Equinoxes at right. Solstices at left. In each of the images, Earth’s rotational axis is perpendicular (straight up and down), with the North Pole at top and South Pole at bottom. Images via Geosync.
It’s often said that days and nights are equal lengths twice a year, at the March and September equinoxes, for everyone worldwide. Is it true? Does everyone receive 12 hours of day and 12 hours of night at the equinoxes? Not exactly. There’s actually more day than night on the day of an equinox. At mid-temperate latitudes, there are approximately 8 more minutes of daylight on the day of an equinox. There are two big reasons why:
1. The sun is a disk, not a point.
2. Atmospheric refraction.
Read more about the September 2020 equinox: All you need to know
Contemplating the sunset on the Philippine island of Leyte. Photo by Abie Oquias Baybay.
1. The sun is a disk, not a point. Watch any sunset, and you know the sun appears in Earth’s sky as a disk.
It’s not pointlike, as stars are, and yet – by definition – most almanacs regard sunrise as when the leading edge of the sun first touches the eastern horizon. They define sunset as when the sun’s trailing edge finally touches the western horizon.
This in itself provides an extra 2 1/2 to 3 minutes of daylight at mid-temperate latitudes.
Atmospheric refraction raises the sun about 1/2 degree upward in our sky at both sunrise and sunset. This advances the time of actual sunrise, while delaying the time of actual sunset. The result is several minutes of extra daylight, not just at an equinox, but every day. Image via Wikipedia.
2. Atmospheric refraction. The Earth’s atmosphere acts like a lens or prism, uplifting the sun about 0.5 degrees from its true geometrical position whenever the sun nears the horizon. Coincidentally, the sun’s angular diameter spans about 0.5 degrees, as well.
In other words, when you see the sun on the horizon, it’s actually just below the horizon geometrically.
What does atmospheric refraction mean for the length of daylight? It advances the sunrise and delays the sunset, adding nearly another six minutes of daylight at mid-temperate latitudes. Hence, more daylight than night at the equinox.
Astronomical almanacs usually don’t give sunrise or sunset times to the second. That’s because atmospheric refraction varies somewhat, depending on air temperature, humidity and barometric pressure. Lower temperature, higher humidity and higher barometric pressure all increase atmospheric refraction.
On the day of the equinox, the center of the sun would set about 12 hours after rising – given a level horizon, as at sea, and no atmospheric refraction.
Illustrations like this one make it seem as if day and night should be equal at the equinox. In fact, they aren’t exactly equal.
Bottom line: There’s slightly more day than night on the day of an equinox. That’s because the sun is a disk, not a point of light, and because Earth’s atmosphere refracts (bends) sunlight.
from EarthSky https://ift.tt/2uatFvW
Equinoxes at right. Solstices at left. In each of the images, Earth’s rotational axis is perpendicular (straight up and down), with the North Pole at top and South Pole at bottom. Images via Geosync.
It’s often said that days and nights are equal lengths twice a year, at the March and September equinoxes, for everyone worldwide. Is it true? Does everyone receive 12 hours of day and 12 hours of night at the equinoxes? Not exactly. There’s actually more day than night on the day of an equinox. At mid-temperate latitudes, there are approximately 8 more minutes of daylight on the day of an equinox. There are two big reasons why:
1. The sun is a disk, not a point.
2. Atmospheric refraction.
Read more about the September 2020 equinox: All you need to know
Contemplating the sunset on the Philippine island of Leyte. Photo by Abie Oquias Baybay.
1. The sun is a disk, not a point. Watch any sunset, and you know the sun appears in Earth’s sky as a disk.
It’s not pointlike, as stars are, and yet – by definition – most almanacs regard sunrise as when the leading edge of the sun first touches the eastern horizon. They define sunset as when the sun’s trailing edge finally touches the western horizon.
This in itself provides an extra 2 1/2 to 3 minutes of daylight at mid-temperate latitudes.
Atmospheric refraction raises the sun about 1/2 degree upward in our sky at both sunrise and sunset. This advances the time of actual sunrise, while delaying the time of actual sunset. The result is several minutes of extra daylight, not just at an equinox, but every day. Image via Wikipedia.
2. Atmospheric refraction. The Earth’s atmosphere acts like a lens or prism, uplifting the sun about 0.5 degrees from its true geometrical position whenever the sun nears the horizon. Coincidentally, the sun’s angular diameter spans about 0.5 degrees, as well.
In other words, when you see the sun on the horizon, it’s actually just below the horizon geometrically.
What does atmospheric refraction mean for the length of daylight? It advances the sunrise and delays the sunset, adding nearly another six minutes of daylight at mid-temperate latitudes. Hence, more daylight than night at the equinox.
Astronomical almanacs usually don’t give sunrise or sunset times to the second. That’s because atmospheric refraction varies somewhat, depending on air temperature, humidity and barometric pressure. Lower temperature, higher humidity and higher barometric pressure all increase atmospheric refraction.
On the day of the equinox, the center of the sun would set about 12 hours after rising – given a level horizon, as at sea, and no atmospheric refraction.
Illustrations like this one make it seem as if day and night should be equal at the equinox. In fact, they aren’t exactly equal.
Bottom line: There’s slightly more day than night on the day of an equinox. That’s because the sun is a disk, not a point of light, and because Earth’s atmosphere refracts (bends) sunlight.
from EarthSky https://ift.tt/2uatFvW
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