By Muhammad F.J. Moledina (San Diego)
Introduction:
Muslims do not have a definite calendar at this time and some of them feel that it is impossible to make one because there is something unpredictable about the motion of the Moon. This causes a great deal of argument and confusion in the community and is most embarrassing for all of us.
In reality, the calendar based on the Holy Qur'an that was established during the days of Prophet Muhammad (SAW) is quite definite and can be used internationally if we get organized and agree upon certain conventions.
Because this has not been done we often have conflicting interpretations on what the actual Islamic date is. Europeans went through a similar phase in the last century. As they started travelling over great distances quickly, there was confusion over dates. Soon a date line was agreed upon among the leaders and the common man was merely informed after the decision.
With advancements in technology Muslims have been debating on a possible lunar calendar. The lunar cycle is not as obvious as the solar cycle of day and night. Many people do not realize that just as we cannot ascertain whether it is a day or night at our location by calling relatives in distant countries, we cannot be certain about a lunar date confirmed by someone half a world away. One has to look at the local sky. Every Muslim is not an astronomer, he cannot rationally decide about such matters. His decisions are often based on emotional and devotional reasons and are often scientifically wrong. It is essential that the community pay urgent attention to solve this problem.
The Motion of Heavenly Bodies
A few simple laws commonly known as Newton's laws of motion and the law of gravity govern the relative motion of all heavenly bodies.
The carved path of a body through the heavens is such that gravitational force is always balanced against (i.e. equal and opposite to) the centrifugal force due to the curvature of the path and the inertia force due to the acceleration of the body. As a result planets generally move in an elliptic orbit around the sun such that the sun remains at one of the foci of the ellipse. A circular orbit is just a special case of an ellipse when the two foci are congruent. The two equations that have to be solved simultaneously for computing the orbit of a planet are as follow:
For a two-body system the method of solution is quite simple. As the number of body's increase it becomes increasingly difficult to solve these equations. But modern computers have done all this and tables giving relative positions of bodies in our solar system are available in most libraries. To construct a calendar all one has to do is to learn to read and use them.
The Earth, Moon and Sun System
The Holy Qur'an states that the Sun and the Moon move according to "hisab" i.e. governed by laws. We have known these laws for sometime in enough detail to be confident about their location in the heavens at any time. This is all that is needed for constructing prayer timetables and calendars. A calendar can be considered merely as a special prayer timetable that tells us when to say Morning or Noon prayers, etc.
Motion of the Earth
The earth basically moves in three ways:
a) It spins on its axis to produce day and night. This takes 24 hours. Its spin axis is tilted about 23.5 degree in relation to its orbital plane.
b) It revolves around the Sun in a near circular orbit. Because of the tilt of its rotational axis the Sun appears to move north and south through the year. This change in the angle of incidence of light from the Sun is responsible for the change of seasons. Its rotational period is about 365.25 days or roughly one year.
c) A third less known motion of the Earth is called precession. The axis of rotation describes a cone in space over a period of about 25,800 years. This necessitates frequent correction in the length of the solar year to prevent the seasons from sliding through the year. This is done by means of a system of having or not having leap years (366 days) every four, one hundred, four hundred, and four thousand years. This also corrects for fractional number of days in a solar year.
Motion of the Moon
The Moon moves under the influence of gravity, mainly, of the Sun and the Earth. It is also disturbed slightly by other planets. Its orbit around Earth is only approximately elliptic because the Sun disturbs it. Its orbital plane is tilted in relation to the Earth's orbital plane (ecliptic) by about five degrees. The Moon's orbital plane is not fixed in space in relation to the stars. Even though it maintains a constant tilt in relation to the Earth's orbital plane, the line of intersection of these two planes (nodes) rotates slowly in space. Because of the tilt of its orbit the Moon appears to move to the north and south of the Sun, and the new crescent is sometimes seen to the left and at other times to the right of the setting sun.
Viewed from space the path of the Moon is like a sine wave super-imposed over the near circular path of the Earth.
The Moon produces no light of its own and shines due to Sun light. Like Earth only half of the surface of the Moon's sphere is illuminated by the Sun at any time. Depending upon the relative position of the three bodies we can see varying portions of the illuminated side of the Moon from the Earth. When the Moon is between the Earth and the Sun only the far side of the Moon is illuminated and we cannot see it. This position is called astronomical New Moon. When the Earth is between the Moon and the Sun and the side of Moon facing us is illuminated by the Sun we see full Moon. Viewed from the Moon, the Earth appears to change phases much like the Moon.
Figure-1 Below show the sphere of the Moon and the relationship of the directions of the Sun and the Earth at the time of first visibility of the Crescent.
Criterion to Determine Crescent Visibility:
It is apparent that Muslims have to use astronomical facts and data for fixing the beginning of Islamic new month and in particular the holy month of Ramadhan date. Astronomers have being trying to find a simple criterion to determine when a crescent will always be visible, but their efforts did not succeed until mid-1980s when a renewed interest in the visible moon forced some prominent western astronomers like:
L.E. Doggett, B.E. Schaeffer and others from the most respected Observatories like the US Naval Observatory, Royal Greenwich Observatory and others started collecting and evaluating "verifiable" observation reports from around the world. Mr. B.D. Yallop of Royal Greenwich Observatory uses moon's 10-degree altitude at sunset to accept earliest visibility. Others have recommended and angle of 12-degree at sunset for the Moon to be visible to naked-eye observers. Muslims experts who have gathered sighting reports for the last 16 years in North America have found that a 12-degree angle of the Moon's separation from the Sun along with an altitude of 10-degree at sunset is a more reliable criteria.
First Sighting of the Crescent in 1998
The table below outlines the calculated date, time and place of the first sighting of the crescent in the year 1998. The calculations are based on the criteria that the best place and time for making the first crescent sighting is where the crescent is vertically above the Sun at sunset, i.e., their relative azimuth is zero, and where the apparent altitude of the moon is 10-degrees.
In addition the Table provides the civil date and local time at sunset for the first sighting, time difference from universal time (UT), the time between sunset and moonset in minutes.
The longitude and latitude of the best place and age of the Moon since its birth or commonly referred as astronomical New Moon.
It should be noted the age is shortest when the Moon is nearest to the Earth and longest when the Moon is furthest from the Earth. The last column of the Table provides a approximate geographical description of the place.
SPACE FOR TABLE
What the Holy Qur'an Says
The Holy Qur'an says that the Sun and the Moon move according to laws and are for the reckoning of time. There has always been a tendency among common people of all faiths to ascribe some mystical meaning or significance to their appearance and motion. The Qur'an does not support such a view. To us they are nature's clocks to determine time.
Islamic conventions about time-keeping are different from those in common use in the Western world today. For example, to a Muslim a day (like Thursday or Friday, etc.) begins at sunset not midnight. A week, month, and year are similarly affected.
The 12-Islamic months are not of fixed duration. Muharram can sometime be 29 days and sometime 30 days, etc. The month starts with the visibility phase of the Moon and not with the birth of the Moon or astronomical New Moon as stated in some other calendars.
Calendars can be based on either the motion of the Moon or the Sun or both. Most lunar calendars are luni solar, that is although they count a month based on lunar cycle, they introduce some corrections to keep their calendar synchronized with the seasons that depend on solar cycle. Adding a 13th month every few years usually does this. This is known as intercalation.
The Qur'an forbids intercalation and clearly states that a year has 12 months. Since a lunar month on the average has about 29.5 days, the Islamic year has only 354 days. This injunction leaves the Islamic year about 11.25 days shorter than the average solar year. For this reason our observances move through the seasons and come about 11 days earlier in each succeeding year.
International Lunar Date Line
Just as there is a solar date line (180-degrees longitude) to mark the start of a date on a solar calendar, a lunar date line can be constructed to indicate the start of a lunar date. Because an Islamic Month begins in the evening with the first visibility of the Crescent, these points of first visibility can be considered to constitute the lunar date line.
For a given solar dip angle and angular separation between the Moon and the Sun, the Moon appears higher above the horizon at sunset near the equator than at high latitudes. For this reason higher contrast is achieved in the tropical region. As we move to higher latitudes, because of reduced contrast, bigger crescent is required for visibility and visibility is delayed. For this reason the line of first visibility appears like a parabola enlarging towards the west.
