You may have seen plenty of people sharing Ramadan messages today - it's a special time. But here's something you may not know: not all Muslims have started Ramadan yet.

Let me explain.

There are different interpretations on when a new Islamic month starts:
a) The moon has to be seen in the country you live in
b) The moon has to be seen in a country that "shares the night" with you
c) Astronomy tells us the moon *can* be sighted under one of those conditions

(There's more!)
d) Astronomy tells us there's a new moon
e) Saudi Arabia has declared it Ramadan so you should follow.

What this typically means is that Muslims across the world start (and end) fasting on different dates - and that's OK.

This all means a few things. In the days leading to Ramadan, many people will be devouring charts like this, because it tells you where the moon is possible to be seen.

Look below: The areas without colour is where the moon is not visible on Thursday night.

The proliferation of these charts means that there's more healthy skepticism when a country says someone pious saw the moon when the astronomical charts are clear that it can't be seen.

But, crucially, the decision to start Ramadan is a deeply personal one.

Final tweet (and I might just delete this whole thread anyway): here's how long Muslims will be fasting in Ramadan around the world.

This month will be challenging in so many different ways, but here's a shout out to those health workers putting in long shifts while fasting.