What exactly counts as sunrise
Ask ten people when the sun rose this morning and you will get a range of answers. One saw the first spark of light over a ridge, another checked a weather app, a third remembers the sky going pale long before. All of them are describing something real, but only one of those moments is the sunrise that astronomers, almanacs and navigation tables agree on.
The upper edge, not the centre
The standard definition of sunrise is the instant when the upper edge of the sun’s disc appears on an ideal, flat horizon. Not the centre of the disc, and not the moment the whole sun is visible. The disc is about half a degree wide, so waiting for its centre would push sunrise a minute or two later at mid-latitudes, and much later near the poles, where the sun climbs at a shallow angle.
The atmosphere lifts the sun
The second ingredient is refraction. Air bends light, and it bends it most when light skims along the horizon through the thickest layer of atmosphere. Near the horizon the effect is about 34 arcminutes, a little more than the width of the sun itself. In other words, when you see the sun’s lower edge sitting on a clear sea horizon, the real sun is already entirely below it.
Combine the two corrections, half the disc (about 16 arcminutes) and standard refraction (about 34 arcminutes), and you get the number used by the US National Oceanic and Atmospheric Administration and most published tables: sunrise is when the geometric centre of the sun is 0.833° below the horizon. Sunset uses the same rule in reverse.
Why two sources disagree by a minute
If you compare a phone’s weather app with an almanac you will sometimes see a one-minute difference. There are a few honest reasons:
- Rounding. Some sources truncate seconds, others round them.
- Location. A city’s sunrise is usually calculated for its centre. On the eastern edge of a large city the sun can rise noticeably earlier than on the western edge.
- Refraction varies. The 34-arcminute figure is an average. Very cold air over the sea can bend light more strongly; warm air over desert, less.
- Elevation. Standard sunrise assumes you stand at sea level with a flat horizon. From a mountain summit, the horizon dips and you see the sun earlier.
None of these change what the moment is. They change how precisely a given person, standing in a given place, will see it.
Clouds do not move sunrise
A cloudy morning still has a sunrise. The astronomical event happens at the calculated instant whether or not anybody can see the disc. This matters more than it seems: a calendar, a tide table or a dedication tied to a sunrise needs a moment that is fixed in advance and does not depend on the weather.
Where the 0.833° comes from
The figure is older than any app. Nautical almanacs have long tabulated sunrise and sunset for the moment the sun’s centre is 50 arcminutes below a sea-level horizon: 16 arcminutes for the radius of the disc and 34 for refraction. Fifty arcminutes is 0.833°. Navigators needed a fixed convention so that a time in a table meant the same thing on every ship, and the convention stuck. Modern algorithms, including NOAA’s, compute the sun’s position far more precisely than the old tables, but they still ask the same question: when does the centre of the sun reach 0.833° below the horizon?
How long a sunrise takes
Sunrise is defined as an instant, but the sun does not appear all at once. The time from the first sliver to the full disc depends on how steeply the sun climbs:
- At the equator, where the sun rises almost vertically, the whole disc clears the horizon in about two and a half minutes.
- In Paris in October, the path is slanted and it takes nearly four minutes.
- In Tromsø in October, at almost 70° north, the sun climbs so obliquely that the disc needs close to eight minutes to rise completely.
The calculated time always marks the first of those moments, when the upper edge appears.
Height changes the time you see
Standard sunrise assumes an eye at sea level and a flat horizon. Climb, and the horizon drops below eye level. From 100 metres it lies about a third of a degree down; from a 3,000-metre summit, about 1.6°. Seeing over that lower horizon lets you catch the sun earlier: roughly eight minutes earlier at the equator from a high summit, around twelve in Paris’s latitude in autumn, and more than twenty in the far north. Mountains on the eastern skyline work the other way and delay the first direct light, sometimes by a long time in deep valleys.
That is why two people a few kilometres apart can honestly report different sunrise times. Neither is wrong; they are standing on different horizons.
What sunrise is not
Two nearby events are often confused with it. Dawn, in the astronomical sense, begins when the sun is 18° below the horizon and the sky starts to brighten. Civil dawn, at 6° below, is when there is enough light for most outdoor activity. Both come well before sunrise: in Paris in October, civil dawn is about half an hour earlier. A photograph of a bright, colourful sky is almost always taken in that half hour, not at the moment of sunrise itself.
How Resthe uses the definition
Every Resthe sunrise and sunset is calculated with this same rule: the NOAA solar algorithm, the upper edge of the sun and standard refraction of 0.833°, for the exact point you pin on the map. The time is converted to the local time zone of that point, using its historical time-zone rules. The calculated moment is intended to be accurate to about one minute, and it is the one printed on the certificate.
If you want to dedicate a real sunrise to someone, the rule above is the one that decides when it happens. You can choose the place and see the time before you pay with The Sunrise, or start straight away in the gift wizard.