“I am 52 and till now I thought dawn and sunrise the same thing!”

That comment was left on a blog post written by a physicist and teacher explaining the meanings of dawn, dusk, sunrise, sunset and twilight.

I admit being a bit foggy about how these parts of the process of the beginning and ending of each day fit together, so thought I should share the info.

Sunrise and sunset are easy. They occur when the top edge of the sun is on the horizon, coming and going.

But there’s a lengthy period of twilight before sunrise and after sunset that is divided into three categories.

Here’s a long excerpt from the explanation provided by the U.S. Naval Observatory.

Twilight: Before sunrise and again after sunset there are intervals of time, twilight, during which there is natural light provided by the upper atmosphere, which does receive direct sunlight and reflects part of it toward the Earth’s surface. Some outdoor activities may be conducted without artificial illumination during these intervals, and it is useful to have some means to set limits beyond which a certain activity should be assisted by artificial lighting. The major determinants of the amount of natural light during twilight are the state of the atmosphere generally and local weather conditions in particular. Atmospheric conditions are best determined at the actual time and place of events. Nevertheless, it is possible to establish useful, though necessarily approximate, limits applicable to large classes of activities by considering only the position of the Sun below the local horizon. Reasonable and convenient definitions have evolved.

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.

Here are today’s times of sunrise, and the beginning of the several stages of twilight in Honolulu, as calculated by the Naval Observatory.

Astronomical Twilight — 5:42 a.m.

Nautical Twilight — 6:08 a.m.

Civil Twilight — 6:34 a.m.

Sunrise — 6:56 a.m.

I know that for many of you, this early morning stuff is all theoretical. But we go walking in the early morning, so it makes a big difference to us. And as I chase sunrise photos, the best times range from the start of civil twilight, and extend through some minutes after sunrise, depending on the clouds and atmospheric conditions. And what are the “atmospheric conditions” that make the difference? I haven’t figured that out yet, but mostly it seems related to the presence of clouds and how they are layered as it gets close to sunrise. I suppose that I should be keeping a record of the day-to-day differences in sunrise colors, or their absence, in order to get more information. Or perhaps I should look online, since someone has probably already done this.