On the nature of time: Daylight, Sunrise/Sunset, Twilight, and Dawn/Dusk

Note: The writer is in North America, and is writing primarily about conventions in North America using American English for this post. If and when I get more information on this particular topic in other cultures, I will add that to the series.

Why does it matter what we name a unit of time (second, minute, hour), or call a particular point in time (daybreak, sunset, Noon)? Frankly, it's to facilitate communication. If I want to meet someone at a particular point in time, I might tell them that I want to meet them at 1300 hours, or 1:00 PM, or an hour after Noon, and I would have a reasonable expectation that we would be talking about the same thing if we had the same convention for referring to a particular time. When we don't have the same convention for referring to a particular time, we need to know that so that we can agree on when to meet.

In my last post on time (On the nature of time: hours), I noted that some systems of measuring time start the day at nightfall/sunset, daybreak/sunrise, or at midnight mean solar time. I also mentioned that saying nightfall or sunset may not be saying the same thing.

Since the hours of daylight are usually defined according to sunrise and sunset, the unequal hours time systems have to have an agreement on just what that means.

Let’s start this at the easiest point to deal with: Sunset (also called sundown in some areas). This is, from the term, the time of day when the sun, well, sets. As a point in time, sunset is usually defined as that time of the day when the sun's trailing edge disappears below the horizon at the end of the daylight period.

Sunrise, likewise, is that point of time when the sun's leading edge first appears above the horizon at the beginning of the daylight period.

Because these conventions refer to the position of the trailing and leading edge of the sun in relation to daytime, not the center of the sun's disk, that means that at the Vernal and Autumnal Equinox, when the path of the Sun crosses the equatorial plane of the Earth, the defined period of daylight, using the definitions of sunset and sunrise above, is actually slightly longer than the defined period of darkness.

Even if you do measure the daylight period as the period when the center of the sun's disk is at or above the horizon, we can still see the sun when it is actually physically below the horizon due to refraction of light in the atmosphere, and again, the day when observed daylight and nighttime are the same length does not fall on the equinox.

This can lead to some very interesting conversations as some folk who understand the sunrise/sunset convention of defining the daylight period, or the effect of refraction, try to explain to others that the days that daytime and nighttime are the same length don't actually occur on the day of the equinox.

We call that period when the sun is not visible but there is still light in the sky twilight. In the morning, twilight begins at dawn, and in the evening, twilight ends at dusk.

So, what are dawn and dusk? There are three common definitions in use.

EDIT NOTE: In the following definitions, it is important to note that the sun's disk appears to be ½° wide, and that during all periods of twilight, the sun's disk is completely below the horizon.

Civil or civilian twilight: That period of time between when the center of the sun's disk is less than 6° below the horizon and the upper limb of the sun's disk is visible above the horizon (sunrise/sunset). The beginning of the morning twilight is called civil dawn and ends at sunrise, and the end of the evening twilight period is called civil dusk and begins at sunset. It is also sometimes referred to as the elapsed half hour before sunrise and the elapsed half hour after sunset, for simplicity's sake.

Nautical twilight: That period of time between when the center of the sun's disk is less than 12° below the horizon but greater than or equal to 6° below the horizon. The beginning of the morning twilight period is called nautical dawn and the end of the evening twilight period is called nautical dusk.

Astronomical twilight: That period of time between when the center of the sun's disk is less than 18° below the horizon but greater than or equal to 12° below the horizon. The beginning of the morning twilight period is called astronomical dawn and the end of the evening twilight period is called astronomical dusk.

Each type of twilight is often thought of by what kind of activity you can undertake during that period. During civil twilight, you can still perform regular outdoor activities without artificial light (one convention is that if you can no longer read, it is no longer twilight). During nautical twilight, you can still distinguish large objects at a distance, and there is sufficient darkness to take sightings of brighter, known stars and compare their positions to the horizon for navigation purposes (there are some additional practical definitions of nautical twilight regarding whether or not you can still see the rings of a target at a fixed distance). During astronomical twilight, you can observe most stars, but some of the dimmer objects, such as nebulae, are not distinguishable.

A moment's reflection should be sufficient to realize that sunrise and sunset, dawn and dusk, and the periods of twilight are all dependent on time of year and geography (including but not limited to the observer's latitude and longitude). They are local conventions.

Here's to local conventions.