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Fall begins tonight, but why?

And are day and night actually equal on the equinox?
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Posted at 3:36 PM, Sep 22, 2022
and last updated 2022-09-22 15:36:12-04

CLEVELAND — Seasons are changing! Fall officially begins on Thursday at 9:04 pm. Fall will continue for 90 days until December 21 at 4:48 p.m. when winter starts.

WHY DO WE EVEN HAVE SEASONS?

The Earth makes a complete revolution around the sun once every 365 days, following an orbit that is elliptical in shape. This means that the distance between the Earth and Sun, which is 93 million miles on average, varies throughout the year.

Interestingly, the Earth is closest to the sun during winter in the Northern hemisphere! During the first week in January, the Earth is about 1.6 million miles closer to the sun. This is referred to as the perihelion or the point when Earth is closest to the sun.

And the aphelion, or the point at which the Earth is about 1.6 million miles farther away from the sun, occurs during the first week of July. This fact may sound weird to us, but the difference is not significant in terms of climate and is NOT the reason why we have seasons.

Seasons are caused by the fact that the Earth is tilted on its axis by 23.5°.  The tilt's orientation with respect to space does not change during the year; thus, the Northern Hemisphere is tilted toward the sun in June and away from the sun in December, as illustrated in the graphic below.

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NOT EQUAL ON THE EQUINOX....

We have been losing daylight rapidly over the last few weeks. Today, there will be about 12 hours and 9 minutes of daylight. The sun came up around 7:15 a.m. in Cleveland on Thursday and will set around 7:24 pm. But...shouldn't the sunrise and sunset be exactly 12 hours apart on the Equinox (which comes from the Latin term meaning "equal night")?

While the geometric center of the Sun is above the horizon for 12 hours today, sunrise is defined as when the edge of the Sun's disk becomes visible, not when the center of the Sun is visible. The atmosphere also causes the light to bend ("refraction"), affecting when the Sun starts to be seen versus when it actually crosses the horizon. The combination of these factors will cause the equal day-night (known as "equilux") to vary by latitude.

The date when there are equal parts day and night happens a few days after the autumnal equinox. The specific dates for this occurrence are different for different latitudes. For our latitude, this will happen on September 25 with 12 hours and 1 minute of daylight. By September 26, we will lose another 2 minutes and 45 seconds of daylight.

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