Ramadan is upon us but when exactly does it start and finish?

And why is there such uncertainty over the date of such an important religous festival?

The reason is that the Islamic calendar is based on the moon so dates move around from year to year.

And since the Gregorian calendar used by much of the western world is based on the sun, Islamic festivals slide back by around 11 days a year.

The dates for the start of each month in the Muslim calendar aren't known until there is a confirmed sighting of the first crescent of the new moon, known as Hilal - although some organisations stick to astronomical charts so they can plan ahead.

Now a UK Government agency has weighed in with its own predictions.

The HM Nautical Almanac Office has given information on sightings of the moon for the start of Ramadan and Eid al-Fitr .

The office provides astronomical data for the police, Royal Navy, British Army and many other organisations including schools, diary and calendar manufacturers, religious groups and film-makers.

For Ramadan - the month of daily fasting - the agency says the new moon begins on Tuesday, May 15, but no naked eye sightings are likely anywhere in the world.

But for observers in the Middle East, North Africa and most of the United Kingdom except Scotland, easy sightings of the first crescent of the new moon should be possible on Wednesday, May 16.

Indian Muslims offer prayers on Juma-tul-wida or the last Friday of Ramadan at Jama Masjid, or the Grand Mosque, in New Delhi

People in Scotland will have to wait until Thursday, May 17, to see the moon, when it will be easy to see across the whole of the world.

Muslim days start at sunset so Ramadan is expected to begin on the evening of May 15 or 16 following confirmation of a sighting of the moon.

Despite predictions that the moon won't be seen on Tuesday night, The Supreme Court of Saudi Arabia is asking people to look out for it using the naked eye or binoculars.

Many Muslim organisations are already taking the start of Ramadan as the evening of May 15.

Mosques in Birmingham have unveiled their timetables for prayers, fasting, suhoor and iftar during Ramadan and you can see those below:

Visibility of the New Crescent Moon for Ramadan (times in UK BST)BradfordTimes in BSTi.e. GMT+1

Not Visible

Easily VisibleSS: 21:04MS: 22:13

Easily VisibleSS: 21:06MS: 23:27

Easily VisibleSS: 21:07MS: **:**

LeedsTimes in BSTi.e. GMT+1

Not Visible

Easily VisibleSS: 21:04MS: 22:13

Easily VisibleSS: 21:05MS: 23:26

Easily VisibleSS: 21:07MS: **:**

BelfastTimes in BSTi.e. GMT+1

Not Visible

Easily VisibleSS: 21:25MS: 22:34

Easily VisibleSS: 21:26MS: 23:48

Easily VisibleSS: 21:28MS: **:**

NewcastleTimes in BSTi.e. GMT+1

Not Visible

Easily VisibleSS: 21:09MS: 22:18

Easily VisibleSS: 21:11MS: 23:32

Easily VisibleSS: 21:13MS: **:**

GlasgowTimes in BSTi.e. GMT+1

Not Visible

Visible Under Excellent ConditionsSS: 21:25MS: 22:34

Easily VisibleSS: 21:27MS: 23:48

Easily VisibleSS: 21:28MS: **:**

When Ramadan ends, the next month (Shawwal) begins with a festival called Eid al-Fitr celebrating the end of the fasting.

The almanac office says the new moon will begin on Wednesday, June 13, but it won't be visible.

For observers in parts of Saudi Arabia and North Africa, it may be possible to see the crescent with the naked eye - if there are excellent conditions with clear skies - on Thursday, June 14.

Easy sightings of the new crescent moon should be possible in the Middle East, North Africa and the United Kingdom on Friday, June 15.

If conditions are good enough for the moon to be seen on June 14, it means Ramadan has ended and Shawwal begins.

If the moon isn't seen, Ramadan will go on for another day and start on June 15.

Visibility of the New Crescent Moon for Eid al-Fitr (times in UK BST)See latest deals on flights to Mecca with Skyscanner and Tripadvisor.

Download our all-new mobile app to get the latest news, sport and what's on. Click here for iPhone and here for Android

Looking for an older story? Search our archives

Search for jobs, motors and property, or place an advert or family notice here.