The daily cycle of light and darkness is a fundamental rhythm of life, yet the precise moments of sunrise and sunset vary considerably depending on where you are on Earth and the time of year. Sunrise and sunset are specific, defined instants when the visible edge of the Sun crosses the horizon. The science behind these constantly changing times involves a complex interplay of Earth’s movement, atmospheric effects, and human-made adjustments. Understanding these factors provides clarity on why your local sunrise and sunset times change daily or seasonally.
Defining the Astronomical Moment
The official timing of sunrise and sunset is based on a precise astronomical definition, which is not the moment the Sun’s center aligns with the horizon. Instead, the event is timed when the upper limb, or the topmost visible edge of the Sun’s disk, appears to touch the horizon line. This definition accounts for the Sun’s own apparent size and atmospheric refraction.
Atmospheric refraction is the bending of light by Earth’s atmosphere. Light rays from the Sun are bent downward as they pass through the layers of air, which makes the Sun appear higher in the sky than its true geometric position. This refraction effect is so pronounced near the horizon that when you observe the Sun setting, it is actually already entirely below the horizon. This phenomenon effectively causes the Sun to appear to rise earlier and set later than it would if Earth had no atmosphere, adding several minutes to the total duration of daylight.
How Latitude and Season Dictate Change
The primary drivers of the daily and annual variations in sunrise and sunset times are Earth’s 23.5-degree axial tilt and the observer’s latitude. This constant tilt, relative to Earth’s orbit around the Sun, is what creates the seasons. As the Earth travels along its orbit, the hemisphere tilted toward the Sun receives more direct sunlight and experiences longer daylight hours, causing the Sun to follow a different path across the sky each day.
This change in the Sun’s path is most dramatically observed at higher latitudes. Near the poles, the variation is extreme, leading to the midnight sun in summer and periods where the Sun does not rise at all in winter. For observers near the equator, the Sun’s angle changes very little throughout the year, resulting in day and night lengths that remain nearly equal.
The daily change in sunrise and sunset times is also affected by the angle at which the Sun’s path crosses the horizon. In spring and autumn, the Sun is moving more directly up or down relative to the horizon, causing the times to change rapidly from one day to the next. During the summer and winter solstices, the Sun’s path is more angled, meaning the daily time change is much smaller.
Understanding the Phases of Twilight
The period of ambient light that occurs before sunrise and after sunset is known as twilight, and it is officially divided into three distinct phases based on how far the center of the Sun is positioned below the horizon. The duration of all three twilight phases varies significantly with latitude, lasting only about 24 minutes total near the equator but extending for hours or even weeks near the poles.
Civil Twilight
The first phase, civil twilight, begins at sunset and lasts until the Sun is 6 degrees below the horizon. During this time, there is enough natural light for objects to be clearly seen, and many jurisdictions base regulations, like when car headlights must be turned on, on the limits of civil twilight.
Nautical Twilight
Nautical twilight follows, occurring when the Sun is between 6 and 12 degrees below the horizon. Mariners historically used the horizon and stars for navigation during this time, as the horizon line becomes difficult to distinguish from the sky. Artificial light is generally required for most outdoor activities.
Astronomical Twilight
The darkest phase is astronomical twilight, which lasts until the Sun is 18 degrees below the horizon. Once the Sun drops past this 18-degree mark, the sky is considered truly dark because the remaining illumination from the Sun’s scattered light is less than that from starlight.
The Impact of Daylight Saving Time
While the astronomical factors determine the actual moment of sunrise and sunset, the times published in local newspapers or apps are often altered by the human-made convention of Daylight Saving Time (DST). DST is a practice that artificially shifts the clock hour forward, typically by one hour, during the summer months. This adjustment is a change to civil time, not a change to the physical movement of the Earth or Sun.
When clocks “spring forward” for DST, the solar events still occur at the same astronomical time, but the time displayed on the clock is one hour later. For instance, a sunrise that occurred at 6:00 a.m. Standard Time will be listed as 7:00 a.m. DST, and sunset will also shift an hour later by the clock. This practice is designed to better align daylight hours with typical human schedules, providing more light in the evening.
When DST ends and clocks “fall back” to Standard Time, the reverse occurs, causing sunrise and sunset times to abruptly shift one hour earlier on the clock. The DST adjustment creates the sudden, hour-long jump that frequently confuses people tracking the Sun’s daily schedule.