NWMI’s Approach to Determining the Start of a Lunar Month

The Quranic basis for the start of the month in the Islamic calendar

Determination of the start of a lunar month is based on two ayahs in the Quran:

…فَمَن شَهِدَ مِنكُمُ الشَّهْرَ فَلْيَصُمْهُ…

…so whoever from you witnesses the month, so he shall fast it… (2:185)

يَسْأَلُونَكَ عَنِ الْأَهِلَّةِ قُلْ هِيَ مَوَاقِيتُ لِلنَّاسِ وَالْحَجِّ 

They ask you about the crescent moons; say ‘They are demarcations of time for the people and the Hajj…' (2:189) 

In 2:185, interpretations of “shahida” range from the literal understanding of seeing with one’s physical eyes to broader understandings of knowing through inference or other forms of ascertainment. Ayah 2:189 provides the lunar basis for the Islamic calendar.

Decision points in ascertaining the new lunar month

Based on the Quranic verses, there are several methods to ascertain the beginning of a lunar month. These methods are generally distinguished by two decision points:

The first decision point is how the crescent moon is “witnessed.” The moon may be sighted physically, or if 30 days have passed since the beginning of the lunar month without a sighting, the next day must be the start of the new month because lunar months are always 29 or 30 days. In addition, the presence of the new moon may be calculated according to various parameters (knowledge without sighting). 

If the moon must be sighted physically, then the second decision point is how far away other trustworthy and competent individuals can be to accept their verified sighting of the moon. There are two main opinions on this matter. The more local criterion is the known as the Common Horizon (or Multiplicity of Horizons) criterion, defined as follows:

“If the first day of a month is proved in a city, it is also proved in other cities if they are united in their horizon. And the meaning of having a common horizon in this matter is that if new moon was sighted in a city, there would be a distinct possibility of sighting it in the other cities, if there were no impediments, like, the clouds etc.”


In other words, if it is astronomically possible to see the moon in locations A and B, but the sighting is obscured in location A, the Common Horizon criterion says individuals living in location A can accept the sighting of location B. The Common Horizon criterion stipulates that the moon should appear similarly in both locations with respect to its altitude above the horizon and degree of illumination; hence, the horizons of the two locations are similar.

The broader criterion is known as the Common Night (or Unity of Horizons) criterion, which is defined as follows:

“If the first date of a month is proved in a city it is also proved in other city whether they are near or far and whether or not they have a common horizon, provided they have a common night, even though when it is the first part of the night in one of them it is the last part of the night in the other.”


According to the Common Night criterion, a sighting in location B can be accepted by individuals living in location A as long as both A and B are under the darkness of night at the same time. It is not necessary for the moon to be sightable in location A, and even if it were possible to see the moon in location A, it would not need to appear similar to that in location B with regard to altitude and illumination.

Sighting of the moon

Calculations of the movements of the Sun, Earth, and Moon in relation to each other, and refinements of those calculations through scientific observation, have led to algorithms that can predict the likelihood of seeing the crescent moon for any given date and location. The ability to see the moon is dependent on a variety of factors, such as the amount of time that has elapsed since the new moon, the amount of sunlight in the sky, and the elevation of the moon above the horizon. The following figures show areas of the Earth where the crescent moon marking the beginning of Ramadan 2017 may be sighted:

According to this figure, the moon first becomes visible by naked eye sighting on the evening of May 26, 2017 in eastern Africa. As more time passes following the birth of the moon, it becomes visible over progressively greater areas of Earth with widespread visibility across the Americas. Ramadan in these areas began the following day, May 27.

Predictions for sighting of the crescent moon marking the beginning of Shawwal 2017 (that is, Eid-al-Fitr), are displayed in the following figures:

The first figure indicates that the moon first becomes visible by naked eye sighting in Central and South America, Mexico, the Caribbean, and the southernmost areas of the United States in the evening on June 24, 2017, with progressively greater visibility across the Pacific. According to the second figure, the moon is widely visible across the rest of the globe on the evening of June 25, 2017 (local date).

Calculation of the new moon

Calculation systems are based on various astronomical parameters, such as altitude above the horizon, minimum angle of separation with the Sun, or simply that the Moon is born. The most commonly used calculation system in North America is that used by the Fiqh Council of North America, which has determined that the first day of Shawwal 1438 is June 25, 2017.

Applying the Common Horizon criterion

The Common Horizon criterion requires the moon to be sightable with similar appearance in two locations if one location is to accept the sighting of the other. This requires that the locations have geographic proximity. The first day of Shawwal for most of North America according to the Common Horizon criterion is likely to be June 26, 2017.

Applying the Common Night criterion

The Common Night criterion says that two locations that are both covered by the darkness of night at the same time can accept each other's sighting of the moon, regardless of whether or not the moon can be sighted in one location. The following figure indicates the areas of Earth under the darkness of night when Washington, D.C. is about to enter Fajr (dawn) on June 25, 2017:


When Washington, D.C. is in the last moments of night in the pre-dawn moments of June 25, the cover of darkness extends as far as the eastern portions of Australia. Moon sightings in any of the darkened areas are acceptable for Washington, D.C. to celebrate Eid on June 25 according to the Common Night criterion.

NWMI’s system for determining the start of a lunar month and rationale

After having deliberated at length over the merits of each system for determining the start of lunar months, NWMI decided to adopt the Common Night criterion because 1) there is consensus within NWMI that the crescent moon should be physically sighted, and 2) the Common Night criterion most closely reflects NWMI's vision of a diverse Ummah, allowing us to be informed by verified sightings from fellow Muslims across continents. NWMI consults sightings from around the globe as reported on such websites as www.moonsighting.com, www.moonsighting.info, and www.crescentwatch.org.

Adoption of the Common Night criterion does not mean that NWMI follows the rulings of a single jurist (marja'). NWMI is comprised of a variety of Muslims with varying preferences in matters of jurisprudence (fiqh). NWMI respects each individual's personal choices in these matters and regards such differences as a reflection of the diversity of the Muslim Ummah.

As for Eid-al-Adha, NWMI celebrates with the hajjis and holds salat and celebrations on the day after they stand at Arafat.