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Civil, Nautical, or Astronomical?
The light in the middle of the day can be harsh and unflattering. But the light just before and after sunrise and sunset can be marvelous. If you've tried to look up when sunrise and sunset occur though you may have ended up a bit confused since many sources list times for three different kinds of twilight. If you've wondered what's up with civil twilight, nautical twilight and astronomical twilight, read on.
 At first glance, it would seem that the sun comes up when it crosses the horizon — before that it's dark, and after that it's daylight. But the sun is no small object so it takes a while to cross the horizon. Does daylight officially begin when the sun starts to come up, or when the whole thing is above the horizon? The color of the sky changes quite a bit during this interval, so if you want to be in the right place at the right time, these things matter. But it gets even more interesting than that since it actually starts to get light even before the sun peeks over the horizon. Remember, not only is the sun big, but it's also rather bright. This whole sunrise and sunset thing can become a bit complicated when you get right down to it.
The truth is, precisely where the sun is at when someone officially declares that sunrise or sunset has happened depends on who is doing the declaring, and why they are doing it. Generally speaking, there are three different "official" answers.
Civil twilight officially begins in the morning and ends at night when the center of the sun is six degrees below the horizon line. At such times, the brightest starts will just be visible under normal atmospheric conditions, and you should still be able to see terrestrial objects clearly. This is also about the time you would generally turn on your car's headlights or need artificial lights to see in the evening. It's the time most people call "dusk." It can be easy to shoot a lot of images during civil twilight because the light changes so quickly. By the time civil twilight has occurred in the evening, most people will have called it a night and gone inside thinking that the magic light is over, but the second act is only just starting.
The boundaries of nautical twilight occur when the center of the sun is twelve degrees below the horizon. Depending on your latitude and the time of the year, this will be roughly half an hour before civil twilight ends in the morning and half an hour after it ends in the evening. Thus, nautical twilight is darker than civil. Between nautical and civil twilight you can generally discern the shape of objects but not their details. "First light" and "last light" generally refer to the nautical twilight. When color first shows in the east in the morning and disappears from the western sky at night you can say that nautical twilight has occurred. Beyond this, most people would consider it to be dark. Photography during nautical twilight can result in vivid colors ranging from indigo through pink but you will need to be prepared for long exposure times to get them.
 Astronomical twilight is darker still and happens when the center of the sun is a full eighteen degrees below the horizon. As the name implies, this is mostly of interest to astronomers. There is no color in the sky during astronomical twilight, but there is still enough faint light to make nebula, galaxies and other diffuse objects hard to see in a telescope. Star trail photographs will register some background glow near the horizon during this time, although this will often be the case these days near any major city all night long due to light pollution. Even die hard nature photographers will probably have called it quits for the night by this point.
What I plan for depends on what I am shooting. Subjects that are still recognizable in silhouette or are large enough to remain interesting in low light can work quite well during nautical twilight. Sea stacks, mountains, rock formations and landmarks, and even large trees can all be good candidates. Smaller subjects and those with few major discernable features tend to have their limits with civil twilight. No matter how beautiful the sky may be beyond that, the low contrast light that remains just can't adequately yield enough detail in such subjects. Of course, judicious use of fill flash can allow you to extend things.
So, take a nap at mid-day if you have to, but be out there for the magical light of sunrise and sunset. This is prime time for outdoor photography. Just what time that will be depends on what kind of photos you are after. When in doubt of course, get there early for sunrise, and plan on staying late for sunset.
Oh, and do remember to bring a flashlight so you can get to where you want to be for sunrise and find your way back when you do call it quits at night.
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