| U.S. Naval Observatory | Astronomical Applications Department |
|
|
Crescent Moon Visibility |
The Islamic calendar is based on lunar months, which begin when the thin crescent Moon is actually sighted in the western sky after sunset within a day or so after New Moon. The ancient Hebrew calendar was also based on actual lunar crescent sightings, although the modern Hebrew calendar is calculated.
The 12 months of the Islamic calendar are: Muharram, Safar, Rabi'a I, Rabi'a II, Jumada I, Jumada II, Rajab, Sha'ban, Ramadan, Shawwal, Dhu al-Q'adah, Dhu al-Hijjah. Since 12 lunar months are, on average, 11 days shorter than the (Gregorian) civil year, the Islamic year shifts earlier in each civil year by about this amount. The count of years for the Islamic calendar begins in 622 CE; specificaly, 1 Muharram 1 AH corresponds to 16 July 622 CE (Julian calendar).
A tabular Islamic calendar has been established for some non-religious purposes in which the lengths of the months alternate between 29 and 30 days; in leap years an extra day is added to the last month, Dhu al-Hijjah. This calendar consists of a 30-year cycle in which 11 of the 30 years are leap years. Civil dates corresponding to important Islamic dates in this tabular calendar are:
Islamic New Year 1425 - 2004 February 22The Islamic dates begin at sunset on the previous evening and end at sunset on the date listed above. These dates may or may not correspond to the evenings on which the crescent Moon is first visible, and it is the visibility of the crescent Moon that determines when the religious observance begins. The Moon's visibility at these times varies with location; generally, the visibility increases to the west, and locations in the tropics are favored over those in middle or high latitudes.
First day of Ramadan - 2004 October 15
Islamic New Year 1426 - 2005 February 10
First day of Ramadan - 2005 October 4
Islamic New Year 1427 - 2006 January 31
First day of Ramadan - 2006 September 24
Islamic New Year 1428 - 2007 January 20
First day of Ramadan - 2007 September 13
The visibility of the lunar crescent as a function of the Moon's "age" - the time counted from New Moon - is obviously of great importance to Muslims. The date and time of each New Moon can be computed exactly (see, for example, Phases of the Moon in Data Services) but the time that the Moon first becomes visible after the New Moon depends on many factors and cannot be predicted with certainty. In the first two days after New Moon, the young crescent Moon appears very low in the western sky after sunset, and must be viewed through bright twilight. It sets shortly after sunset. The sighting of the lunar crescent within one day of New Moon is usually difficult. The crescent at this time is quite thin, has a low surface brightness, and can easily be lost in the twilight. Generally, the lunar crescent will become visible to suitably-located, experienced observers with good sky conditions about one day after New Moon. However, the time that the crescent actually becomes visible varies quite a bit from one month to another. The record for an early sighting of a lunar crescent, with a telescope, is 12.1 hours after New Moon; for naked-eye sightings, the record is 15.5 hours from New Moon. These are exceptional observations and crescent sightings this early in the lunar month should not be expected as the norm. For Islamic calendar purposes, the sighting must be made with the unaided eye.
Obviously, the visibility of the young lunar crescent depends on sky conditions and the location, experience, and preparation of the observer. Generally, low latitude and high altitude observers who know exactly where and when to look will be favored. For observers at mid-northern latitudes, months near the spring equinox are also favored, because the ecliptic makes a relatively steep angle to the western horizon at sunset during these months (tending to make the Moon's altitude greater).
If we ignore local conditions for the moment, and visualize the problem from outside the Earth's atmosphere, the size and brightness of the lunar crescent depend on only one astronomical quantity - the elongation of the Moon from the Sun, which is the apparent angular distance between their centers. For this reason the elongation has also been called the arc of light. If we know the value of the elongation at any instant, we can immediately compute the width of the crescent.
What is the value of the elongation when the Moon's age is one day? It varies, depending on several factors:
-
(1) The elongation at New Moon. The Moon can pass directly in front of the Sun at New Moon (when a solar eclipse will occur) or can pass as far as five degrees away. That is, the Moon can start the month with an elongation ranging from zero to five degrees. A minor complicating factor involves the definition of New Moon in the almanacs. Astronomical New Moon is defined to occur when the Sun and Moon have the same geocentric ecliptic longitude, which may not occur precisely when the Sun and Moon are closest together in the sky.
(2) The speed of the Moon in its orbit. The Moon's orbit is elliptical, and its speed is greatest when it is near perigee, least near apogee. If perigee occurs near New Moon, the Moon will appear to be moving away from the Sun in the sky at a greater than average rate.
(3) The distance of the Moon. Again, because of its elliptical orbit, the distance of the Moon varies, so even if the Moon moved with a constant speed, its angular motion as viewed from the Earth would be greater when the Moon is near perigee.
(4) The location of the observer. If the observer is located in the tropics such that the one-day-old-Moon is observed just before it sets, its elongation as seen by the observer will be about a degree less than that seen by a fictitious observer at the center of the Earth, which is the basis for most almanac calculations. This decrease in observed elongation is less for observers at middle or high latitudes (although other geometric factors are less favorable for these observers).
Factors (2) and (3) are linked by Kepler's second law, which predicts that the angular speed of the Moon as seen from the Earth will vary by about 22%. If we combine all these factors we find that geocentric elongation of the Moon from the Sun at an age of one day can vary between about 10 and 15 degrees.
This large range of possible elongations in the one-day-old Moon is critical, because at this time the width of the crescent is increasing with the square of the elongation, and the surface brightness of the crescent is also rapidly increasing. Some of the earliest reliable sightings of the crescent occur near elongations of around 10 degrees. Obviously, simply specifying the age of the Moon cannot tell the whole story. Of course, the elongation of the Moon does not tell the full story, either. But, of the two parameters, the elongation is a much more reliable parameter to use as a starting point in assessing the lunar crescent visibility at any given date and time.
The prediction of the first sighting of the early crescent Moon is an interesting problem because it simultaneously involves a number of highly non-linear effects. Stated in less technical language, a lot of things are changing very rapidly. Effects to be considered are the geometry of the Sun, Moon, and horizon; the width and surface brightness of the crescent; the absorption of the Moon's light and the scattering of the Sun's light in the Earth's atmosphere; and the physiology of human vision. The problem has a rich literature. Some modern astronomical references are:
-
Schaefer, B. E., 1988: "Visibility of the Lunar Crescent", Quarterly Journal of the Royal Astronomical Society, Vol. 29, pp. 511-523.
Schaefer, B. E., Ahmad, I. A., Doggett, L. E., 1993: "Records for Young Moon Sightings", Quarterly Journal of the Royal Astronomical Society, Vol. 34, pp. 53-56.
Ilyas, M., 1994: "Lunar Crescent Visibility Criterion and Islamic Calendar", Quarterly Journal of the Royal Astronomical Society, Vol. 35, pp. 425-461.
Doggett, L. E., Schaefer, B. E., 1994: "Lunar Crescent Visibility", Icarus, Vol. 107, pp. 388-403.
M. B. Pepin, 1996: "In Quest of the Youngest Moon", Sky & Telescope, December 1996, pp. 104-106.
Her Majesty's Nautical Almanac Office computes and distributes predictions of lunar crescent visibility. The Astronomical Calendar by Guy Ottewell (published annually by Furman University, Greenville, SC) includes good diagrams of the positions of young and old Moons during the year (drawn for the eastern U.S.) and an explanation of the factors affecting their visibility.
Related information on these web pages includes:
Phases of the Moon and Percent of the Moon Illuminated (definitions) in FAQ
Phases of the Moon in Data Services
Complete Sun and Moon Data for One Day in Data Services
- G. H. Kaplan
[News | Site Map | Index | Home]
|
Last modified March 28, 2005 at 16:05 |
***************************************************************************
{Invite (mankind, O Muhammad ) to the Way of your Lord (i.e. Islam) with wisdom (i.e. with the Divine Inspiration and the Qur'an) and fair preaching, and argue with them in a way that is better. Truly, your Lord knows best who has gone astray from His Path, and He is the Best Aware of those who are guided.}
(Holy Quran-16:125)
{And who is better in speech than he who [says: "My Lord is Allah (believes in His Oneness)," and then stands straight (acts upon His Order), and] invites (men) to Allah's (Islamic Monotheism), and does righteous deeds, and says: "I am one of the Muslims."} (Holy Quran-41:33)
The prophet (peace and blessings of Allah be upon him) said: "By Allah, if Allah guides one person by you, it is better for you than the best types of camels." [al-Bukhaaree, Muslim]
The prophet (peace and blessings of Allah be upon him) also said, "Whoever calls to guidance will have a reward similar to the reward of the one who follows him, without the reward of either of them being lessened at all."
[Muslim, Ahmad, Aboo Daawood, an-Nasaa'ee, at-Tirmidhee, Ibn Maajah]
--------------------------------------------------------------------------
All views expressed herein belong to the individuals concerned and do not in any way reflect the official views of IslamCity unless sanctioned or approved otherwise.
If your mailbox clogged with mails from IslamCity, you may wish to get a daily digest of emails by logging-on to http://www.yahoogroups.com to change your mail delivery settings or email the moderators at [EMAIL PROTECTED] with the title "change to daily digest".
| Holy quran | Converts to islam | Islam |
| Comparative religion | Women in islam |
YAHOO! GROUPS LINKS
- Visit your group "islamcity" on the web.
- To unsubscribe from this group, send an email to:
[EMAIL PROTECTED]
- Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service.
