Don’t be fooled – the calendar says the Spring equinox is on the March 20th, but in Chicago, we come closest to equal amounts of daylight and darkness on the 17th. I thought this oddity would have an easy explanation, but after a few hours or research, l learned there’s a lot to it.
The reason why we have an equinox in the first place is because of the Earth’s tilt. Relative to is path around the sun, the Earth is tilted at a roughly 23.4 degree angle, all year round. And, if you were to draw an arrow straight up from the North pole, and watched the Earth revolve around the world from a distance, that line would point at the same angle all year. That is to say, the Earth tilts in the same direction regardless of where it is in its yearly path. As a result, in Chicago’s summer, the northern hemisphere tilts towards the sun and receives direct sunlight, and in the winter, it tilts away and the southern hemisphere gets its time in the sun’s direct light.
On the summer solstice in the northern hemisphere, the sun directly shines on the tropic of Cancer, a line of latitude that is 23 degrees and 26 minutes north of the equator, which cuts through the middle of Mexico. In Chicago at this point, the sun will appear as at the highest point in the sky it will all year – for those in Mexico, the sun will appear directly overhead. The sun, of course, rises and sets at the same frequency and amount each day, so the fact that it peaks higher in the sky on the summer solstice means that we’re just seeing more of its typical journey, giving us extra daylight. In Chicago, this counts for a total of about 15 hours and 13 minutes of daylight.
Meanwhile, on the winter solstice, the sun shines directly on the the tropic of Capricorn, which falls along the same distance south of the equator, around southern Brazil, and the Sun will peak at the lowest point it will all year. We don’t see much of the sun during this part of the year – it doesn’t appear above the horizon until right before it peaks and right after, giving us short days. On the winter solstice in Chicago, our daylight hours shrink to about nine hours and seven minutes. (If you’re keeping score, that’s a difference of six hours and six minutes between summer and winter solstices).
The spring and autumn equinoxes, by comparison, will be rather average days, from a scientific point of view. On the equinoxes, we are exactly halfway between the winter solstice (the shortest day of the year) and the summer solstice (the longest). It’s not the coldest time of year, nor the warmest – it’s just sort of, you know, average.
On the spring and autumn equinoxes, the Earth is tilting sideways relative to the sun, meaning that if you were standing on the sun, it wouldn’t be tipping towards you or away from you. As a result, the sun shines directly on the entire Earth. On these days, those living near the equator (in northern Brazil, for instance) will see the sun lie directly overhead at noon. But for us in Chicago, the sun will reach an intermediate point in the sky–not as high as in summer, but not as low as in winter.
So you’ve probably deduced that if the entire Earth receives direct sunlight on the equinox, which is halfway between the longest and shortest days of the year, at least someone on Earth would see exactly 12 hours of sunlight and 12 hours of darkness. Right?
To start off, those living on the equator never see exactly 12 hours of daylight. On the equinox, in fact, they see light for a few minutes over 12 hours. This happens for two reasons: one semantic, and one physical.
First, the semantic reason: Astronomers define “sunrise” as the time when the top of the Sun first peeks over the horizon, and “sunset” as the time when the entire sun completely disappears below the horizon. Therefore, the time the Sun spends completely clearing the horizon in the morning and completely sinking below count as “day,” giving us a bit more than 12 hours of “daytime.” Not cool.
The physical reason is more interesting. The sky refracts, or bends, light towards the Earth. When the sun is sitting below the horizon in the morning, its light bends around the Earth and gives us some daytime before the Sun is directly visible. This time is what we call “dawn.” Likewise, after the Sun sets, some of its light bends around the Earth like tail, giving us “twilight,” or some residual daylight to finish off the day. At the equator, dawn and twilight give people more than 12 hours of daylight.
If you’re living in Chicago, you get close to 12 hours of daylight twice a year, but it’s not on the equinoxes. Why is this?
Chicago gets extra daylight for the same reason cities on the equator – semantics and refraction. This extra daylight means Chicago gets a head start in finally achieving 12 hours of daylight after a long winter. So, while the official equinox falls on the 20th this year, Chicago will actually see exactly 12 hours of daylight three days early, on the 17th.