Explore Jewish time through astronomy.
Atmospheric Refraction and Apparent Altitude
Why atmospheric refraction affects apparent celestial altitude and calculated sunrise, sunset and observation times.
The term
Atmospheric refraction is the bending of light through air of changing density. An object’s apparent position can differ from its geometric direction. The effect is particularly important near the horizon.
Model and conditions
Refraction models approximate an atmosphere. Pressure, temperature and the air along the line of sight affect the observation; unusual atmospheric layers can depart from a simple model. Extra displayed decimal places do not remove this uncertainty.
Use in Jewish Cosmology
Solar and observation settings expose refraction assumptions along with observer elevation and horizon choices. These affect the apparent-altitude or limb thresholds used for event times. Refraction is an astronomical modelling choice; it does not decide which threshold should define a halachic boundary.
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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.