Sunrise is when the sun’s upper edge appears on the horizon, and sunset is when it disappears below. These times are not fixed; they shift each day in a predictable pattern. This daily change results from the Earth’s rotation and its orbit around the sun, which explains the constant, gradual flux in the length of daylight.
Why Sunrise and Sunset Times Change Daily
The primary reason for the daily change in sunrise and sunset times is the Earth’s axial tilt. Our planet is tilted at approximately 23.5 degrees relative to its orbital plane. This tilt means that as the Earth revolves around the sun, different parts of the planet receive more direct sunlight. During summer in the Northern Hemisphere, it is tilted toward the sun, resulting in longer days and shorter nights. Conversely, during winter, this hemisphere is tilted away, leading to shorter days.
A secondary factor is the Earth’s elliptical orbit around the sun. This oval-shaped path means the Earth’s orbital speed varies throughout the year, causing the sun to appear to move across the sky at slightly different speeds daily. The combination of the axial tilt and the elliptical orbit causes the sun to follow a different path across the sky each day, leading to continuous changes in sunrise and sunset times.
How Location Affects Sunrise and Sunset
Your geographic location, specifically your latitude, influences the times you experience sunrise and sunset. Latitude, your distance from the equator, determines the variation in daylight hours throughout the year. Locations closer to the equator experience roughly 12 hours of daylight year-round, while areas nearer the poles see very long days in summer and very short days in winter.
Longitude, your east-west position, also plays a part in relation to time zones. The Earth rotates from west to east, so locations farther east experience sunrise earlier than those to the west. Within a single time zone, a person on the eastern edge will see the sun rise several minutes earlier than someone on the western edge. For every 70 miles you travel west within your time zone, the sunrise occurs about four minutes later.
The Different Phases of Twilight
Twilight is the period when the sun is below the horizon, but its scattered light still illuminates the sky. It is divided into three distinct phases based on the sun’s position below the horizon. These phases occur in reverse order before sunrise.
The first phase after sunset is civil twilight, ending when the sun’s center is 6 degrees below the horizon. During this time, there is enough natural light for outdoor activities without artificial illumination.
Following civil twilight is nautical twilight, which concludes when the sun is 12 degrees below the horizon. At this stage, sailors can see the horizon and use bright stars for navigation.
The final stage is astronomical twilight, lasting until the sun is 18 degrees below the horizon. By the end of this phase, the sky is almost completely dark, and the faintest stars become visible for astronomical observations.