Explore Jewish time through astronomy.
Shkiyah: Sunset and the Observer Horizon
How sunset geometry, refraction and horizon settings relate to shkiyah and Hebrew date boundaries.
The term
Shkiyah refers to sunset in Jewish time terminology. Astronomically, sunset describes the Sun crossing a specified western horizon. The solar disc has a finite size, so a centre crossing and an upper-limb crossing are different events.
Astronomy and interpretation
Atmospheric refraction and the observer’s elevation affect apparent sunset. Terrain or a building can obscure the Sun before it crosses an unobstructed horizon. Which boundary defines a halachic time must be stated separately from the calculation.
Use in Jewish Cosmology
Sun and observation tools expose solar-limb, refraction, observer and obstruction settings. Zmanim use the selected definitions, and date readouts explain when their Hebrew day advances. Compare the settings before interpreting differences between displayed sunset times.
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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.
Models, terminology and limitationsHalachic 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.