Why Days Shorten Fastest at the Equinox, Not the Solstice
The sun's path crosses the horizon at its steepest angle near the equinoxes and nearly flattens out near the solstices — and Chicago's sunrise-sunset data show just how much that speeds up, then stalls, the loss of daylight.
Chicago lost 28 minutes of daylight in the 10 days after this year's autumnal equinox. It will take the entire 10 days surrounding the winter solstice combined to lose 2 more minutes. Same city, same planet, wildly different pace. The reason has nothing to do with clouds, latitude drift or anything the weather did this week.
The equinox arrived at 7:05 p.m. Central time on Tuesday, Sept. 22, 2026, the moment the sun crossed directly over Earth's equator on its way south for the season. NASA describes that instant as when the center of the Sun crosses the plane of our planet's equator.
Three days later, the effect is easy to feel in Chicago: sunrise creeps later, sunset arrives earlier, and it happens fast. That speed isn't a coincidence of timing. It's the sharpest the daylight clock moves all year, and it won't move that fast again until spring.
The Angle That Actually Drives the Clock
Earth's axis is tilted 23.5 degrees relative to its orbital plane, and that tilt is the entire story. As the National Radio Astronomy Observatory explains, how fast sunrise and sunset times shift each day depends on how steeply the sun's apparent path crosses the horizon. Near the equinoxes, that crossing is close to vertical: the sun cuts through the celestial equator, and the horizon, at close to a right angle. Near the solstices, the same path runs almost parallel to the horizon, gliding sideways along it instead of cutting through it.
A steep crossing means a small shift in the sun's north-south position translates into a big shift in when it clears the horizon. A shallow crossing does the opposite: the sun's position keeps changing day to day, but the horizon-crossing time barely budges, because the sun is skimming along the horizon rather than dropping through it. That's why the fastest change in day length all year clusters around late September and late March, not around the longest and shortest days themselves.
Chicago's Daylight Ledger, Minute by Minute
Sunrise and sunset data from the U.S. Naval Observatory show exactly how that plays out at 41.88 degrees north. On Sept. 20, the sun rose over Chicago at 6:36 a.m. and set at 6:51 p.m., 12 hours and 15 minutes of daylight. Ten days later, on Sept. 30, sunrise had slid to 6:47 a.m. and sunset to 6:34 p.m., for 11 hours and 47 minutes. That's a loss of 28 minutes in 10 days, or roughly 2.8 minutes disappearing every single day. The same math applied to the March equinox produces almost the identical number in reverse, about 2.8 minutes gained per day, because the geometry is symmetric on both sides of the year.
Run the clock forward to the solstice and it nearly stops. From Dec. 18 through Dec. 28, Chicago's day length holds at 9 hours and 8 minutes for more than a week before nudging up by a single minute. Net change over that 10-day window: 2 minutes. That's roughly 14 times slower than the equinox rate. The sun hasn't stopped moving. The horizon-crossing math has simply gone flat.
Why the Peak Shifts Depending on Where You Stand
The rate is symmetric across both equinoxes and flat at both solstices, but it isn't identical everywhere, and it isn't locked to the calendar either. Meteorologists at the University of Wisconsin-Madison's satellite research institute, writing on the Weather Guys blog, note that while the equinoxes mark the fastest change in raw minutes lost or gained per day, the fastest change in percentage terms (daylight lost relative to how much remains) actually peaks a few weeks later, in late October and early November at midlatitudes. By then the days are already shorter, so the same 2- or 3-minute swing is a bigger slice of a smaller pie. Move the whole effect toward the poles and it intensifies further; move it toward the equator and it nearly disappears, since day length there barely varies across the year at all.
None of this should be confused with the equinox's other myth. As Daybreak Wire explained after this year's equinox, the day-night split on Sept. 22 wasn't exactly even, either. Atmospheric refraction and the width of the sun's disk pushed the true 12-hour, 12-hour balance to a separate date entirely. The equinox is the fastest-changing day of the year. It was never the most evenly split one.
Chicago has its own visual marker for the underlying geometry. Twice a year, near the equinoxes, the setting sun lines up with the city's street grid, an effect local television has nicknamed Chicagohenge. This week's alignment was the year's last until the sun swings back across the equator in March.
The local forecast published around this year's equinox put the immediate loss at 2 to 3 minutes of daylight per day through the end of September, consistent independently with the Naval Observatory figures above. That's the number worth remembering this week, not the one usually attached to the season: it isn't the winter solstice, the shortest day of the year, that pulls daylight away the fastest. It's right now.
The clock won't stay this fast. By the time the sun reaches its lowest arc on Dec. 21, Chicago's sunrise and sunset will have gone nearly still, holding within a minute of each other for more than a week. Sometime after New Year's, the whole cycle reverses. Daylight returns at a crawl at first, then accelerates into March at that same steep angle, handing back everything September just took away.
