How many people know that Easter is celebrated on the first Sunday after the first full moon of the spring equinox?

Very few I suspect, but fail to see that it matters in the least.

3/4 correct.  You left out "as observed from Jerusalem".  Due to time zone differences can be Sunday in Jerusalem and still Saturday in (for example) North America.

I know it now. Thank you. 

It is actually the first Sunday after the Paschal Full Moon, which is not exactly the same thing.

The equinox you are referring to is actually the March equinox. In my country the March equinox is the autumn equinox and the September equinox is the spring equinox. But the ecclesiastical calendar considers the March equinox to always fall on the 21st of March, regardless of when it actually falls.

Also the date of the Full Moon is determined by an ecclesiastical lunar calendar, not by actual observation. It is always the 14th day of a lunar month, according to an ecclesiastical lunar calendar. So there may be some variation.

Finally the Western churches use the Gregorian calendar, which is the calendar we all commonly use. But Orthodox churches use the Julian calendar. The Julian calendar says that leap years where February has 29 days always occur every 4 years. The Gregorian calendar says that three out of four century years are NOT leap years. So the year 1900 was not a leap year, and the year 2100 won't be a leap year either. This has led to some progressive differences in dates between the Julian and the Gregorian calendar.

The Julian calendar will consider 21st March to fall on the date we consider to be the 3rd of April in the Gregorian calendar. And this year western churches consider that Easter Sunday falls on the 4th of April, but Orthodox churches think it falls on the 2nd of May (as per the Gregorian calendar).

I would guess most people that celebrate Easter or have celebrated it in the past know that.

Practically everyone.

I can only speak for myself.

I know that Easter is a stolen pagan ritual. 

Probably more than you realize!

I’m aware, but... someone thinks there’s a *winter* equinox...? 

Not many.  And down where I live it is not the Spring Equinox, It is the Winter Equinox.