Zmanim · The moments of the day
What are the seasonal hours (sha'ot zmaniyot)?
The seasonal hours, in Hebrew sha'ot zmaniyot, are found by dividing the daylight — from sunrise to sunset — into twelve equal parts. Each part is one proportional "hour," longer in summer and shorter in winter.
An hour that stretches with the season
The halachic day is measured by the sun, not by the wall clock. From sunrise to sunset the daylight is divided into twelve equal parts, and each part is a שעה זמנית, sha'ah zmanit (a proportional hour). Because summer days are long and winter days are short, these hours lengthen and contract with the season — the third such hour of a June morning falls much later on the clock than the third hour of a December morning.
Where this is taught
The Mishnah Berurah, in chapter 58, explains that the deadline for reciting the morning Shema (the twice-daily declaration, "Hear, O Israel") is reckoned by these seasonal hours, and not by a fixed sixty-minute count.
What to do
When you want to know how late you may still say the morning Shema, or when other daily moments fall, measure by the seasonal hours of your own place and season rather than by an ordinary clock. Since sunrise and sunset differ by city, the length of each hour is local.
Source: Mishnah Berurah, chapter 58.
To read your day in its true proportional hours, and to catch the Shema before its time passes, let Kimá (the Jewish clock: your city's zmanim and candle times) do the reckoning for you.
Open Kimá