As salaamu alaykum wa Rahmatullahi wa Barakatahu:
There is a possibility that the moon of Shawwal may not be sighted in the Eastern Hemisphere on Wednesday, August 7, 2013 and subsequently many Muslim countries may announce Eid al-Fitr on Friday, August 9th. For instance on Wednesday, August 7th, sunset at Makkah is 6:56 p.m. and moonset is at 7:07 p.m. The moon will stay on the Makkan horizon just for 11 minutes after the sunset. This may not be sufficient enough of a time period for the moon to be seen.
The Fiqh Council of North America (FCNA) has already announced that Eid al-Fitr will be on Thursday, August 8, 2013. Following are the reasons for this decision:
1: The Astronomical New Moon is on August 6, 2013 (Tuesday) at 21:51 Universal Time. The moon will be almost 29 hours of age by the time it is on the Los Angeles, California horizon. It will stay in the horizon about 17 minutes after the sunset.
2: The FCNA takes scientifically guaranteed astronomical calculations as the valid method of lunar month's affirmation. The calculations ascertain that the new moon of 29 hours old will be on Southern California horizon on August 7, 2013, therefore we accept that certain knowledge as a Sharai' reason to declare that Thursday, August 8, 2013 will be the first day of Shawwal.
3: Additionally the new moon will be sight-able by a telescope in California. That possibility of a telescopic sighting in California strengthens FCNA's position. Many Muslim jurists accept telescopic sighting as a genuine Sharai' sighting.
4: The moon should easily be sighted in Chile, South America, by the naked eye. Chile is in the same timezone as the US Eastern timezone and shares the night with us. The definite news of naked-eye sightings should arrive to the US by 7:00 pm PDT, long before the sunset in California. That naked-eye sighting report will additionally substantiate the FCNA's position as we share the night with Chile.
5: Both California's telescopic and Chile's naked eye sighting will also fulfill the global sighting criterion which is the original juristic criterion for the affirmation of lunar months. All four Imams have agreed that moon sighting anywhere in the globe is sufficient of a Sharai' reason to declare the new Islamic month. That criterion will be fulfilled on Wednesday, August 7th as the new moon will be sight-able by telescope in California and by the naked eye in Chile.
6: Many established Sharai' criteria of the new month's declaration are being fulfilled on Wednesday, August 7, 2013. Therefore, as announced earlier, Eid al-Fitr will be on Thursday, August 8, 2013.
Eid al-Fitr Mubarak!