How refraction affects calculated moonrise and moonset
Moonrise and moonset are crossing events, not simple checks for a center altitude of zero. The Moon has an apparent radius, and the atmosphere bends light near the horizon. A reproducible calculation must state which limb is used, what horizon is assumed, and how refraction enters the event search.
The position tool uses an ideal sea-level observer and the engine's rise/set convention for the visible upper limb with standard horizon refraction. It also reports geometric and normally refracted center altitudes so the instantaneous coordinate result is not confused with the event definition.
A horizon crossing has a convention
For a body with a visible disc, first contact above the horizon occurs before its center reaches a geometric altitude of zero. The search accounts for the Moon's apparent radius and uses a standard 34-arcminute horizon-refraction assumption. Rise is the upward crossing and set is the downward crossing; the solver follows the changing geometry rather than rounding a table of altitudes.
The returned azimuth describes the direction of the modeled event from true north. The timestamp is an instant and can be formatted in the selected time zone. If the solver finds no crossing within the 40-day forward window, the interface returns no event rather than fabricating a date from an average daily interval.
Instantaneous refraction is a separate output
Geometric altitude is computed without atmospheric bending. The normally refracted value applies the library's standard model near the horizon, where the correction grows strongly. More than one degree below the horizon the library tapers an extrapolated correction toward zero at the nadir; the interface suppresses that deep-below-horizon refracted reading because it is not a useful observational claim.
Real refraction varies with pressure, temperature, weather, elevation, and the actual horizon. Therefore the modeled rise or set is a reference calculation, not a guarantee of the moment a user will first or last see the Moon. Terrain and buildings can delay appearance or advance disappearance independently of atmospheric bending.
Worked rise-and-set example
For Greenwich at 51.4779° north, 0° longitude, starting 26 August 2026 at 12:00 UTC, the forward search returns:
- Next moonrise
- 26 August 2026, 18:29:42 UTC at azimuth 117.80°
- Rise center altitude
- −0.82° geometric; −0.20° normally refracted
- Next moonset
- 27 August 2026, 03:58:40 UTC at azimuth 245.60°
- Next upper transit
- 26 August 2026, 23:08:17 UTC
The Moon's center remains geometrically below zero at the modeled upper-limb rise. That is expected under the stated disc and refraction convention; forcing the event to center altitude zero would answer a different question.
Calculation limits
- The horizon is ideal and does not include hills, buildings, trees, sea dip, or a custom elevation profile.
- Standard refraction is not a live atmospheric measurement and can differ from observed conditions.
- Upper transit is a meridian crossing and may occur below the horizon; it is not synonymous with peak visibility.
Use the working calculator
Open the Moon Position Calculator to calculate another exact instant with the same production logic. The calculator accepts the relevant input and keeps result state on its single canonical tool page.