The experience of looking outside at 7 AM and finding it still dark is common. This phenomenon often prompts questions about why daylight varies significantly throughout the year. The timing of sunrise is a complex interplay of astronomical factors and human-made conventions.
Earth’s Axial Tilt and Seasonal Changes
A primary reason for varying daylight length throughout the year is Earth’s axial tilt. Our planet’s rotational axis is tilted by approximately 23.5 degrees relative to its orbital path around the Sun. As Earth revolves around the Sun, this consistent tilt angles different hemispheres toward or away from our star.
When a hemisphere tilts toward the Sun, it receives more direct sunlight, leading to summer with longer days and shorter nights. Conversely, tilting away from the Sun results in less direct sunlight, causing shorter daylight hours and longer nights, marking the arrival of winter.
During winter months, the angle at which sunlight strikes the surface is lower. This leads to the sun appearing lower in the sky, and the period of daylight becomes shorter. Shorter daylight hours mean sunrise occurs later by the clock, contributing to the perception of darkness at 7 AM.
Our Position on Earth
Beyond Earth’s axial tilt, a location’s latitude significantly influences how day and night length varies throughout the year. Regions closer to the equator, at lower latitudes, experience consistent day lengths, close to 12 hours of daylight and 12 hours of darkness year-round. This is because the sun’s path across the sky does not change dramatically for these areas.
Moving toward higher latitudes, the axial tilt’s effects become more pronounced. These areas experience greater variations in daylight hours between summer and winter.
In winter, the sun’s low angle means it takes longer to rise above the horizon, resulting in later sunrises and shorter days.
For example, a northern latitude city will see sunrises shift later in winter compared to an equatorial city. This amplification of shorter daylight at higher latitudes contributes to the observation of darkness at 7 AM.
The Impact of Time Zones and Daylight Saving
Human-imposed systems also play a substantial role in when 7 AM feels dark. Time zones divide the world into longitudinal bands, standardizing local time. Each time zone generally represents about 15 degrees of longitude, corresponding to one hour of Earth’s rotation. This system ensures communities within a region operate on a unified clock time.
Time zones do not perfectly align with solar time. For locations on the western edge of a time zone, sunrise occurs later by the clock compared to those on the eastern edge. This is because Earth rotates from west to east, bringing the sun’s light to eastern longitudes first.
Daylight Saving Time (DST) further complicates early morning darkness. DST involves advancing clocks by one hour during warmer months to shift an hour of daylight from morning to evening. When DST ends in autumn, clocks are “set back” one hour to standard time.
This “falling back” effectively makes sunrise occur an hour earlier in solar time, but the clock has also moved back an hour. Consequently, 7 AM by the clock aligns with what was previously 6 AM, making the morning appear darker. The practice was initially adopted to conserve energy, especially during wartime.