Issue 57 / January - March 2007
The Moon Sighting and the Lunar Calendar
Bruce C. Williams
The Islamic calendar is based on lunar months, which begin when a new crescent is sighted in the western sky after sunset within a day or so after the New Moon. Hence, the month is either 29 days or 30 days. There are 12 months in an Islamic year, which is either 354 days or 355 days long, compared to the Gregorian calendar year of 365 or 366 days. Since the Islamic lunar year has 12 lunar months, it is on an average 11 days shorter than the Gregorian year; the Islamic year shifts by about 11 days to an early date according to the Gregorian calendar.
The starting date of a month in the Islamic lunar calendar is very important in the Islamic world as it determines the essential religious days, like the first and last days of fasting, and Muslim holidays.
It takes about 27 days and 8 hours for the Moon to complete one revolution around the Earth. At the same time, while revolving around the Earth, the Moon rotates around itself. Because of this, we always see the same side of the Moon. In the picture below, we see the phases of the Moon. For simplicity, the sunlight is fixed, but this is not a problem as the Earth and the Moon are revolving around the sun together. The position of the Moon with the Earth according to sunlight determines how we see the Moon.
The Moonâ€™s position does not change greatly in one day relative to that of the Earth, but the Earthâ€™s rotation around itself changes our position a great deal. One might ask then why do we see the Moon as the same shape throughout the night. As the figures below suggest, the distance from the Moon to the Earth is much greater than the diameter of the Earth. While the closest distance of the Moon to the Earth is around 350 thousand kilometers, the diameter of the Earth is around 13 thousand kilometers. Because of this huge difference, we see almost the same shape of the Moon anywhere in the world- this â€śalmostâ€ť will cause many problems, when we determine the dates in lunar calendar. On the other hand, the place where we see the Moon in the sky depends on our parallel on the Earth. In other words, the northern observer sees the Moon at a lower place in the sky, while the southern one sees the Moon at a higher place in the sky. This is because the Earth is round and our horizon changes if we go to the north or the south.
Now, letâ€™s examine another point. We have just said that the Moon completes one revolution around the Earth in 27 days and 8 hours. However, the time between two New Moons is around 29 and a half days. How does this happen? The reason is not very hard to understand. While the Moon is revolving around the Earth, the Earth revolves around the Sun. Hence, when the Moon completes one full revolution around the Earth and comes to the same point from where it had started, the position of the sunlight changes and the Moonâ€™s shape, as we perceive it, has changed.
the Moon completes one full revolution around the Earth and comes to the same point from where it had started, the position of the sunlight changes and the Moonâ€™s shape, as we perceive it, has changed.
Now, letâ€™s make some computations. How much does the angle of the sun change in 27 days and 8 hours? The Earth completes one revolution around the Sun in 365 days, so there are 365 days in one year. So the angle change should be about 27 days /365 days x 360 ~ 27 degrees. So, how long does it take to complete 27 degrees in the revolution of the Moon around the Earth? One complete revolution takes 27 days and 8 hours. Hence, 27 days x 27/360 ~ 2 days. So, we add 2 days to 27 days and 8 hours, we get about 29 days and a half. Therefore, in this time period, the Moon comes to the same position as the Earth according to sunlight, and we see the same shape of the Moon in the sky. When we do an exact calculation, we get 29 days, 12 hours, 44 minutes, and 3 seconds for the time period between two new Moons.
The Islamic lunar calendar
The earliest calendars in human history were based on the phases of the Moon. Historically, this was used by southern Europeans, as well as Jewish, Chinese, Muslim, and African civilizations. Some of these civilizations are still using semi-lunar calendars, but the Muslim calendar is the only purely lunar calendar in widespread use today. In the Qurâ€™an, there are many references to the moon as a means of measuring time (see Rahman 55:5, Yunus 10:5; Baqara 2:189).
In a lunar calendar, a month is the period between the two consecutive new Moons. Since the exact period between two new Moons is 29 days, 12 hours, 44 minutes, and 3 seconds, a month in a lunar calendar is either 29 days, or 30 days, depending on the appearance of the Moon at night.
Now, letâ€™s examine another point. We have just said that the Moon completes one revolution around the Earth in 27 days and 8 hours. However, the time between two New Moons is around 29 and a half days. How does this happen? The reason is not very hard to understand. While the Moon is revolving around the Earth, the Earth revolves around the Sun. Hence, when
The Islamic calendar was introduced in 632 by the Prophet, peace be upon him; however the beginning of the Islamic era for the count of Islamic years was considered and discussed in 639, during the time of the fourth year of the Caliphate of Umar. For the Islamic lunar calendar, the time that a month started was decided according to the following hadith. The Prophet said: â€śFast when you see the Moon and break your fast when you see the next Moon. But if you cannot see it, then complete the 30th day [of fasting].â€ť Hence, in the Islamic lunar calendar, the first day of a month is the day after the first night on which you can see the new crescent Moon. Since the Moonâ€™s orbit around the Earth is 29 days and a half, Islamic months are either 29 or 30 days long. If there is doubt or if the new crescent Moon is not sighted due to clouds or other atmospheric conditions, then it has been agreed upon by Muslim scholars that the previous month is completed after 30 days.
Now, letâ€™s turn to the controversy in determining the first day of a new month in the Islamic lunar calendar. There are essentially two main issues, which are â€śLocal vs. Non-local Sighting,â€ť and â€śNaked Eye Observation vs. Astronomical Data.â€ť
i. Local vs. non-local sighting
Since the cycles of the Moon and the sun are not completely synchronized, the new crescent Moon might not be observed everywhere in the world on the same night. As we mentioned above, the shape of the Moon is seen almost the same anywhere in the world, but not exactly same. On the same night, Spain might observe the new crescent Moon while Japan might see nothing in the sky. The only thing we can say about these observations is that if one city observes the new crescent Moon, than any city to the west of the original one should be able to observe the same on a clear night, as the Earth rotates about itself from west to east. This means the new crescent Moon can be observed on different nights in different places in the world. There are essentially two different opinions on which Moon sighting should be taken as the basis. The first opinion asserts that all Muslims in the world should begin the new month if the new crescent Moon is sighted in any Muslim country, and the second one says that the new month starts only with a local Moon sighting.
The scholars defending the non-local Moon sighting give evidence from other Islamic practices. They claim that all Muslims should start the fasting and the Eid on the same day, as many practices of Islam suggest that the unity of Muslims is important in Islam; hence Muslims should celebrate these days in unity. So, according to this view, if the Moon is sighted, for example, in Saudi Arabia, all Muslims should start fasting. This is the common opinion among the followers of the Sunni Hanbali school. On the other hand, some other Islamic scholars assert that the starting day of a month in the Islamic lunar calendar might be different depending on local observations. Those scholars derive this result from the principle of simplicity in Islam. All people should be able to perform the practices of Islam by themselves without the need of any outside input. In other words, a Muslim should be able to decide by themselves when Ramadan starts by observing the Moon. A Muslim in Brazil might not get the information of the sighting in Mecca, and this contradicts the simplicity of practices in Islam. A Muslim can determine the prayer times by observing the sunâ€™s position in the sky, and similarly, therefore, they should be able to decide about the starting time and ending time of the fast.
So, the first controversial point in Moon sighting is about whether it is local or non-local. The second issue is about how we make the observations.
ii. Naked Eye Observation vs. Astronomical Data
We are living in the 21st century, and astronomy is a very well-developed science. Man can go to space, observe the distant galaxies, and even observe the expansion of the universe. Hence, at such a time, calculating the times of the visibility of the Moon for a given location should be very straightforward for astronomers. So, why is still there an ambiguity about the starting date of a month in the Islamic lunar calendar?
In the Islamic lunar calendar, the month starts with the visibility of the Moon, not the scientific phase of the Moon. In other words, â€śTheoretical Visibilityâ€ť is not the same as â€śReal Visibility.â€ť If the Moon was a perfectly round sphere with a perfectly flat surface, if the atmospheric conditions were always very clear and did not affect our vision, or if there were no other physical conditions affecting the Moon sighting, then â€śTheoretical Visibilityâ€ť would be the same as â€śReal Visibility,â€ť and we would be able to compute the exact times when we could sight the new crescent Moon by using astronomical data. However, there is an important difference between theory and real life; hence, the problem lies in this difference. Even though the astronomical data tell you the positions of the Moon, the Earth and the Sun, the visibility of the Moon from the Earth depends on many different factors.
This difference between â€śTheoretical Visibilityâ€ť and â€śReal Visibilityâ€ť is the origin of the second controversy about the starting day of a month. Some Muslim scholars interpret the theoretical visibility as being sufficient to start the month, and as they determine the dates of the lunar calendar by using astronomical data, they do not bother with sighting the Moon. That is, they know the dates for Ramadan for the following 100 years, and they practice according to this calendar depending only on the astronomical data. However, some other scholars assert that people must be able to sight the new crescent Moon with the naked eye for the month to start. Since astronomical data and naked eye observation are not always in sync, some Muslims might start fasting in Ramadan on a different day from other Muslims.
Now, letâ€™s return to the question as to why astronomical data and naked eye observation are not in sync. Scientifically, there are two main factors affecting the visibility of the Moon other than atmospheric conditions or clouds. The first one is the Moonâ€™s altitude measured from the horizon. The second one is the age of the Moon, that is, the time after the new Moon (when the Sun, Earth, and Moon are in line). To see the Moon in the sky, it should be â€śhighâ€ť enough above the horizon, and it should be â€śoldâ€ť enough (the crescent should be sufficiently pronounced).
The first important factor is the Moonâ€™s altitude above the horizon. If the thickness of the crescent is sufficient, but if it is not sufficiently above the horizon, then it cannot be seen. This happens in the month of September and October in the UK and the USA, when the Moon is in the southern hemisphere, therefore, it cannot be seen from northern hemisphere. If the Moon is above the horizon, but close to it, falling within the glare of the Sun, then it may not be visible. A crescent within an altitude of 10 degrees is usually not visible.
The second important factor is the age of the Moon. When the New Moon has moved away at least about 7 degrees and it is about 20 hours old, then it becomes visible. Until this angle becomes 7 degrees, no light of the sun reflected by the Moon can come to the Earth, because the mountains on the surface of the Moon block the sunâ€™s light. This angle must be about 10-12 degrees for sufficient sunlight to reflect off the Moon and come to the Earth, making a thin crescent which can be seen. Sometimes, this crescent is very thin and very low on the horizon, so it will not be seen, as it fades due to the glare of the sun, even though it may remain above the horizon for 20-30 minutes after sunset. There are many other factors involved in visibility. Without going into technical details, due to other factors, even a 36 hour old Moon may not be visible. Therefore, human knowledge so far has not reached the stage at which it can inform us of the Moonâ€™s visibility in different countries with an accuracy of minutes and seconds, but calculations can be made within a few hours difference.
So, these facts show that astronomical data provide a good means for estimating the starting day of the new month, but it cannot tell us exactly when it begins. There are some Muslim experts who are working on this problem. By using data from the past, they are trying to calculate more accurately the times of the visible Moon for all locations in the world.
On the other hand, this uncertainty of the starting day makes Muslims more aware, and keeps their attention on Ramadan and other festivities. In many countries, Muslims go to hilltops to observe the new crescent in the night, watching the Moon-sighting news on the internet, or calling their friends about it to welcome Ramadan. Hence, the uncertainty of the day causes the Muslim community to be more aware about Ramadan. There are some Muslim experts who have been working on this problem. By using data from the past, they are trying to calculate more accurately the times of the visible Moon for all locations in the world. Finally in 2006, they released an algorithm which can detect the visibility of the Moon by using astronomical data; ISNA (the Islamic Society of North America) has adopted this method to determine the starting day of the Islamic Lunar month. This algorithm makes use of several thousand controlled observations, collected over a period of 150 years in different locations of the world. Moreover, these visibility calculations have been confirmed by observations of the Moon made every month since 1993. Again, this algorithm does not depend upon purely astronomical data, but rather the observational data of the last 150 years. The researchers, Khalid Shaukat et al., used these observations, and comparing them with the astronomical data of the same time were able to attain information about the degree of the Moon and at which geographical position of the Earth the Moon became visible. For more information, please visit www.Moonsighting.com.
To sum up, we have provided an overview of the controversies about the starting day of the month in the Islamic lunar calendar. There are essentially two points of controversy which are â€śLocal vs. Non-local Sighting,â€ť and â€śNaked Eye Observation vs. Astronomical Data.â€ť We have attempted to show that these controversies are not as simple as many people might think. So, if there are different starting days for Ramadan, or different celebration dates for Islamic holidays for different Muslim communities, we should not see this situation as a huge controversy caused over a small issue. As the Prophet Muhammad said: â€śThe difference of opinion among my followers is a mercy.â€ť
1. Dr Jamie Love, The Phases of the Moon, http://www.synapses. co.uk/astro/Moon2.html
2. Syed Khalid Shaukat, What is Islamic Calendar, http://www. Moonsighting.com/islamcal.html
3. Syed Khalid Shaukat, The Science of Moon Sighting, http:// www.ummah.net/Moonsighting/science/Moonscie.htm
4. Zahoor Niazi, The Question of Moon Sighting, http://www. Moonsighting.com/question-rooyat-hilal.mht
5. The Moon, http://www.enchantedlearning.com/subjects/astronomy/ Moon/index.shtml
6. Moon Calendars, http://www.riverocean.org.uk/lunarcalendar/ calendar.htm