Zmanim · Shabbat and the candles
How is the end of Shabbat calculated?
The end of Shabbat is calculated from tzet hakochavim — the emergence of three stars, which is nightfall. Different customs reach that moment in different ways: some measure how many degrees the sun has sunk below the horizon, others use a fixed number of minutes after sunset — commonly forty-two, fifty, or seventy-two. Because both depend on your place and the date, a local calculation is what makes it precise.
Two honored ways of measuring nightfall
The sign itself is fixed — three stars — but the sky does not print a clock time on itself, so the tradition offers ways to reckon it. One approach is astronomical: nightfall is when the sun reaches a certain depth below the horizon, and that depth translates into a time for your exact latitude. The other is the minute-count: a set interval after sunset, forty-two or seventy-two minutes being the most widespread. Both are aiming at the same darkness in the sky.
What the sources teach
The Shulchan Aruch, in Orach Chaim 293, anchors the close of Shabbat in tzet hakochavim, and the Mishnah Berurah (the widely followed early-twentieth-century commentary) is where the practical stringencies and the different minute-counts are weighed. The variety is not confusion; it is communities guarding the day carefully, each within its received custom.
What to do
Use a calculation set for your own city and date rather than a number borrowed from somewhere else — the same seventy-two minutes means one darkness in the tropics and another far north. Then follow the count your community keeps. When a real decision hangs on it, your rabbi is the address for which custom is yours.
Source: Shulchan Aruch, Orach Chaim 293; Mishnah Berurah.
To have that reckoning done for your own latitude and week rather than a borrowed number, let Kimá (the Jewish clock: your city's zmanim and candle times) work out nightfall where you are.
Open Kimá