Crepusclar celestial events refer to everything when the sun is "close" to the horizon. This is the time when you get the most beautiful colors in the sky as the sun's rays become filtered in through the atmosphere. So why not learn about those actual phases so you can know more precisely what's going on when you see that magical palette of rich oranges and reds off your favorite beach at sunset?!
First off, sunrises and sunsets.
This one is simple. Sunset is the exact moment trailing edge, which would be the "upper edge" of the sun sphere disappears below the horizon. The moment you can no longer see the "orb of the sun" is the last second of a sunset and "sunset phase" is over. Therefore, a sunset lasts exactly how long it takes for the trailing edge of the sun to disappear from the horizon after the leading edge of the sun first hits the horizon. Sunrise, the exact opposite, is the exact moment the trailing edge of the sun full emerges from the horizon. A sunrise "commences" when the leading edge first appears on the horizon, but the sphere is not yet visible, and a sunset "commences" when the leading edge first touches the horizon, but the full sphere is totally visible. Technically sunrise and sunset is when the geometric zenith distance of center of the Sun is 90.8333 degrees, but that's "hyper technical".
The reason why I mentioned sunrises and sunsets were so simple is because there's only one definition of them. Whether your into civil, nautical or astronomical "sky stuff", a sunset and sunrise has a universal definition.
Things get a little more complicated with twilight, dusk, and dawn. Those special times of day are when cats totally freakout because they'e crepuscular! Okay, huge non-sequitor, but I love cats and you'll have to read my post on Cat Vocalizations to learn more about cat sleep patterns. Back to the complications with twilight, dusk, and dawn. There are three types of "getting dark" or "getting light" phases. All measurements agree that day is when the sun is in the sky, night is when their's no light from the sun visible, and sunrise and sunset when the visible sun orb is appearing or disappearing off the horizon. But when the sun disapears off the horizon, after sunset, for example, there's still light! The sun's illumination is not an on-off light switch! Light fades. It dims or slowly illuminates! So, let's start with the categorizations of light. But before we do let's just define the three terms of twilight, dusk, and dawn.
The Sequence of "low horizon" patters of the sun are:
- Dawn
- Twilight
- Sunrise
- Sunset
- Twilight
- Dusk
Now there exist three key types of measurement and each of these have their own unique definition of daytime twilight, nighttime twilight, dusk, and dawn. Fortunately, the only difference is the angle measurement of the sun's position off the horizon. In other words, the different between civil, nautical, and astronomical solar "in between light" hour measurements is purely mathematical!! Phew! I bet you thought we'd have to go through ridiculous parables like "When the sun's bun can no longer be won, then do not fright without light" or some idiotic imprecise, fable. Nope, don't need to worry about that. These solar measurements are precise! So let's get started on those!
Twilight -- The "in between time" when there's scattered sunlight in the sky off the horizon, but the actual sun orb technically just set or has not yet risen. Therefore, twilight kicks in immediately after sunset and ends immediately at dusk, while daytime twilight kicks in immediately after dawn and ends immediately at sunrise.
Civil -- I don't really care about civil twilight because of it's inaccuracy and misleading measurements. Technically, its "dusk" isn't really dusk and it's "dawn" isn't really dawn because there's still light. Nevertheless, civil dusk is when the sun is 6° below the horizon in the evening, while dawn is obviously 6° below the horizon in the morning. Because of its misleading measurements, that's all we'll discuss regarding civil measurments.
Nautical -- Nautical crepuscular measurements are more precise. They reflect "real darkness" and "real morning". Simply, or should I say mathematically, put , nautical twilight is the time period when the sun is between 6° and 12° below the horizon. If the center of the sun is between 6° and 12° below the horizon and it's night, that's nighttime nautical twilight; if it's morning, that's daytime nautical twilight.
Astronomical -- This is the measurement I like the most because it represents "real" light. Civil is purely misleading and Nautical is great because it represents visible objects. The nautical dusk, for example, is when you can't see anything well enough to navigate! Pretty useful. But for those of your who just are appreciating solar light, you've gotta use astronomical! It's the real times of day and night that reflects exact amounts of solar light! So astronomical twilight is when the sun is between 12° and 18° below the horizon; in the morning that's daytime twilight, in the night, that's nocturnal twilight.
Dusk and Dawn -- contrary to popular belief dusk and dawn only occur for a few seconds!! It's morning twilight and nighttime twilight that actual last for as a half hour each depending on which measurement you use. Again, there are three definitions of dusk and dawn: civil, nautical, and astronomical. But again, fortunately, it's all mathemtical clear and clean-cut.
Civil Dusk and Dawn -- Dawn is the exact moment when the sun's ascending center hit's 6° below the horizon, while dusk is the exact moment when the sun's descending center hits 6° below the horizon.
Nautical Dusk and Dawn -- Dawn is the exact moment when the sun's ascending center hit's 12° below the horizon, while dusk is the exact moment when the sun's descending center hits 12° below the horizon.
Astronomical -- Dawn is the exact moment when the sun's ascending center hit's 18° below the horizon, while dusk is the exact moment when the sun's descending center hits 18° below the horizon.
So to our same skeleton framework from before we can add:
- Dawn -- Astronomical
- Twilight -- Astronomical
- Dawn -- Nautical
- Twilight Nautical
- Dawn -- Civil
- Dawn -- Civil
- Sunrise
- Sunset
- Twilight -- Civil
- Dusk -- Civil
- Twilight -- Nautrical
- Dusk -- Nautical
- Twilight -- Astronomical
- Dusk -- Astronomical
So now let's put all of this together to visualize it starting with the first peep of light to the last.
- It's Night. The Sun is > 18° below the horizon.
- As the ascending sun hits 18° below the horizon, Astronomical Dawn Occurs.
- Astronomical Twilight Starts.
- As the sun hits 12° below the horizon, Astronomical Twilight Ends & Nautical Dawn Occurs.
- Nautical Twilight Starts.
- As the sun hits 6° below the horizon, Nautical Twilight Ends & Civil Dawn Occurs.
- Civil Twilight Starts.
- Civil Twilight Ends & Sunrise Starts as the suns upper limb first appears on the horizon.
- Sunrise ends as the sun's lower limb ascends off the horizon & Daytime starts.
- As the Sun hits the zenith, it is temporally furthest and equally away from sunrise and sunset. The daily zenith is analogous to the annual equinox! This means that the time after the daily sunrise and before the daily sunset are equal at the zenith, just as the time from sunrise to sunset is equal to the time after sunset to sunrise, which is the definition of an equinox. Far out! Symmetry between daily zenith and annual equinoxes! See my note on "Zenith and Nadir".
- Sunset starts as the lower limb of the sun is tangent with the horizon
- Sunset end as the suns upper limb is tangent with the horizon.
- Civil twilight starts.
- As the sun hits 6° below the horizon Civil twilight Ends and Nautical Dusk Occurs.
- Nautical Twilight Starts.
- As the sun hits 12° below the horizon, Nautical Twilight Ends and Astronomical Dusk occurs.
- Astronomical Twilight Starts.
- As the sun hits 18° below the horizon, Astronomical Twilight Ends and zero light is visible in the sky.
Note: Zenith and Nadir. While on the topic of zenith's and nadirs, it's important to note that regardless of civil, nautical, or astronical measurements, daytime length will always be the same for those three measurements because the cut-off point is the sunrise and sunset. In other words, if you're using civil (god forbid!), nautical, or astronomical measurements, the time from sunrise to zenith and from zenith to sunset will be identical. However, flip over the sphere and focus on the nadir. Now, it changes. The time from dusk to nadir and nadir to dawn will always be equal, but those arcs, although identical in length, will be shortest for astronomical and longest for civil. This is just a geometrically precise (which I feel is just more lucid) method of saying that civil night is longer than astronomical night.
Here's a very VERY informal diagram I made of the 14 key phases of crescular solar activity:
Let's talk about Dawn and Dusk. If you switch to astronomical dawn and dusk, something I highly recommend, you have "real light" time measurements to work with. It's true that you can be out "doing stuff" outside without flashlights before sunrise and after sunset! Astronomical dawn and dusk gives you more of a safe estimate of how much light you still have to work with. In other words, civil dawn and dusk is so pointless because it's still light out at civil dawn and dusk! Nautical dawn and dusk is a most practical for knowing exactly when you can see well enough to navigate without man-made light, but it's still the best to know when the first peep of light is illuminated and when it's "extinguished" and astronomical celestial measurements is the only way to do that! Generally -- and mind you this is severely grotesque generalizations -- astronomical dawn gives you VERY roughly 1.5 hours of light before sunrise and after sunset, nautical gives you about 50 minutes of of light before sunrise and after sunset; and civil gives you a mere 30 minutes of light at best, which is just highly false, before sunrise and after sunset. Try it out. Get your local sunrise times, subtract 1.5 hours and you'll see astronomical dawn around that time! Subtract 50 minutes and nautical dawn will kick in abou then! Don't even bother with civil, though, nautical and astronomical are precise measurements and all you need to be concerned about!
I just needed to revise something here, people. I heralded astronomical dusk and dawn measurements because, after all, those truly are the MOST accurate absolute measurements of knowing precisely when the first sun ray of light illuminates the dawn or when the last inkling of light is "extringuished" at dusk. However, let me say, Nautical dusk is DARK!! I was just out hiking on a trail in Santa Barbara, CA called Cold Springs Trail. I watched the sunset at 5:34pm and then around 6pm it was DARK! Like I could barely see the rocks and trail infront of me in highly-tree-covered areas. Blotches of moonlight shone through on the trail, but by 6:15 I was fortunate to have reached a better known part of the trail. I finally found the main road at around 6:25pm, Nautical dusk was at 6:29, so 4 minutes before Nautical dusk. Let me tell you, Nautical dusk is IT! If you're out manuevering around -- surfing, walking, hiking, jogging, any of those activities -- without a man-made light source PAST Nautical dusk, you're screwed for visibility! So I just wanted to revise my previous statements that Nautical dusk and dawn is the measurements we should be using all the time. Feel free to use civil dusk and dawn, but don't plan on being MY friend! haha! Civil dusk and dawn is just ridiculous because you can still see and navigate well past, often up to half an hour at least, past civil dusk and dawn, but if you're out past nautical dusk in astronomical twilight, without a manmade light source, you'll be visibly blind! Astronomical solar low-horizon measurements should only be considered for observing actual celestial bodies and needing the night sky. If you're using stars to navigate or looking at constellations, Astronomical dusk and dawn is all that matters, but most of the time we're navigating so Nautical low-horizon solar measurements are the most practical and the most necessary. That said, I still LOVE the precision of knowing the EXACT moment light emerges or submerges off the horizon via astronomical measurements, but nautical low-horizon measurements should be the defeault. Civil measurements are absurdly stupid because of their misleading inaccuracy (why label something as "dusk" when there's still a lot of light around? It's not cool and dumb); nautical should be the default; and then if you go into "star-gazing" mode, then check out the astronomical measurements.
So Bottom-Line: Although Astronomical Measurements are the coolest because of their precision. There exist a thousand and more reasons why astronomy is awesome. Books have been devoted to that discussion. The top reason, though, is astronomy is INVARIATES! There are few things in life that are constant! Moon phases, star constellations, and celestial-astronomical events are alive, real-time, but consistent and reliable! However, Nautical low-horizon solar measurements should be our default for living on planet earth. Use Nautical Measurements!! Vine Vidi Vici!
Technically, you could turn to your best friend and say "let's watch the astronomical twilight" even though you can't see any orb in the sky. That's pretty cool! Additionally, "astronomical celestial measurers" start their day the earliest and end the latest. They have the longest days. They soak up the marrow out the light they experience because their "day" reflects the total amount of light down to the last drop. There's no cutting corners with astronomical measurements. It's quite cool to base sunrise and sunset on such precise measurements! Additionally, use this example. We just had a Winter Solstice a few months ago in December. It's february now. I'm in the Northerne Hemisphere in California. Tonight, on February 11, 2008 sunset occurred at 5:33 pm and then civil dawn kicked in 25 minutes later at 5:58. If you were on a civil solar clock, you're "day" would be over at 5:58 before 6pm. Flat done, no more light for you. You only get 25 minutes of twilight, if you use civil measurements today! However, if you were using astronomical solar measurements, dusk is 6:57pm! That means that instead of a 25 minute twilight period, you get a a 1 hour and 24 minute twilight period! Your "day" is 59 minutes longer, too! So astronomical "night" therefore is always much shorter than civil night! Day is technically immediately after dawn and immediately before dusk. Night is immediately after dusk and immediately up until dawn. Astronomical "night" therefore, today, is only 10 hours, 20 minutes, but civil "night" is 12 hours, 18 minutes; nearly a 2 hour difference! The astronomical "day" therefore is 13:40 and the civil "day" is a mere 11:42! So with astronomical celestial measurements you get real-time solar light calculations, instead of just when it "seems" to start to get dark or light. Let's remove "seems" from these calculations. Astronomical dusk, dawn, and twilight is exact!! So you get increases measurement accuracy, real-time solar light calculations, longer days, shorter nights, and, personally, I think the coolest thing is to be able to look out over the horizon after soaking up and witnessing an amazing sunset and to have the knowledge to turn to your friend and say "Ahhh! Isn't that astronomical nocturnal twilight that just followed the nautical and civil twilight, all of which was preceded by that amazing array of solar rays through the atmospheric prism....beautiful?!!!" :)
