Which part of the Sun crosses?
Standard sunrise tables commonly refer to the apparent upper edge at the horizon with average atmospheric refraction. USNO’s conventional centre altitude is about −0.8333°. Atlas’s paran sunrise-day preset instead uses the apparent topocentric centre at unrefracted altitude 0°.
These settings answer different questions, so matching the date and city is not enough to compare two published times. Check the selected solar edge, refraction, elevation and horizon.
U.S. Naval Observatory: rise, set and twilight definitions ↗Swiss Ephemeris: rise and transit functions ↗
A worked sunrise-to-sunrise day
For London on 29 March 2026, the independent Skyfield reference places geometric sunrise near 05:48:15 UTC and the next near 05:45:59 UTC. The resulting interval is about 23.962 hours. The local civil date is 23 hours that day, illustrating why the two windows select different sets of events.
Atlas begins with the first geometric sunrise inside the chosen local date. The first sunrise is included and the next is excluded from the window; nearby events can still appear as explicitly marked padding for a pair comparison. The Sun supplies these boundaries even if you choose only other bodies in the event list.
Swiss Ephemeris: rise and transit functions ↗Skyfield: astronomical event search ↗
Polar dates and interpretive traditions
Some high-latitude dates have no sunrise. A following sunrise can also be too distant to define an ordinary daily search. Atlas returns an unavailable window if there is no sunrise in the chosen date or no following one within 48 hours.
Zyntara distinguishes actual moving planets from rotating fixed natal degrees, and offers several ways to begin the astrological day. Atlas’s named geometric-sunrise preset makes one choice explicit; it does not establish exact agreement with a practitioner’s dawn, visibility phases or full natal interpretation.
U.S. Naval Observatory: rise, set and twilight definitions ↗Zyntara: day conventions and actual moving bodies ↗
Word origin · etymology
English compound of sun and rise.
Merriam-Webster records sunrise from the 15th century. This word history does not specify a computational altitude or an ancient astrological day boundary.
How we know · epistemology
Astronomical geometry with an adopted horizon, limb and atmospheric model.
A computed crossing is reproducible within those assumptions. Actual sighting also depends on terrain, observer height and atmospheric conditions.
U.S. Naval Observatory: rise, set and twilight definitions ↗
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