorter than the solar year. In lunar calendars, which do not make use of lunisolar calendars' intercalation, the lunar months cycle through all the seasons of a solar year over the course of a 33–34 lunar-year cycle (see, e.g., list of Islamic years). The details of when months begin vary from calendar to calendar, with some using new, full, or crescent moons and others employing detailed calculations. The term lunar new year is the first day of lunar calendar but is also widely used, especially in the US, of lunisolar new years.

History Scholars have argued that ancient hunters conducted regular astronomical observations of the Moon back in the Upper Palaeolithic. Samuel L. Macey dates the earliest uses of the Moon as a time-measuring device back to 28,000–30,000 years ago.

Start of the lunar month

Lunar and lunisolar calendars differ as to which day is the first day of the month. Some are based on the first sighting of the lunar crescent, such as the Hijri calendar observed by most of Islam. Alternatively, in some lunisolar calendars, such as the Hebrew calendar and Chinese calendar, the first day of a month is the day when an astronomical new moon occurs in a particular time zone. In others, such as some Hindu calendars, each month begins on the day after the full moon.

Length of the lunar month The length of each lunar cycle varies slightly from the average value. In addition, observations are subject to uncertainty and weather conditions – in Sunni Islam, the month begins with Hilal, the actual observation of the first crescent. Thus, to minimise uncertainty, there have been attempts to create fixed arithmetical rules to determine the start of each calendar month. The best known of these is the Tabular Islamic calendar: in brief, it has a 30-year cycle with 11 leap years of 355 days and 19 years of 354 days. In the long term, it is accurate to one day in about 2,500 solar years or 2,570 lunar years. It also deviates from observation by up to about one or two days in the short term. The algorithm was introduced by Muslim astronomers in the 8th century to predict the approximate date of the first crescent moon, which is used to determine the first day of each month in the Islamic lunar calendar.

List of lunar calendars Islamic Hijri calendar Javanese calendar The prehistoric Roman calendar is believed to have been a purely lunar calendar that later evolved into a lunisolar one, which in turn was replaced outright by the purely solar Julian calendar.

Lunisolar calendars

Most calendars referred to as "lunar" calendars are actually lunisolar calendars. Their months are based on observations of the lunar cycle, with periodic intercalation being used to restore them into general agreement with the solar year. The solar "civic calendar" that was used in ancient Egypt showed traces of its origin in the earlier lunar calendar, which continued to be used alongside it for religious and agricultural purposes. Present-day lunisolar calendars include the Chinese, Korean, Vietnamese, Hindu, Hebrew and Thai calendars. The most common form of intercalation is to add an additional month every second or third year. Some lunisolar calendars are also calibrated by annual natural events which are affected by lunar cycles as well as the solar cycle. An example of this is the lunisolar calendar of the Banks Islands, which includes three months in which the edible palolo worms mass on the beaches. These events occur at the last quarter of the lunar month, as the reproductive cycle of the palolos is synchronized with the moon.

Illustrations

Worked examples

Example 1 — a first encounter with Lunar calendar

Start with the simplest possible case. Write down what Lunar calendar claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Lunar calendar before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Lunar calendar ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Lunar calendar

In research: Lunar calendar appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Lunar calendar in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Lunar calendar is common in secondary-school and first-year university syllabi. It links to neighbouring topics Lunar calendars, Time measurement systems, so understanding it makes those chapters shorter.
In everyday life: Look for Lunar calendar outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Lunar calendar in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Lunar calendar means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Lunar calendar out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Lunar calendar in simple terms?

A lunar calendar is a calendar whose months record the cycles of the Moon's phases (synodic months, lunations). This in contrast to solar calendars, whose annual cycles are based on the solar year (which is about 11 to 12 days longer than twelve lunar months).

Why does Lunar calendar matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Lunar calendar?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Lunar calendar.