Coordinates and time
Refraction and horizon dip: why rise and set times shift
At a glance
The atmosphere bends light and makes celestial objects appear higher, with a typical horizon effect of around thirty-four arcminutes. An elevated observer can also see a horizon below the local horizontal plane: horizon dip. Both change visible rise and set. Apphemeride includes refraction and uses observer elevation to account for dip in horizon-event calculations.
Light follows a curved path through air
Atmospheric density and refractive properties vary with height. Incoming light follows a slightly curved path rather than the straight line of a vacuum model. The eye extends its final direction backwards and places the object above its geometrical direction.
The amount depends strongly on altitude. Typical standard-atmosphere orders of magnitude are:
| Apparent altitude | Approximate refraction |
|---|---|
| Zenith, 90° | Zero |
| 45° | About 1 arcminute |
| 10° | About 5 arcminutes |
| Horizon | About 34 arcminutes |
These are reference values, not a live atmospheric measurement. Near the horizon, the effect changes rapidly with altitude. The lower solar edge is lifted more than the upper, compressing the disc vertically and making it look flattened.
Pressure, temperature and atmospheric layering matter. Cold dense air, warm layers over cold water and unusual gradients can produce stronger effects or mirages. A standard model cannot reproduce every local horizon appearance.
The visible Sun rises before its centre reaches the horizon
The usual sunrise convention uses the upper solar edge. The disc radius is roughly sixteen arcminutes, and standard horizon refraction adds roughly thirty-four. The centre can therefore remain around fifty arcminutes below the mathematical horizon when the upper edge appears.
That convention corresponds to a centre altitude of about −0.833°. It is a definition for comparison, not evidence that the Sun has risen “incorrectly”. Without atmosphere and disc size, a centre-crossing event would occur at a different time.
The same atmospheric principle applies to planets and stars, with suitable object-specific treatment. For the Moon, parallax is also large because it is close to Earth. A lunar rise cannot be obtained by treating the Moon as a distant star with a solar-sized correction.
The timing effect depends on how steeply the track crosses the horizon. At high latitudes a small altitude shift can translate into a larger time difference than at a steep tropical crossing.
Horizon dip from an elevated viewpoint
The mathematical horizon is perpendicular to the local vertical. From a height above an unobstructed sea or lower plain, Earth’s curved surface drops away and the visible distant boundary lies below that plane. The angle is **horizon dip**.
Dip increases with elevation, but not in direct proportion. A tower, ship’s deck or summit can give a noticeably lower distant horizon than a point near the surface. Atmospheric bending along the path also modifies the visible terrestrial horizon.
The essential condition is an open view to a lower distant surface. A summit facing another higher ridge can have an elevated obstructing horizon instead. The number of metres above sea level does not by itself describe every direction of terrain.
This distinction matters at the Grand Canyon: being high above the river does not guarantee that the opposite rim is below your ideal horizon. A coastal tower and a canyon overlook can share an elevation yet have very different first-sunlight experiences.
When precision matters
For casual recognition, standard refraction is often sufficient. Photographic alignments, occultation timing and low-altitude observations expose differences more clearly.
A calculation needs consistent treatment of centre or limb, refraction, parallax and observer position. The real horizon remains a separate input to your observing judgement. Combining a standard formula with an unmodelled mountain does not produce a precise first-visible-ray prediction.
Weather-driven refraction uncertainty can exceed the numerical refinement of the calculated event. It is therefore useful to distinguish a finely located model crossing from what the atmosphere and foreground will actually show.
For solar work, never verify the position by looking directly without protection. A flattened or reddened horizon Sun is still unsafe to observe unprotected, especially through magnifying optics.
How Apphemeride answers this
How Apphemeride answers this
The relevant solar-system Corrections cards show atmospheric refraction alongside other positional contributions. Observer elevation supplies the dip correction used for rise and set times. Event times are refined iteratively rather than left at a coarse estimate.
Sun and Moon horizon data and GPS location are free. Manual observer elevation, address or coordinate entry and time travel belong to the full version, available after a three-day trial.
Use your real viewpoint and inspect the foreground. The elevation capability should not be read as a promise that the app automatically maps every tree, building or ridge. Explore location and observer elevation.
Frequently asked questions
How much refraction occurs at the horizon?
A typical standard value is about thirty-four arcminutes, slightly more than the solar angular diameter. Real atmospheric conditions can differ significantly, particularly close to the horizon.
What is horizon dip?
The angle by which an unobstructed visible horizon lies below the local mathematical horizontal plane for an elevated observer. It depends on height and atmospheric effects, and is different from the height of a mountain blocking the view.
Does the Sun rise earlier on a mountain?
It can, when the view extends over a sufficiently low distant horizon. A nearby ridge can instead delay visibility. Elevation alone does not guarantee an earlier first glimpse in every direction.
Why does the low Sun look flattened?
The lower edge is refracted more strongly than the upper edge, compressing the apparent vertical diameter. Unusual atmospheric layers can create more complex distortions and mirages.
Related guides
Features
- Time and location – Time travel, location selection, observer elevation and sidereal time.
- Glossary
