Explore Jewish time through astronomy.

About Jewish Cosmology

Jewish Cosmology is an interactive research and educational web application connecting Hebrew dates, traditional Jewish calendar calculations and halachic concepts of time with the observable motions of the Sun, Moon and planets, astronomical seasons and eclipses.

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What is Jewish Cosmology?

What can I explore?

Explore Hebrew dates and year structure alongside solar and lunar events, planetary motion, eclipses and seasonal calculations from selected locations and moments. Unlike a calendar that only lists dates, the application compares traditional calendar arithmetic with physical astronomical circumstances.

Creator

Rabbi Daniel Freedman created Jewish Cosmology to explore the relationship between the Jewish calendar, halachic time and the motions of the Sun, Moon and planets.

Calendar calculations

The calendar includes molados, keviyah, the four dechiyos, leap years, month lengths, traditional time units, and 19-year, 28-year and Shemittah cycles. Traditional calculations and astronomical events are kept conceptually distinct.

Experimental calendars

The year experiment lets you compare changes to molad basis, postponements, leap years and month lengths with the standard calendar. A separate sighting chronology carries months forward from a declared epoch, Nissan anchor or named custom JDN anchor. It compares the spring equinox with the candidate Nissan boundary and applies selected sighting and month-length constraints. Its Hebrew date labels can change while the JDN comparison axis remains fixed. These experiments illustrate assumptions; they do not certify sightings, historical chronology or halachic decisions.

Supported period

The principal Hebrew-year explorers accept years 1–10000. Individual tools and astronomical catalogues have narrower ranges. A selectable year is not a claim of historical calendar practice or reliable astronomical accuracy throughout that period.

Astronomical foundations

VSOP2000 is used in significant solar and planetary paths and is the shared solar default. VSOP87-based solar comparison is also available. The default Moon model is the project’s EP2MM-C implementation (model-elpmpp02c, getX2000_C), also catalogued as ELPMM02 / ELPMOR82-C. Other implemented choices include ELP/MPP02 DE, ELP82B, Meeus-based lunar position and a VSOP2000 Moon source. Model families and references include Bretagnon, Francou and Moisson’s planetary theories; Chapront-Touzé, Chapront and collaborators’ ELP lunar theories; The Astronomical Almanac; the Explanatory Supplement (1992, Seidelmann; 2013, Urban and Seidelmann); and Jean Meeus’s Astronomical Algorithms and Mathematical Astronomy Morsels. Some legacy calculations remain for comparison.

Limitations and practical use

Halachic times are unverified, have no rabbinic approval, and must not be relied upon for practical halachic decisions unless and until an appropriate authority approves them. Displayed decimal places or seconds describe numeric resolution, not guaranteed scientific accuracy. Model validity, coordinate conventions and observational assumptions limit interpretation. ΔT (the difference between terrestrial and Earth-rotation time) is increasingly uncertain for ancient and distant-future dates. Ancient computed eclipses are research estimates, not historical confirmation. Calculation profiles can intentionally differ between research views. Atmospheric, terrain and visibility assumptions must be considered; computed geometry does not certify an actual observation.

Copyright

© Rabbi Daniel Freedman. Original site content, design and code. Astronomical theories, coefficient data, imagery and third-party software retain their respective authors’ credits, licences and notices.

Calculation terminology

JDN — Jewish Day Number: In this project’s UI, JDN is Jewish Day Number, not astronomical Julian Day Number. Today advances it with the selected Hebrew date boundary: Jerusalem sunset (default), local sunset, or 18:00 in the selected timezone. If a required sunset is unavailable, no civil-day fallback is applied. Calendar tables use the selected civil date’s daytime Hebrew date: SDN minus 347997. In the sighting chronology, JDN remains the fixed comparison axis while the assigned experimental Hebrew date may change. Its starting declaration uses 18:00 Jerusalem standard time (UTC+02:00, without DST); this is distinct from the selectable boundary used by Today.
SDN — Scaliger Day Number: The integer civil-calendar date identifier used by the application. Civil date selection changes at local midnight. It is distinct from fractional astronomical JD.
JD — Julian Date: A continuous astronomical day count with a conventional noon boundary. The Today clock displays UTC-based fractional JD; legacy JDE fields retain their model’s time-scale convention. It does not reset at local sunset.
Molad: The traditional fixed mean conjunction used in Hebrew calendar arithmetic, based on 29 days, 12 hours and 793 chalakim per month. It is distinct from a calculated physical Sun–Moon conjunction (astronomical new moon), and neither alone guarantees crescent visibility.
Tekufah / tekufot / tekufos: A seasonal turning point. Shmuel and Rav Adda are traditional seasonal reckonings; the astronomical comparison solves solar-longitude crossings for equinoxes and solstices. These are distinct calculations.
Sunrise / sunset: Research definitions vary by solar centre or limb, refraction, elevation, horizon, obstruction and observer position. Shared settings and temporary experimental overrides need not produce identical sunrise or sunset times.
Keviyah and dechiyot / dechiyos: Keviyah describes the Hebrew year pattern; dechiyos are the four postponement rules in fixed-calendar arithmetic. Leap years follow a 19-year cycle in the standard calendar.
Shaos zmanios: Proportional hours divide a chosen day interval into twelve parts. The chosen start and end definitions affect their duration; they are not automatically equal clock hours.