This question came up in early January. I wake up early most days, because the shift I work runs 5 am to 1:30 pm Monday through Friday, so “sleeping in” has become “sleeping to 6 am”. As a result, I’m sitting out in the living room when my son comes staggering out at 7 am, wearing his flannel pyjamas and clutching his favorite fleece blanket and clearly not quite awake. He flops down on the couch next to me, rests his head on my leg, and glances at the sliding glass doors. “Is it still night?” he asks, talking through a jaw-cracking yawn.
“No,” I tell him. “It’s seven in the morning.”
“But the sun’s not up!” he manages.
Which, if you think about it, makes perfect sense. Shouldn’t “night” be when the sun isn’t visible, and “day” be when it is visible?
What is night?
The first three definitions of “night” are as follows:
- the period of darkness between sunset and sunrise.
- the beginning of this period; nightfall.
- the darkness of night; the dark.
Interestingly enough, there is also a legal definition of night found in Title 14 of the Code of Federal Regulations, Part 1, Section 1.1: “Night means the time between the end of evening civil twilight and the beginning of morning civil twilight, as published in the Air Almanac, converted to local time”.
It turns out that there are three different types of twilight: civil twilight, nautical twilight, and astronomical twilight. Which one you use depends on what you are trying to achieve.
Civil twilight is defined to begin in the morning, and to end in the evening when the center of the Sun is geometrically 6 degrees below the horizon. This is the limit at which twilight illumination is sufficient, under good weather conditions, for terrestrial objects to be clearly distinguished; at the beginning of morning civil twilight, or end of evening civil twilight, the horizon is clearly defined and the brightest stars are visible under good atmospheric conditions in the absence of moonlight or other illumination. In the morning before the beginning of civil twilight and in the evening after the end of civil twilight, artificial illumination is normally required to carry on ordinary outdoor activities.
Nautical twilight is defined to begin in the morning, and to end in the evening, when the center of the sun is geometrically 12 degrees below the horizon. At the beginning or end of nautical twilight, under good atmospheric conditions and in the absence of other illumination, general outlines of ground objects may be distinguishable, but detailed outdoor operations are not possible. During nautical twilight the illumination level is such that the horizon is still visible even on a Moonless night allowing mariners to take reliable star sights for navigational purposes, hence the name.
Astronomical twilight is defined to begin in the morning, and to end in the evening when the center of the Sun is geometrically 18 degrees below the horizon. Before the beginning of astronomical twilight in the morning and after the end of astronomical twilight in the evening, scattered light from the Sun is less than that from starlight and other natural sources. For a considerable interval after the beginning of morning twilight and before the end of evening twilight, sky illumination is so faint that it is practically imperceptible.vening twilight, sky illumination from the Sun is so faint that it is practically imperceptible.
That’s the “conventional” definition. The technical definition is similar, but involves more geometry:
Twilight. There are three kinds of twilight defined: civil twilight, nautical twilight, and astronomical twilight. For computational purposes, civil twilight begins before sunrise and ends after sunset when the geometric zenith distance of the center of the Sun is 96 degrees – 6 degrees below a horizontal plane. The corresponding solar zenith distances for nautical and astronomical twilight are 102 and 108 degrees, respectively. That is, at the dark limit of nautical twilight, the center of the Sun is geometrically 12 degrees below a horizontal plane; and at the dark limit of astronomical twilight, the center of the Sun is geometrically 18 degrees below a horizontal plane.
Sunrise and Sunset
The United States Naval Observatory also defines sunrise and sunset: “Sunrise and sunset conventionally refer to the times when the upper edge of the disk of the Sun is on the horizon. Atmospheric conditions are assumed to be average, and the location is in a level region on the Earth’s surface.” They also provide a technical definition:
Sunrise and sunset. For computational purposes, sunrise or sunset is defined to occur when the geometric zenith distance of center of the Sun is 90.8333 degrees. That is, the center of the Sun is geometrically 50 arcminutes below a horizontal plane. For an observer at sea level with a level, unobstructed horizon, under average atmospheric conditions, the upper limb of the Sun will then appear to be tangent to the horizon. The 50-arcminute geometric depression of the Sun’s center used for the computations is obtained by adding the average apparent radius of the Sun (16 arcminutes) to the average amount of atmospheric refraction at the horizon (34 arcminutes).
So, is it still night?
Assuming you’re using civil twilight, dawn twilight begins when the center of the Sun is 6 degrees below the horizon and ends when the center of the Sun is 0.8333 degrees below the horizon. How long it takes to move from night through twilight to day depends on your latitude and the time of year. But ultimately, until the sun rises, it is still night.
Interestingly enough, having checked a sunrise and sunset table for Cincinnati, I was wrong. It was 7 am, and I said it was daytime and the sun was up. It wasn’t. The earliest the sun rose in Cincinnati in January 2016 was 7:46 am, so it was actually twilight.