How refraction affects calculated moonrise and moonset

Moonrise and moonset are horizon-crossing events. The Moon has an apparent radius, and the atmosphere bends light near the horizon, so the stated convention includes the visible limb, horizon, and refraction assumption.

The position tool uses an ideal sea-level observer and the visible upper limb with standard horizon refraction for rise and set. It also reports geometric and normally refracted center altitudes so the instantaneous coordinate result remains distinct from the event definition.

Night observatory horizon dome with brass altitude and azimuth controls

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 calculation 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 returned azimuth describes the direction of the event from true north. The timestamp is one instant and can be displayed in the selected time zone. If no upcoming crossing is available for the location, no event is shown.

Instantaneous refraction is a separate output

Geometric altitude is calculated without atmospheric bending. The normally refracted value applies a standard near-horizon correction and is shown only where that comparison is useful.

Refraction varies with pressure, temperature, weather, elevation, and the actual horizon. Terrain and buildings can also change the first or last visible moment.

Worked example

Worked rise-and-set example

For Greenwich at 51.4779° north, 0° longitude, after 26 August 2026 at 12:00 UTC, the next events are:

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 when its upper limb rises under the stated disc and refraction convention. Center altitude zero describes a different crossing.

Read before comparing

Reference conditions

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.

Next step

Use the working calculator

Calculate another exact instant with the same reference frame and conventions described on this working sheet.

Open Moon Position Calculator