The evenings begin to shorten considerably as summer moves toward autumn, leading many to wonder when this seasonal shift in daylight truly begins. Although the change in daylight hours is a constant, predictable astronomical event, people often notice the days getting darker much later than the scientific starting point. This confusion arises because the initial reduction in sunlight is subtle and easily overlooked. Understanding this process requires examining the movement of the Earth relative to the Sun, which dictates the precise timing of our daylight hours.
The Astronomical Starting Point
The longest day of the year marks the scientific starting point for the evenings to begin shortening. This event is the Summer Solstice, which typically occurs around June 20th or 21st in the Northern Hemisphere. The Solstice happens when the Earth’s axial tilt angles the hemisphere most directly toward the Sun, resulting in the maximum period of daylight.
Immediately following this peak, the Earth’s orbit and tilt cause the sun’s angle to decline, meaning the sun sets a fraction of a second earlier each day. This shift is a direct consequence of the Earth’s 23.5-degree axial tilt, which governs the changing position of the sun. Although the sun sets progressively earlier after the Solstice, the initial change is so minute that it is virtually undetectable.
When the Shifting Darkness Becomes Noticeable
Although the sun starts setting earlier in late June, the change is initially too slow to be felt in daily life. In the days immediately following the Summer Solstice, the loss of daylight may only amount to a few seconds per day. This subtle rate of change continues through early summer, making the bright evenings of July feel nearly constant.
The change in evening darkness becomes much more apparent as the year progresses toward the Autumnal Equinox in late September. This is when the rate of daylight loss reaches its maximum speed, which can be around three minutes of total daylight lost per day in mid-latitude locations. In northern locations, the total loss of daylight throughout September alone can exceed 100 minutes. This rapid daily reduction causes people to notice the evening darkness setting in much earlier than they recall from summer.
The quickening rate of change is due to the geometry of the Earth’s tilt relative to the sun around the equinox. At this time, the Earth’s position creates the fastest change in the sun’s angle, translating to a greater daily difference in day length. Unlike the solstices, the period around the equinox involves the most dynamic change in the amount of light received. This peak rate of shortening days is why most people perceive the evenings as truly getting darker in September, rather than in July.
The Impact of Daylight Saving Time
Daylight Saving Time (DST) dramatically affects the perception of when it gets darker earlier in the fall. In the Northern Hemisphere, DST generally ends in early November when clocks are moved back one hour to revert to standard time. This change artificially shifts the entire schedule of sunrise and sunset by 60 minutes.
The “fall back” causes the sunset time to instantly appear one hour earlier on the clock, creating a sudden increase in perceived evening darkness. This abrupt jump often overshadows the weeks of gradual astronomical darkening that preceded it. People may feel the evenings have suddenly become much shorter, even though daylight has been diminishing steadily since June. Reverting to standard time in autumn returns the clock to a more astronomically aligned schedule, but the sudden shift registers most strongly in public consciousness.
Why Morning and Evening Shifts Are Asymmetrical
The earliest sunset and the latest sunrise do not occur on the same day as the Winter Solstice, the shortest day of the year. This asymmetry results from the Equation of Time, which accounts for the difference between clock time and solar time. Standard clocks assume every day is exactly 24 hours long, but the actual time between one solar noon and the next varies slightly throughout the year.
This variation is caused by two factors: the Earth’s elliptical orbit and its axial tilt. The elliptical path means the Earth speeds up and slows down slightly as it orbits the sun. This effect causes the earliest sunset to occur several weeks before the Winter Solstice, typically in early to mid-December. Conversely, the latest sunrise occurs a few weeks after the Solstice, usually in early January. Therefore, leading up to the shortest day, the sunset gets earlier while the sunrise moves slightly later, an effect reversed after the solstice.