The sunset time listed in a weather app or almanac refers to the moment the sun finishes setting, not when it starts. Specifically, it marks the instant the very top edge of the sun’s disc disappears below the horizon. So if your weather forecast says sunset is at 7:42 PM, the sun will be completely gone from view at that moment, and the process of watching it sink will have started a few minutes earlier. The distinction matters more than you might think, particularly for photographers, pilots, and anyone tracking religious observances tied to sundown.
What the Sunset Time Actually Marks
Astronomers and meteorological agencies define sunset as the moment the sun’s “upper limb” vanishes below an idealized, unobstructed horizon. The upper limb is just the topmost sliver of the sun’s disc. Sunrise works the same way in reverse: the listed sunrise time is the instant the upper limb first peeks above the horizon. Both definitions focus on the edge of the disc that touches the horizon last (at sunset) or first (at sunrise), not the center of the sun.
This upper-limb convention is standard across the U.S. Naval Observatory, the UK’s HM Nautical Almanac Office, and essentially every weather service worldwide. It is not some arbitrary choice. The top edge of the sun is the most visually distinct marker an observer can identify with the naked eye. The center of the disc, by contrast, is already half-hidden behind the horizon and impossible to pinpoint exactly. So the convention aligns with what a person standing outside would naturally notice.
One wrinkle worth knowing: the official calculation also bakes in atmospheric refraction. When the sun appears to sit right on the horizon, its geometric center is actually about 50 arcminutes (a little under one degree) below it. The atmosphere bends light upward, so the sun you see touching the horizon is an image of a sun that has already physically dipped below the geometric line. This means sunset times are slightly later than they would be in a universe with no atmosphere, by roughly two to three minutes depending on conditions.
How Long the Setting Process Takes
Since the published sunset time is the final moment of a process, you might wonder how much time you get to watch the sun go down. Timed observations from a mid-latitude location near Rome, at about 42 degrees north, recorded the full visible descent taking roughly three minutes. One sunset in February lasted about 190 seconds, while an August sunset at the same latitude took closer to 178 seconds, the difference coming from the sun’s angle of approach relative to the horizon at different times of year.1arXiv. SUNSETS AND SOLAR DIAMETER MEASUREMENT – Section: 3 The Formula and Results
Three minutes is a useful ballpark for people living at mid-latitudes in the United States, Europe, or similar zones. But the duration varies widely. Near the equator, the sun drops almost straight down through the horizon, and the whole thing is over in about two minutes. Near the Arctic or Antarctic, the sun meets the horizon at such a shallow angle that it can spend ten minutes or more sliding along it before disappearing. On the solstice at very high latitudes, the sun may barely dip below the horizon at all before bobbing back up, making “sunset” and “sunrise” almost meaningless as distinct events.
Why You Are Watching a Sun That Already Set
Atmospheric refraction is the reason sunset times run a few minutes later than pure geometry would predict, and it creates a genuinely strange situation. At the instant the sun’s upper limb appears to touch the horizon, the entire physical disc of the sun is already below the geometric horizon. You are seeing it only because the atmosphere bends the sun’s light around Earth’s curvature toward your eyes. In a real sense, every sunset you have ever watched was of a sun that was no longer “there.”
The standard amount of refraction assumed in official calculations is 34 arcminutes at the horizon. The sun’s apparent diameter is about 32 arcminutes. So the refraction is slightly larger than the sun itself, which is why the entire disc can be visible even after the geometric sun has fully set. Weather services plug this 34-arcminute correction into their algorithms, treating it as a constant. In practice, the actual refraction on any given evening depends on temperature, humidity, and the structure of the atmosphere between you and the horizon.
This is also why the sun sometimes looks squashed or flattened right at the horizon. Refraction is stronger at lower angles, so the bottom edge of the sun’s disc gets “lifted” more than the top edge. The bottom compresses upward into the top, giving the sun an oval or mushroom-like shape. The effect is entirely atmospheric; the sun itself hasn’t changed shape. You can watch the disc distort in real time during those final couple of minutes.
Mirages and the Green Flash
The atmosphere does stranger things to the setting sun than just bending it upward. The temperature structure of the air near the surface can create mirages that distort the sun’s lower edge in dramatic ways. Near-surface temperature gradients can produce an “inferior mirage” that greatly magnifies the apparent size of the lowest portion of the atmosphere, making the bottom of the sun appear to stretch, duplicate, or develop a gap between the disc and the horizon.2Applied Optics. Sunset science. II. A useful diagram If you have ever seen the sun appear to form an hourglass shape or a notch near the horizon, you were watching a mirage layer at work.
The green flash is perhaps the most famous optical phenomenon tied to the final seconds of sunset. Just as the upper limb vanishes below the horizon, a brief flash of green or emerald light sometimes appears where the sun’s last sliver used to be. The effect typically lasts only two to three seconds. It happens because atmospheric refraction is accompanied by a small amount of dispersion, the same effect that splits white light through a prism. The shorter-wavelength green and blue components of sunlight are refracted slightly more than the reds and oranges, so the green image of the sun’s upper limb is the last to disappear.3Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character. Normal atmospheric dispersion as the cause of the “green flash” at sunset, with illustrative experiments The blue component is usually scattered away before it reaches your eyes, leaving green as the last color visible. A clear, sharp horizon, like an ocean horizon, gives you the best chance of seeing one.
The Three Stages of Twilight
The published sunset time marks the start of twilight, not the start of darkness. This catches people off guard regularly. After the upper limb vanishes, you still have a significant amount of usable light, and the sky goes through three recognized stages before true night arrives.
Civil twilight comes first. It lasts from the moment of sunset until the sun’s center is 6 degrees below the horizon. During civil twilight, the sky is still bright enough to carry out ordinary outdoor activities without artificial light. Street lights typically come on toward the end of this phase. At mid-latitudes, civil twilight lasts roughly 20 to 35 minutes, depending on season.
Nautical twilight follows, lasting until the sun is 12 degrees below the horizon. The general outlines of objects on the ground are still visible, and the horizon at sea is still distinguishable, which is why sailors historically used this window for celestial navigation. This phase adds another 25 to 35 minutes.
Astronomical twilight continues until the sun is 18 degrees below the horizon. To the casual observer, it looks dark outside during most of this period, but the sky still carries a faint glow that interferes with observing dim stars and deep-sky objects. Once astronomical twilight ends, full night has arrived. In total, you might wait well over an hour after the stated sunset time before the sky is truly dark.
At high latitudes in summer, the sun never gets 18 degrees below the horizon, and at the highest latitudes it never gets 6 degrees below. This means astronomical twilight, or even civil twilight, can last all night. The “white nights” of St. Petersburg and similar high-latitude cities happen because the sun never dips far enough for any of these twilight stages to end before it starts rising again.
When the Precise Definition Matters
For most daily purposes, the difference between “the sun starts touching the horizon” and “the sun fully disappears” is a few minutes, and nobody cares. But there are contexts where that distinction has real consequences.
In aviation, the transition from day to night flying involves specific regulatory requirements that hinge on the official sunset time. Pilots operating under visual flight rules in the United States must have position lights on from sunset to sunrise. The FAA defines sunset using the same upper-limb convention. A pilot who thinks sunset means the beginning of the process rather than the end could technically be flying without required lighting for several minutes.
In Islam and Judaism, important daily events are tied to sunset. The start of the Sabbath in Jewish law, for example, traditionally begins at sunset on Friday evening, though communities often add a buffer of 18 to 40 minutes before the calculated time. Ramadan fasting ends each day at sunset. The question of whether “sunset” means the moment the sun touches the horizon or the moment it vanishes has been debated in religious scholarship for centuries. Most contemporary religious authorities align with the astronomical definition: the upper limb fully below the horizon. But some traditions use additional markers, like the appearance of stars, which corresponds more closely to the end of civil or nautical twilight.
For photographers chasing “golden hour” light, the sunset time is a useful anchor. The warm, diffuse light that photographers prize begins well before the published sunset time, typically starting when the sun is about 6 degrees above the horizon, roughly 30 to 40 minutes before sunset at mid-latitudes. The light continues to be useful during civil twilight after sunset. Knowing that sunset time marks the end of the sun’s disappearance, not the beginning, helps with planning. If sunset is at 7:42, you want to be in position by 7:00 or earlier, and you can keep shooting until about 8:10.
Why Published Times Can Be Off by a Few Minutes
Even if you understand the definition perfectly, the sunset time on your phone might not match what you observe outside. Several things cause discrepancies.
Elevation is one. Published sunset times assume you are standing at sea level on a flat plain with an unobstructed horizon. If you are on a hilltop, a tall building, or flying in an airplane, your effective horizon is lower, and you can see the sun for longer. A person at 100 meters of elevation sees sunset roughly two minutes later than someone at sea level in the same location. This is why you can watch the sun set, then quickly ride an elevator to a high floor and catch the last sliver setting again.
Local terrain also matters. Mountains, buildings, or a tree line to the west will block the sun before it reaches the true horizon, making your personal sunset earlier than the published time. Weather apps don’t account for local obstructions because they calculate based on a theoretical smooth horizon.
Atmospheric conditions introduce another source of variation. The standard 34-arcminute refraction value is an average. On a particularly hot day with a strong temperature inversion near the surface, actual refraction can be noticeably higher, delaying the apparent sunset. On a very cold, dry evening with stable air, refraction may be lower. These variations are usually small, on the order of 30 seconds to a minute, but they are real and unpredictable from a forecast.
How Location Changes the Entire Experience
The angle at which the sun approaches the horizon determines almost everything about how sunset looks and feels. At the equator, the sun drops nearly perpendicular to the horizon year-round. Sunset is fast, twilight is brief, and darkness arrives quickly. The entire transition from full sun to near-darkness can take under half an hour.
At mid-latitudes, the angle varies with the season. In summer, the sun’s path meets the horizon at a shallower angle, stretching both the setting process and the twilight period. In winter, the approach is steeper, and both are shorter. This is why summer evenings feel so long and winter ones feel abrupt, even if you can’t articulate why. The actual duration of usable light after sunset shifts by a significant margin between June and December.
At extreme latitudes, the geometry breaks down entirely. Above the Arctic Circle in summer, the sun can skim along the horizon for hours without setting at all. On the days bracketing the midnight sun period, the setting process can last an extraordinarily long time, with the sun barely dipping below the horizon before rising again. In those conditions, “sunset time” as printed in an almanac barely captures the visual experience.
Longitude within a time zone also produces a gap between the clock and what you see. Two cities in the same time zone can have sunset times that differ by 30 minutes or more if they sit at opposite edges of the zone. The published times account for this by being location-specific, but people who travel east or west within a zone and expect sunset to happen at “about the same time” are often surprised.
The Sun’s Apparent Size at Sunset
A common belief is that the sun looks larger at the horizon than it does high in the sky. In terms of its measured angular size, this isn’t quite right. The sun’s angular diameter is about 32 arcminutes whether it’s overhead or on the horizon. Atmospheric refraction actually compresses the vertical dimension of the disc, making it slightly smaller in that direction, not larger. The perceived enlargement is an optical illusion, known as the moon illusion when it happens to the moon, and its exact cause is still debated among vision researchers. The most widely accepted explanations involve the brain interpreting the horizon sun in the context of terrestrial reference points like buildings and trees, which makes it seem larger by comparison. High in an empty sky, the brain has no reference and reads it as smaller.
What does change at sunset is color. The thick layer of atmosphere the light passes through at low angles scatters away shorter blue wavelengths, leaving the reds and oranges that make sunsets vivid. On evenings with thin high clouds or volcanic aerosols in the stratosphere, the scattering becomes more complex and can produce unusually intense purples and crimsons. The specific color palette of any given sunset is as much about what is in the atmosphere that day as it is about the sun itself.