Muslims in Britain - where the night sky tends to be cloudy and or light-polluted - have less chance of spotting the slight, crescent-shaped moon than those in equatorial countries. So most will take the Ramadan cue from sightings in their family's country of origin, or in Mecca itself.
"Schools want to know in advance when Ramadan begins and when the Eid (the end of fasting) will be," says Muhammad Usamah of the Muslim Educational Trust. "We try to advise them about the difficulty of giving precise dates." The Islamic year, determined by lunar months, is about 11 days shorter than the western one, and Ramadan gets earlier every year.
Fasting during Ramadan is one of the five chief duties of Muslims - others include giving alms to the poor (2.5 per cent of annual income), and praying five times a day. They believe Ramadan is the month when the Koran was revealed for the first time to the Prophet Mohammed and that by fasting they will focus on spiritual standards and strenghten their faith. Believers are meant to fast from dawn until sunset.
A winter Ramadan, such as this one, is easier than a summer one where - in parts of Scotland at least - there may be 17 hours of daylight.
Rigorous fasting is not expected to begin until puberty, but many of Britain's estimated 500,000 Muslim schoolchildren will be fasting and praying this Ramadan, some of them for the first time. What can teachers do to help them?
At Belle Vue First School in Bradford, all of the 150 pupils - aged from four to nine - are Muslim. Ramadan is a big event at Belle Vue, says headteacher Jill Jagger. "Things are different at home. Sleeping and eating patterns change as children get up for breakfast before dawn with the rest of the family. The younger children often show signs of being very tired, especially in summer. "
In respect for Islamic strictures, the school doesn't have music during assemblies in Ramadan, just the Koran being intoned on tape. At the end of the month, they have an Eid party to mark the end of fasting when the next new moon is sighted. It is the main celebration of the year when everyone sits down together.
"The children say 'it's our Christmas'," says Jill Jagger. "We say 'no it's not. It's your Eid ul Fitr - which is not the same at all. It's important that they know it's their festival, and they don't have to explain it away."
Some of the pupils at Belle Vue are already beginning to practise fasting when they reach seven, if only for a few days or hours during the month. "It tends to be the boys who are fasting," says Jill Jagger. "There's a lot of kudos attached to the idea of growing up to be an imam, like having a son who's a priest."
During Ramadan, the day begins with a pre-dawn meal called Suhur, followed by prayers. The fast is broken immediately after sunset with Iftar, traditionally a snack of dates and milk or water. A fuller meal is taken later.
Fasting does not begin and end with food, though. "The Islamic fast is not only concerned with material things," according to the Muslim Educational Trust. "It involves total abstinence from all food, drink, tobacco and marital intercourse, but it also requires abstinence from lying, fighting, anger and back-biting."
During Eid, special prayers are performed and it is traditional to visit friends and relatives.
Is there a bottom age limit for children fasting? "Islamically, whenever you feel the child is physically able, you begin for an hour or two," says Fatma Amer, a teacher originally from Egypt. "It's a training. Fasting is a skill like any other, and when they try it children feel responsible and grown-up, and part of the community."
Muslims stress that fasting should not harm health. For instance, women are not meant to fast while menstruating, pregnant or breast-feeding. Those who are chronically sick are exempted, as are those on journeys, the very old and the very young. If fasting makes the individual ill, it is supposed to be abandoned.
"There is ample space for people not to fast," says Fatma Amer. Observant Muslims can also make up for lost fasting days later. Fatma fears that Ramadan is sometimes misunderstood in schools here. "There are some teachers who have no clue about our children's cultural background. Sometimes, they are really scared for them," she says. "They try to get them to have water, or to eat."
She cites the case of a nine-year-old boy who boasted in class that he was fasting. The teacher, apparently out of concern for his wellbeing, insisted he drink water.
"It broke the boy's dignity," says Fatma Amer. "He had wanted to show he could go a whole day. When he came home, he was very angry."
But some schools - particularly those with many Muslim pupils - go out of their way to do the right thing during Ramadan. Michael Worsley has been head for five years at Broadway School in Birmingham. Eighty per cent of the 1,300-plus pupils are Muslim. "We can't and wouldn't wish to ignore Ramadan, " he says. "There are aspects of school life that have to change."
At Broadway School - as in others - the numbers of pupils who pray at lunchtime is greatly increased during Ramadan, and a larger space than the usual prayer room has to be made available. Pupils must also have washing facilities for the ablutions required before prayers. At Broadway, boys and girls will have separate spaces this year - last year it was noticed that the boys dominated the prayers in the sports hall.
Some schools lighten the PE curriculum for fasting pupils during Ramadan. "You have to think in summer about doing athletics on a hot day," says Kevin McGee, head of Manningham Middle School in Bradford. "It would be negligent of us to have children running around becoming exhausted."
Some local education authorities have made the Eid days an authorised absence for Muslim pupils. But Richard Thompson, head of Grange Upper School in Bradford, warns against making allowances for Muslim pupils during Ramadan.
"If one makes excuses, it can be a form of institutionalised racism," he says. "So we tend not to. Muslims say that one must never use Islam to avoid one's duties."
Heads may find it useful to bring in guest speakers or imams to clear up points of information. Muslims in this country come from a wide range of areas, including Asia, the Middle East and north Africa. Different communities have different interpretations of the religion; some have become more fervently observant in this country.
Some pupils have been taught at home that they should not swallow their saliva during Ramadan and so spit into waste-paper baskets. Others believe that swimming is forbidden during the month, or showers.
"Some groups became more religious on coming here and have become extreme, " says Fatma Amer. "Misunderstandings exist both within and without the community. But the action of spitting has no roots in proper Islam."
Different families - rather than different communities - interpret the religion differently. Not all pupils will fast at Ramadan and not all Muslims are in favour of it.
"Fasting is a very terrible thing but people can't publicly say so," says one Sudanese academic. "Committed Muslims will tell you about the spiritual dimensions but in fact people get short-tempered and spend the whole day waiting for that time when they can eat. I think Ramadan is bad for one's health."
Undoubtedly, Muslim pupils face peer pressure from fellow Muslims to fast - and mocking from some others. "Some of the girls can get a bit rough," says Rizwana Abdulrehman, a 16-year-old pupil at a girls' school in east London who describes herself as enthusiastic about her religion. "They stick food in your face and think it's a joke. We just deal with it."
Michael Worsley says Broadway School has an altered atmosphere during Ramadan. "Children are engaged in something highly significant for them, and they have a different attitude to life during this time. A number give up smoking. They're focused on right behaviour and there's a greater effort to behave."
Pupils at Broadway have suggested that non-Muslim staff try a day or two fasting this Ramadan. "Kids will be saying - 'are you fasting Sir?'" says Michael Worsley. "And I shall be saying 'yes I am'."
Fact sheets on Ramadan and Muslim festivals are available free from the Muslim Educational Trust, 130 Stroud Green Road, London N4 3RZ, on receipt of SAE. Tel: 0171 272 8502. The trust has also published British Muslims and Schools, Pounds 2, inc. p p Alexander Omar-Basha is 14 and at school in London. His father is a Syrian Muslim, his mother a Christian from Cyprus. He is a Muslim, and observes the fast at Ramadan. "In my school, about one-third of the pupils are Muslim. You see everyone else fasting and you feel you want to do it as well. It's fun. Then as you get older it's more than fun.
"At my primary school, there were only two or three Muslims. Everyone would ask me 'what's it like fasting'? I felt quite proud. My friends would joke and say 'don't you want a chocolate'?
Sometimes I used to break my fast. Get home and have a biscuit or something. Sometimes you pop a sweet in your mouth without thinking. Then you think 'oh no! I'm fasting'. But if you do it accidentally, it doesn't really matter. It's the intention that counts.
"You appreciate food a lot more when you've been fasting. You think, 'oh I can't wait to get home, I've got a lovely dinner coming'. In Ramadan my Dad does all the cooking - hummous and lamb and everything lovely. "Sometimes people at school are nasty about it, and say 'stupid Muslims fasting'. But there are a few Muslim teachers, so that helps, and we've got a prayer room to congregate in.
"It brings us all together a bit. Because even though we're all the same religion, we still fight with one another.
"But if you fight in Ramadan, it ruins your fast, so you've got to try really hard. I'm just like any kid, I'm naughty sometimes and misbehave. But in Ramadan I won't have as much energy left to misbehave."
Nazmin Khanom is a 16-year-old pupil at a girls' school in east London, and the oldest in a family of five. She was born here; both her parents are from Bangladesh. She will be fasting this year, and began when she was 13.
"Fasting is not really hard. You get used to it -- and if it gets dark quickly, that's OK. Near the end of Ramadan, the day seems to get longer though. I do it because I have to - it's part of my religion and Mohammed did it. It's also like having experience of how poor people feel, not having food. And you have to pray as well.
"There are a lot of Muslim girls at school, so we have a special room at lunchtime. We sit there and pass the time.
"PE is a real big fuss though. Some of the girls try to refuse to have a shower afterwards in case water goes in their mouths. But the teachers are strict and we have to do it. So we shut our mouths. I think they should let us off PE because we get thirsty easily and afterwards you feel like breaking your fast.
"I usually get headaches in the afternoon during Ramadan. We all do. But it would be very bad to cheat. God can see everything you do, and he will write it down. I look forward to Ramadan, but halfway through I sometimes get fed up. You have to concentrate on your work, and it's hard when you're thinking about food. You think about all the delicious food then, when it's time to eat, you can't manage it.
"When you finish keeping Ramadan, you feel proud of yourself, and you know God will be pleased with you.
"You feel like you're part of this religion."