According to the paper, tomorrow’s sunrise will be at 6:39 am, and sunset wil be at 6:48 pm. That’s not 12 hours of day and night, but isn’t it supposed to be? What accounts for the discrepancy?
I think this is because the paper reports apparent sunrise/sunset. Due to atmospheric refraction, sunrise occurs shortly before the sun crosses above the horizon. Light from the sun is bent, or refracted, as it enters earth’s atmosphere. This effect causes the apparent sunrise to be earlier than the actual sunrise. Similarly, apparent sunset occurs slightly later than actual sunset.
Also, sunrise/sunset times refer to the time the first bit of the solar disk arises (or the sun completely disappears below the horizon). Without atmosphere, I’d expect the time between when the center of the solar disk is on the horizon to be 12 hours on the equinox, but a few minutes needs to be added at sunrise to account for the time between first light and the moment the sun is halfway above the horizon (similar at sunset). Atmospheric effects extend this time as described above. I think all are included in apparent sunrise/sunset calculations.
CCJ* has part of it. A pretty simple explanation can found here.
You won’t see that condition unless you’re on the equator. Equinox is defined as the sun being directly overhead at local noon on the equator.
Sorry, but you’re wrong on both points. There will be (roughly) 12 hours of day and night at any latitude, but this is not prescely true at any latititude, including the equator, due to the subtleties that have already been mentioned. And the the equinox is defined as the Sun crossing the celestial equator. This means that the Sun, at its highest point, is approximately at zenith for an observer at the equator, but the equinox is a moment, not a date.
In particular, this equinox is happening — or maybe has happened, by the time you read this — at 22:23 UT today, September 22nd.
That’s 6:23 PM here in North America’s Eastern time zone. And in Ecuador, where you could stand right on the equator if you wished, the equinox will be at 5:23 PM local time.
Happy Equinox! (God bless us, everyone.)
And since the sun crosses the celestial equator in an instant, at that instant it will be precisely at zenith for some point on the equator. A moment later, it will be at zenith for some other point south of the equator.