Experimental Hebrew Calendar

Compare a modified Hebrew year with the standard calendar, using month-length, molad and observation adjustments.

Adjust the experimental year and compare its Hebrew date labels with the standard calendar on the same underlying day. Review month lengths, holiday shifts, molad references and the relationship of Nissan to the calculated spring season.

Observation controls let you examine the Moon at a chosen local time. Experimental date labels do not change the underlying day or astronomical instant. Saved adjustments are exploratory assumptions, not historical evidence of a sighting or a halachic ruling.

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.

JavaScript is required for the interactive explorers and calculations. The descriptions and navigation remain available without JavaScript.