Hilal

Due to positive naked-eye Hilal sighting reports on Tuesday, March 28th, 2017G (positive reports from California, Arizona), HSCNA announces the start of the month of Rajab 1438H from Wednesday, March 29th, 2017G in North America

arrow Home FAQs Monday, 24 April 2017  
Home
News
About Us
Hijri Calendar
Articles
FAQs
Contact Us
Announcements
Hilal Visibility Curves
Criteria
FAQs Print E-mail
Written by Administrator   
Sunday, 04 March 2007 01:44
Hilal (Crescent) Sighting

Frequently Asked Questions


1. Are there some locations on the globe inherently better than others for sighting the new crescent?

2. Could you please clarify a notion that astronomy is forbidden in Islam?

3. Is it possible to see the moon near the sun in the morning and during sunset in the same day? And when does that happen

4. The Prophet (Peace be upon him) went to Arafat on Friday the 9th Dhul Hijjah of 10 AH which was on March 6, 632 CE. Is it correct according to calculation? Give detailed answer with exact time?


5. If we go to higher altitudes like on top of a mountain or in an airplane, does it help for moon-sighting?

6. Was astronomy relied upon thousands of years ago, especially when cloudy weather obstructed visual observation?


7. Is the calculation of solar movement more exact than the calculation of the lunar movement?

8. Is it possible to have several consecutive months of 29 days and to have several consecutive months of 30 days and what is the highest possible number?

9. What is The Astronomical New Moon?

10. What is the relevance of Elongation in crescent sighting?

11. What is the relevance of Sunset in crescent sighting?

12. What is the relevance of Moonset time in crescent sighting?

13. Is there an accepted criterion for crescent sighting?

14. When has the earliest crescent of a new moon been seen after the moon was born?

15. What is the process one must use to correctly sight the moon?

16. Why do we always see the same side of the Moon from Earth?

17. Why does the Moon have phases?

18. Why does the Moon look bigger at the horizon?

19. What is the Moon made of?

20. Why is the distance between Earth and Moon increasing?

21. What causes the bright halo that sometimes surrounds the Moon?


22. What is the difference between a lunar and a solar eclipse?

23. What is the value of the elongation when the Moon's age is one day?


________________________________________

1. Are there some locations on the globe inherently better than others for sighting the new crescent?

YES. These are locations that have factors that are better for optics than other locations.

Locations looking out over the ocean are better than those looking in the direction of a populated city.

Poor air transparency due to molecules and dust suspended in the air is also bad for visibility. The air transparency is better for higher elevations like mountain tops.

Urban and industrial areas are at a disadvantage compared with rural and more arid areas.

The average cloud cover is significantly higher at high latitudes and near equator than in the subtropics and lower temperate zones.

The haze effect also known as light extinction dims the crescent by a large factor. At the low altitude where the most marginal crescents would be seen, only about 5% of the light from the crescent can penetrate the air, and 95 percent is lost because it is scattered away in other directions, even in the cleanest air.

In a humid or polluted environment, much less than 1 percent of the light of a thin crescent comes to the earth.

If the background light from the rest of the sky is brighter than the thin crescent, it would render the crescent invisible.

________________________________________

2. Could you please clarify a notion that astronomy is forbidden in Islam?

The notion that astronomy is prohibited in Islam is a misconception that emerged some 1200 years ago when Astrology and Astronomy were not considered separate science. Then Ilm-un-Nojoom was encompassing both astronomy and astrology. At that time Muslim Ulamaa said something like this that it is prohibited in Islam.

Now it is clearly understood that Astrology is prohibited but Astronomy (Ilm-ul-falakyat) is not. Astronomy is the knowledge of movement and position of cosmos (heavenly bodies), while Astrology is the effect of position of those heavenly bodies on human beings' future. Future is not known to anyone except Allah subhaana wa Taala, so Astrology is considered "Haram" in Islam.

________________________________________

3. Is it possible to see the moon near the sun in the morning and during sunset in the same day? And when does that happen?

No. But it is possible to see the Moon on one morning before sunrise in the East, and then on the next day's evening (i.e. after about 36 hours) it can be seen after sunset in the West. This happens very seldom for a specific location. Usually the Moon remains hidden for two days, a day before new moon, and a day after new moon.

________________________________________

4. The Prophet (Peace be upon him) went to Arafat on Friday the 9th Dhul Hijjah of 10 AH which was on March 6, 632 CE. Is it correct according to calculation? Give detailed answer with exact time?

Very correct. The calculation confirms that new moon occurred at 21:07 GMT on February 25, 632 CE(Tuesday). On the following day, February 26, 632 CE in Medinah, the calculations for the moon show the following results:

Sunset at 6:25 pm, moonset at 7:04 pm. At sunset, the elongation is 10.1, Age of moon 18.3 hours, and Altitude above horizon 7.6°.

This crescent would have been visible on February 26, 632 CE(Wednesday), and the 1st of Dhul Hijjah would have been on February 27, 632 CE(Thursday). Therefore the day of Arafat, 9th Dhul Hijjah would have been on March 6, 632 CE(Friday).

________________________________________

5. If we go to higher altitudes like on top of a mountain or in an airplane, does it help for moon-sighting?

Going to a higher elevation really does not change the situation except for a small time delay added to the setting times for sun and moon. This delay by itself helps very little. However, going to higher elevation helps the visibility, because the higher the elevation, the better is the air transparency, and the lower is the light extinction. But this does not mean that a moon of 10 hours old can be seen from high mountains or even from airplanes.

________________________________________

6. Was astronomy relied upon thousands of years ago, especially when cloudy weather obstructed visual observation?

Thousands of years ago, astronomy was not as accurate as it is today.

But then there were not many man-made objects in the sky either. Now a days people can see many man-made objects and think they saw the moon. We are trying to educate the people, so they understand the basic known facts about the moon, and when it is impossible to see. This way, when a Muslim or a group of Muslims see something and believe it to be the new crescent, we all can differentiate that it was not the crescent.

________________________________________

7. Is the calculation of solar movement more exact than the calculation of the lunar movement?

Calculation for the movement of the Moon is as accurate as that of the

Sun. However, the Sun, being a source of light, can be seen easily, while the Moon, which does not have any light of its own, can only be seen when the Sun is in such a position that its light falling on the Moon can come to the earth.

Many a time the Moon may be above horizon, but it cannot be seen because the Sun is in such a position that its rays coming to the Moon do not make a sufficient thickness of crescent to be seen from earth.

________________________________________

8. Is it possible to have several consecutive months of 29 days and to have several consecutive months of 30 days and what is the highest possible number?

Yes. According to calculations of moon-sighting, and actually observed, up to three consecutive months of 29 days and up to four consecutive months of 30 days are possible. This is nothing unusual. This happens quite often.

________________________________________

9. What is the astronomical New Moon?

The astronomical New Moon, or the invisible New Moon, is the moment when the moon is almost directly between the earth and the sun. At this moment the moon's dark surface faces the earth. Therefore it is completely invisible even if it occurs in the middle of the night. The Astronomical New Moon occurs every 29 days, 12 hours, 44 minutes, and 2.8 seconds. The date and time of this invisible New Moon is commonly found in almanacs, newspapers and calendars.

________________________________________

10. What is the relevance of Elongation in crescent sighting?


Elongation is the angular distance between the moon and the sun. At The Astronomical New Moon this angle is zero and all the sunlight reflected by the moon surface is directed back towards the sun. With passage of time, the moon in its circular motion around the earth, moves away from the alignment with the earth and the sun. The angular distance between the moon and the sun increases, and some 18 to 24 hours after the Astronomical New Moon, the moon will have moved sufficiently for the sunlight reflected off its outer edge to be directed towards the earth. The new crescent moon can now become visible on earth.

Elongation is a critical factor in determining the crescent's visibility, and this depends on more than just time. In the early 1930's, the French astronomer Andre' Danjon deduced (from detailed consideration of the geology of the Moon) that no illuminated portion of the crescent can be seen from the earth when the Moon is 7° or less from the Sun. No sighting can penetrate this barrier, which is set by shadows cast by the rough lunar terrain. This is the limit of our human observational power, and below this the crescent will be impossible to see, even with a telescope

________________________________________

11. What is the relevance of Sunset in crescent sighting?

In the first sixty hours or so after The Astronomical New Moon, the sunlight reflected by the moon is so weak that the crescent cannot be observed while the Sun is in the sky. Generally, for sighting the new crescent, one should start to search the sky about 15 minutes after sunset. One should look in the direction of the setting sun, just above, or up to 30 degrees to the right, or 30 degrees to the left of where the Sun set. One should keep looking until the time of moonset that can be obtained from local newspapers. Binoculars do help.

________________________________________

12. What is the relevance of Moonset time in crescent sighting?

Moonset time is the time when the upper limb of the Moon is on the horizon. If the sky is clear and the horizon is flat, crescent sighting should be possible some 15 minutes after sunset. Quite obviously there cannot be a crescent sighting if this is after the moonset time. The moonset time can be obtained from local newspapers.

________________________________________

13. Is there an accepted criterion for crescent sighting?

It is easy to design a criterion based on certain theories or confirmed sightings. But it is hard for that criterion to withstand the test of actual sighting. We have to be the judge and see if the criterion comes out true to the actual sighting month after month, and year after year. In our own community, records go back to 1932. Month after month, circulars detailing predictions have been regularly published, followed by notification of confirmed sightings. Our community has published calendars from different parts of the world, with remarkable accuracy. This includes the planner from the World Federation.

With many years of experience, we have fine tuned our criteria of prediction, so that our calculations are borne out by observation time and again. Today our main criterion depends on the Moon's angular distance from the Sun at sunset (elongation) and the Moon's altitude (in degrees) above the horizon after sunset. The age of the Moon is recorded as a guideline.

Elongation at sunset, the Moon / Sun angular separation, should be not less than 9°

The Crescent must be well above the horizon when the sun has dropped 5° below the horizon.

Crescent age at sunset is normally not less than 20 hours.

________________________________________

14. When has the earliest crescent of a new moon been seen after the moon was born?

Moon sighting does not depend on age. Non-Muslims have treated this question as a sport, who can see the youngest moon. In the zeal of their competition, they claim early and early sightings, which are in most cases false, as many claims have been refuted with no sighting at places thousands of miles west. The authentic sightings are at about 17.2 hours with the naked eye, and 15.5 hours by observatory telescopes. Remember, in some seasons, earliest moon-sighting takes about 24 hours. So if the moon has become 17 hours old or more, one can not conclude that it is possible to see that moon.

________________________________________

15. What is the process one must use to correctly sight the moon?

If you know that the beginning of the month was based on an authentic crescent sighting, count 29 days from the start date and start looking about 15 minutes after sunset. Look in the direction where the sun set, just above, or up to 30 degrees to the right, or 30 degrees to the left. Keep looking until the time of moonset that you can obtain from local newspapers. Binoculars help. Have one or more persons with you, if possible. If you do this for a few months, you will know yourself about improving your procedure.

You should keep in mind the distinction between occasions when the Crescent is "difficult but DEFINITELY and repeatedly seen", and occasions when the Crescent is "possibly fleetingly glimpsed but NOT DEFINITELY seen." For the former the Crescent should be continuously visible for at least five seconds in a sky location which is consistent from one minute to the next and the shape of the brightening should be consistent with the Moon-Sun angle. If in ANY doubt, accept that you haven't objectively seen the Crescent.
________________________________________

16. Why do we always see the same side of the Moon from Earth?

The Moon always shows us the same face because Earth's gravity has slowed down the Moon's rotational speed. The Moon takes as much time to rotate once on its axis as it takes to complete one orbit of Earth. (Both are about 27.3 Earth days.) In other words, the Moon rotates enough each day to compensate for the angle it sweeps out in its orbit around Earth.

Gravitational forces between Earth and the Moon drain the pair of their rotational energy. We see the effect of the Moon in the ocean tides. Likewise, Earth's gravity creates a detectable bulge -- a 60-foot land tide -- on the Moon. Eons from now, the same sides of Earth and Moon may forever face each other, as if dancing hand in hand, though the Sun may balloon into a red giant, destroying Earth and the Moon, before this happens.

________________________________________

17. Why does the Moon have phases?

The Moon has phases because it orbits Earth, which causes the portion we see illuminated to change. The Moon takes 27.3 days to orbit Earth, but the lunar phase cycle (from new Moon to new Moon) is 29.5 days. The Moon spends the extra 2.2 days "catching up" because Earth travels about 45 million miles around the Sun during the time the Moon completes one orbit around Earth.

At the new Moon phase, the Moon is so close to the Sun in the sky that none of the side facing Earth is illuminated. In other words, the Moon is between Earth and Sun. At first quarter, the half-lit Moon is highest in the sky at sunset, then sets about six hours later. At full Moon, the Moon is behind Earth in space with respect to the Sun. As the Sun sets, the Moon rises with the side that faces Earth fully exposed to sunlight.

You can create a mockup of the relationship between Sun, Earth, and Moon using a bright lamp, a basketball, and a baseball. Mark a spot on the basketball, which represents you as an observer on Earth, then play with various alignments of Earth and Moon in the light of your imaginary Sun.

________________________________________

18. Why does the Moon look bigger at the horizon?

The "Moon illusion," in which the Moon appears larger than normal when close to the horizon, is not the result of magnification by the atmosphere or a change in Earth-Moon distance. Instead, the answer is, as Einstein might say, completely relative.

At most times we see the Moon high in the sky among thousands of stars. We develop our sense of how "big" the Moon ordinarily appears by comparing it with the vast panorama of outer space.

When the Moon is nestled along the horizon, however, we see it surrounded by a foreground of familiar Earth-bound objects -- trees, buildings, or distant landmarks. In comparison with these everyday features, the bright disk of the full Moon appears quite large indeed, and relative to our "normal" sense of the Moon's size, much bigger than we would expect.

________________________________________

19. What is the Moon made of?

The Moon's composition and internal structure are not known in great detail. Most of what we do know comes from studying samples of the lunar surface returned by Apollo missions and from seismic studies performed both remotely from Earth and by astronauts on the Moon. Rocks from the lunar surface are all igneous, having been created below the surface then carried to it by meteor impacts or by the Moon's now-defunct volcanoes. They consist primarily of typical Earth minerals like olivine, feldspar, and quartz, though some minerals found are unique to the Moon -- tranquillityite, armalcolite, and pyroxferroite.

As for the Moon's interior, our understanding of its structure is much more limited. Most scientists agree on the following general concepts: the remains of countless meteor impacts and volcanic flows dominate the first 60 miles below the surface; the next 60 to 100 miles of lunar "crust" is rich in metals. Deeper below the surface, the structure becomes less clear; starting about 100 miles below the lunar surface is a region that appears to have been molten in the past -- how deep this goes remains a subject of debate. Some scientists argue that this region extends through to the center, while others suggest that there is a separate, partially molten iron core as much as 400 miles in radius. Obviously, more work needs to be done to settle these questions.

Rumors of gold and diamonds on the Moon are a bit farfetched. While trace quantities of gold have been found in Moon rocks, they appear to have been deposited by falling meteors -- not a good prospect for potential miners.

________________________________________

20. Why is the distance between Earth and Moon increasing?

Just like a spinning ice skater whose rotation slows as he extends his arms, the Earth-Moon distance is lengthening because Earth is spinning slower each day. The Moon's gravitational influence is slowing Earth's rate of rotation down by one and a half thousandths of a second every 100 years. The loss of rotational energy -- angular momentum, for the physicists in the crowd -- is necessarily matched by an increase in the Moon's angular momentum, which results in a larger orbit for the Moon.

Currently, the Moon moves less than two inches a year farther away from Earth -- a tiny amount, but easily measurable with modern laser-ranging devices.

________________________________________

21. What causes the bright halo that sometimes surrounds the Moon?

This bright ring of light is quite common. It's related to the same process that creates rainbows: refraction. In this case, moonlight shines through a layer of ice crystals high in the atmosphere. The ice crystals act like prisms, splitting the light into a rainbow of colors that surrounds the Moon. If the effect is intense enough, the colors are visible to the unaided eye. If not, then the ring around the Moon looks milky white.

Incidentally, the ring has some practical value, too. The thin clouds that cause the rings often precede cold fronts or storm systems, so they may indicate that rainy weather is on the way.

________________________________________

22. What is the difference between a lunar and a solar eclipse?

From our perspective on Earth, two types of eclipses occur: lunar, the blocking of the Moon by Earth's shadow, and solar, the obstruction of the Sun by the Moon.

When the Moon passes between Sun and Earth, the lunar shadow is seen as a solar eclipse on Earth. When Earth passes directly between Sun and Moon, its shadow creates a lunar eclipse.

Lunar eclipses can only happen when the Moon is opposite the Sun in the sky, a monthly occurrence we know as a full Moon. But lunar eclipses do not occur every month because the Moon's orbit is tilted five degrees from Earth's orbit around the Sun. Without the tilt, lunar eclipses would occur every month.

Lunar and solar eclipses occur with about equal frequency. Lunar eclipses are more widely visible because Earth casts a much larger shadow on the Moon during a lunar eclipse than the Moon casts on Earth during a solar eclipse. As a result, you are more likely to see a lunar eclipse than a solar eclipse.

________________________________________

23. 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.

Last Updated on Sunday, 19 August 2007 06:45
 


Hilal Sighting

Hilal Sighting & Islamic Dates: Issues and Solution by Dr. Salman Shaikh, Coordinator, Hilal Sighting Committee of North America Click here to open
CURRENT MOON