No-one knows exactly when the new Moon appears as it changes in different parts of the world - particularly important if you want to know when Ramadan starts. So this month, scientists are asking the public to help refine the lunar calendar.
For thousands of years, man has looked to the heavens for spiritual solace. Many religions also rely on celestial bodies to determine the timings of their most important annual festivals.
None more so than the Islamic holy month of Ramadan, which begins with the first sighting of the new crescent Moon. But for years it has been beset with debate over whether this first sighting can be accurately measured - and so for some it began on Tuesday night, for others Wednesday.
Ramadan involves fasting, spiritual contemplation and giving to charity. Its end, 29 or 30 days later when the next new Moon is seen, is the festival of Eid, a time of celebration as people reflect on their faith and lives.
Dr Usama Hasan, of the Muslim Council of Britain, says: "I look forward to fasting as a time to get rid of bad habits and concentrate my mind through prayer. As soon as the new Moon becomes visible about 20 minutes after sunset, it signals that fasting begins in the early hours of the next morning.
Easter falls on first Sunday after a full Moon
"The Koran says: 'Begin fasting when you see the new crescent Moon; end fasting when you see it again'. The new Moon only becomes visible when it is the correct distance from the Sun to reflect enough light."
Sky at night
But exactly when this new Moon first appears is the subject of annual controversy - so much so that families split by the continents find themselves divided over when to start the holy month.
While the Ramadan cue in the West tends to come from Muslim nations in the East, in astronomical terms, the moon appears differently around the world.
Secondly, the very faint new Moon can be hard to spot in the UK and northern Europe because of the weather conditions and light pollution in cities.
Ramadan, already underway elsewhere, starts today in the UK
Does this matter? For many, no. They follow Islam's holiest site, Mecca in Saudi Arabia. But for others it does.
Dr Khalil Roberts, of the Islamic Astronomical Society, said that some British Muslims have stopped taking the cue from the East.
"Islamic teachings encourage the use of technology to help establish Islamic dates, hence British Muslims feel it is important to reconcile the two and not feel pressured into following inaccurate decisions made in other parts of the world."
This may sound like an arcane theological discussion to some, but it has financial implications. Ramadan, like Christmas for Christians, is a time of holiday. Like any major festival, it affects the economy.
This is where the Moon Watch scientists come in - and the public, whose help they need.
On Wednesday the Institute of Physics (IoP) and HM Nautical Almanac Office (HMNAO) are launching a nationwide study to work out if sightings of the new Moon match predictions. This Einstein Year project coincides with Ramadan, so the team hope Muslims will help supply the data.
Project leader Dr Steve Bell, of the HMNAO at the Rutherford Appleton Laboratory in Oxfordshire, says the aim is to find out when the naked eye can spot the new Moon.
"We know exactly where the Moon is in the sky - the question is can we see it with our eyes or with an optical aid?"
The new Moon occurs after sunset within three days of the new lunar month - but the timing depends on the relative positions of the Sun, the Moon and the observer.
Data on the Moon's visibility mostly comes from sightings at more southerly latitudes, including the US. But these predictions are not accurate for Europe, hence the project.
Wax and wane
As the Moon passes through the lunar month, some 29 and a half days, its visibility varies according to the amount of its surface illuminated by direct sunlight, and where it sits in the sky relative to Earth's position to the Sun. Click here for the phases of the Moon.
The Moon is not visible at the start of each lunar month. The first sighting is a crescent, which thickens through to first quarter to a full Moon, and then wanes until it can be seen no more.
Special dishes are made
Models used to predict its location are accepted as accurate - but it's the visibility that is in doubt. In the Tropics, the Moon appears higher than locations further north. The further north the observer, the more likely it is to be low in the sky - and therefore obscured by the twilight.
"Over its 19-year cycle, the Moon ranges from -28.5 degrees to +28.5 degrees in its angle to the Earth's equator," says Dr Bell.
"This has a direct bearing on how easy it is to spot the new crescent Moon. Couple this with the relative positions of the Sun and the Moon, and it can be shown that tropical latitudes, such as those in Saudi Arabia or South East Asia, will see the new crescent Moon earlier than elsewhere."
He hopes that the project will allow scientists to check observations against current predictions, providing information to improve the models where necessary.
Those keen to take part can submit their postcode, weather conditions, date, time and orientation of the crescent to the Moon Watch website (see Internet links on right). The team will then incorporate this information into existing astronomical models, which are used to generate dates for the Islamic calendar.
1: new Moon: not visible as unilluminated side faces Earth
2. waxing crescent: an increasing fraction - but less than half - lit by direct sunlight
3. first quarter: half of the Moon lit by direct sunlight, with fraction visible increasing
4. waxing gibbous: more than half illuminated
5. full Moon: whole illuminated side faces Earth
6. waning gibbous: more than half illuminated, but fraction visible decreasing
7. last quarter: half of the Moon lit by direct sunlight
8. waning crescent: a decreasing sliver visible
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