Is sunset (Shkiya) the time the sun hits the horizon or when it sunk completely below it?
What about sunrise?
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Netz HaChamah (Sunrise) = When the center of the sun is at a 0.833 degrees depression angle below the horizon
Shkias Hachamah (Sunset) = When the center of the sun is at a 0.833 degrees depression angle below the horizon
The US Naval Observatory define sunrise/sunset as follows (in their FAQ): Many calendars use the calculations found here for determining halachic times relative to sunrise and sunset.
"Sunrise and sunset conventionally refer to the times when the upper edge of the disk of the Sun is on the horizon, considered unobstructed relative to the location of interest. Atmospheric conditions are assumed to be average, and the location is in a level region on the Earth's surface."
The following post uses the start and end of the day as defined by the Magen Avrohom and the Gr'a. As part of the post you can see how they interpret "sunrise" and "sunset".
Rabbi Avraham "Abaleh" Gombiner (1637–1683) of Kalisz, Poland, the "Magen Avraham," wrote his commentary on "Orach Chayim," the section of the Shulchan Aruch that deals with the "Laws of Daily Life and Holiday Behavior," and includes the "Laws Concerning Rising in the Morning," the various prayers, special prayers, and mitzvot of the holidays. According to Rabbi Avraham, the proportional hour is obtained by dividing by twelve the time interval between "alot hashachar," the pre-sunrise event that occurs when the rays of the sun first become evident and the sky begins to lighten, and "tzeit hakochavim," "starshine," which is defined as that time when three stars of medium brightness become visible in the sky.
Rabbi Eliyahu ben Shlomo Zalman of Vilna (1720–1797), the "Vilna Gaon," used a different calculation. He held that the sha'ah zmanit is derived by dividing by twelve the interval between "hanetz hachamah," sunrise, the moment that the first sliver of the sun's disk becomes visible on the horizon, and "shkiat hachamah," sunset, the time that the disk of the sun disappears completely beneath the horizon. Using our example from above, sunrise would be about 6:30 a.m. and sunset at 6:40 p.m. and one sha'ah zmanit is almost exactly 60 minutes.
Once I wrote a program that calculates the halachik times for some specific cities in diaspora. And I was commanded by the R. Refoel Antin to calculate the time when the sun is completely below the horizon as a time of shkiya.
Same about the sunrise. It is the time when the sun begins to appear above the horizon.
The wiki page about sunrise says:
הָנֵץ החמה הוא ציון זמן בהלכה לרגע שבו נראית בוודאות הנקודה הראשונה מגוף השמש
The sunrise is the time in halacha when the first point of the body of the sun is seen.
They cite Yerushalmi Berachot, chapter 1, halacha 2 and Biur Halacha 58, dibur hamathil "כמו" to backup this statement.