Hebrew date today

A calendar run by the Moon and the Sun

The Hebrew calendar is lunisolar. Its months follow the Moon, its years follow the Sun, and its days follow sunset. Almost every rule in it exists to keep those three in step, which is why Jewish holidays wander across the Gregorian calendar but never leave their season.

Months begin with the new moon: the molad

In the time of the Second Temple, a new month was declared by the Sanhedrin in Jerusalem when witnesses reported seeing the first thin crescent in the west after sunset. Around 359 CE, according to tradition under the patriarch Hillel II, the observation was replaced by a fixed calculation so that communities everywhere could keep the same calendar. That calculation is the molad, the "birth" of the Moon.

The molad uses a mean (average) month of 29 days, 12 hours and 793 chalakim, where a chelek is 1/1080 of an hour (3⅓ seconds). That is 29.530594 days, within half a second of the modern average lunar month. The count starts from Molad BaHaRaD, the letters Bet, Heh, Resh, Dalet standing for day 2 (Monday), 5 hours and 204 chalakim, the molad of the first Tishrei in the traditional reckoning of the year of creation. Add the right number of mean months and you have every molad since. Ours are computed this way and match the published announcements to the chelek: the molad of Tishrei 5787 was Friday, 8:59 PM and 1 chelek.

The real Moon does not keep to an average. Its orbit is an ellipse, so it speeds up near perigee and slows near apogee, and the Sun's pull stretches and shrinks the month further. The astronomical new moon, the moment the Moon passes between the Earth and the Sun (conjunction), can therefore come many hours before or after the molad. Over the 259 months around 5787, the true new moon came between 16 hours before and 12 hours after the molad, 6.9 hours apart on average. Both are announced moments; neither is when you can first see the crescent, which usually takes another day or so.

The molad is given in Jerusalem mean solar time, about 21 minutes ahead of Israel Standard Time. Gap means how much earlier or later the astronomical new moon (computed with astronomy-engine) is than the molad.

Thirteen months seven times in nineteen years

Twelve lunar months make about 354 days, roughly 11 days short of a solar year. Left alone, Passover would slide back through every season in about 33 years, as Ramadan does in the purely lunar Islamic calendar. The Torah requires Passover in the spring (the month of Aviv, "ripening barley"), so the calendar adds a 13th month, Adar I, in 7 of every 19 years.

The numbers come from the Metonic cycle, known to Babylonian astronomers and named after the Greek astronomer Meton (432 BCE): 19 solar years are almost exactly 235 lunar months, and 19 x 12 = 228, so 7 extra months are needed. The leap years are years 3, 6, 8, 11, 14, 17, 19 of each cycle. 5787 is year 11 of cycle 305, so it is a leap year, with Adar I and Adar II.

Year lengths vary too. A common year has 353, 354 or 355 days and a leap year 383, 384 or 385, because Cheshvan and Kislev can each have 29 or 30 days. 5787 has 385 days, a "complete" year. The extra days come from the dehiyyot, the postponement rules that keep Rosh Hashanah off Sunday, Wednesday and Friday (so Yom Kippur never falls next to Shabbat and Hoshana Rabbah never falls on Shabbat) and push it a day later when the molad of Tishrei falls at or after noon. In 5787 the molad of Tishrei fell on Saturday by the Hebrew reckoning, and Rosh Hashanah was held that same day.

Passover at the full moon after the spring equinox

Passover begins on 15 Nisan. Because each month starts at the new moon, the 15th is full moon night, and the first Seder is held under a full or nearly full Moon. In 2027 the March equinox is on Sat, Mar 20, 4:24 PM, the full moon on Tue, Apr 20, 6:27 PM, and the first Seder on the evening of Wednesday, April 21. The same holds for Sukkot on 15 Tishrei, the full moon after the autumn equinox, and for Tu BiShvat and Purim, which fall at or near full moon too.

The calendar's average year, 365.2468 days, is about 6.6 minutes longer than the solar year, so over the centuries Passover has been creeping later, by about a day every 216 years. Today it can fall as much as a month after the equinox in some leap years.

On that evening the Moon is 98% lit and rises at 8:59 PM in New York, close to sunset, as a full moon always does. A Seder table near a window facing east will see it.

Kiddush Levana, the blessing of the new moon

Once a month, on a clear night, Jews go outside to bless the Moon in a short service called Kiddush Levana (sanctification of the Moon) or Birkat HaLevana. It is said while the Moon is growing: from 3 days after the molad (many follow the Kabbalistic custom of waiting 7 days) until the middle of the month, 14 days, 18 hours and 22 minutes after the molad, when the Moon starts to wane. It is customary to say it at the end of Shabbat, in festive clothes, and it needs the Moon to be visible, so in cloudy months the window can close without a chance. This month's window in your time zone is shown with tonight's Moon above.

The day starts at sunset

"And there was evening and there was morning, one day" (Genesis 1). The Hebrew day runs from sunset to sunset, which is why Shabbat candles are lit about 18 minutes before sunset on Friday and every holiday begins the evening before the date on a printed calendar. Jewish law treats the time between sunset and nightfall, bein hashmashot, as uncertain, and nightfall (tzeit hakochavim) as the point when three medium-sized stars are visible, roughly when the Sun is 8.5 degrees below the horizon. A birth, a death or a wedding after sunset counts for the next Hebrew date, which matters for birthdays, bar mitzvahs and yahrzeits.

How to read a Hebrew date

A Hebrew date is written day, month, year: today is 30 Tishrei 5787. In Hebrew the numbers are letters (gematria): ל׳ תשרי תשפ״ז. The year counts from the traditional date of creation, Anno Mundi, and usually drops the thousands, so 5787 is written תשפ״ז. To get the Hebrew year from a Gregorian one, add 3760 before Rosh Hashanah or 3761 after it.

The month names Tishrei, Cheshvan, Kislev and the rest came back with the exiles from Babylon; the Torah itself mostly numbers the months, counting from Nisan as the first month. That is why the year number changes at Rosh Hashanah in Tishrei, the seventh month by the Torah's count, while the festivals are counted from Nisan.

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