Rosh Codesh the "Head of the New (Moon)" ="Head of the Month," is the term for the first day of the month in Jewish Talmudic Calendar

Jewish Talmudic Calendar

The Jewish Talmudic Calendar is a lunisolar calendar using Tishri-years, observed by the Jewish people since the Late Antiquity . While it is based on Nisan-years, which began from the prebiblical Babylonian times , and the Tishri-years was formed in the time of David The Jewish Talmudic Calendar...

. It is the day after the observation of the New Moon

New moon

In astronomical terminology, the new moon is the lunar phase that occurs when the Moon, in its monthly orbital motion around Earth, lies between Earth and the Sun, and is therefore in conjunction with the Sun as seen from Earth...

 at the time of sunset, which in Israel is 18:00 Jerusalem time (16:00 Universal Time

Universal Time

Universal Time is a time scale based on the rotation of the Earth. It is a modern continuation of Greenwich Mean Time , i.e., the mean solar time on the Prime Meridian at Greenwich, and GMT is sometimes used loosely as a synonym for UTC...

).

The time of the New Moons can be calculated today accuarately to 10000 years before or after now. The further it is from today, the bigger is the Delta T

Delta T

ΔT, Delta T, delta-T, deltaT, or DT is the time difference obtained by subtracting Universal Time from Terrestrial Time : ΔT=TT−UT....

, the uncertainty in converting Dynamical Time to Universal Time

Universal Time

Universal Time is a time scale based on the rotation of the Earth. It is a modern continuation of Greenwich Mean Time , i.e., the mean solar time on the Prime Meridian at Greenwich, and GMT is sometimes used loosely as a synonym for UTC...

.

While most astronomical programs used the Julian Calendar

Julian calendar

The Julian calendar began in 45 BC as a reform of the Roman calendar by Julius Caesar. It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year .The Julian calendar has a regular year of 365 days divided into 12 months...

, it is better to sense the season by converting to Gregorian Calendar

Gregorian calendar

The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter...

, since the Equinoxes and Solstices are more fixed in the latter. A convenient program to find the Gregorian dates as well as the week days is CalMaster2000.

While this article is about the astronomically determined Rosh Codesh, Jewish religious rules often allows postponements (Dehioth) of the "head of the month." There is a similar artical on the same issue with a different pronunciation Rosh Chodesh

Rosh Chodesh

Rosh Chodesh or Rosh ḥodesh is the name for the first day of every month in the Hebrew calendar, marked by the appearance of the new moon. The new moon is marked by the day and hour that the new crescent is observed...

, written more from Jewish religious aspect.