Twilight. The word evokes gorgeous colors. But what exactly is twilight? And did you know that there are three types? The Old Farmer’s Almanac now has a calculator that will tell you when twilight begins and ends for your location. Get ready for adventures in the Twilight Zone.
That famous TV show would not have been a hit on Pluto. In fact, creatures on any known planet would be amazed by the unique twilight we Earthlings take for granted. Our world alone experiences the luxuriant palette of colors which mark day’s transition to darkness. So, what is exactly is twilight?
Throughout most of the universe, the sun sets and wham, it’s like a power failure: instant blackness. In this neck of the celestial woods, only Mars has something of a terrestrial twilight. But its lethally thin air is incapable of producing anything that resembles our own planet’s rich hues. That leaves us alone to ponder the phenomenon, along with bats and other crepuscular characters who cleverly avoid both day and night predators.
Crepuscular? A great word that means “pertaining to twilight” and which can describe those fantasyland rays that often stream up from the setting sun, indeed called crepuscular rays. It has such a great sound, I try to use that word as often as possible, even when it’s not appropriate.
Image: Earth at twilight. Blue sunlight fades into darkness. NASA International Space Station.
What is Twilight?
What is the correct definition of twilight? The concept suggest vagueness. But in actuality, it’s a very specific event. And there’s really not one twilight but three! And they’re each so distinct, they have their own names. Below describe the stages of twilight as they occur in the evening. These stages also occur before sunrise, in the reverse order.
- Civil twilight starts at sunset and ends roughly 45 minutes later, when the sun’s center has plunged six degrees below the horizon—equal to 12 times its own width. That’s when streetlights must be on, according to most municipal ordinances.
- Nautical twilight occurs when the sun’s center is between 6 and 12 degrees below the horizon. After this, the horizon vanishes, when a mariner cannot distinguish between sea and sky.
- Astronomical twilight occurs when the sun’s center is between 12 and 18 degrees below the horizon. After this point, the faintest stars emerge. The conclusion of this stage of twilight heralds the arrival of full darkness.
By the way, twilight is different than dusk, which occurs after sunset. (Sunset can be defined as the moment when the top of the sun’s disk has passed the horizon.) As with twilight, there is civil dusk, nautical dusk, and astronomical dusk, occurring at the exact moment when the center of the sun’s disk is at 6°, 12°, and 18° below the horizon, respectively. Stages of dawn occur before sunrise, in the reverse order as described for dusk.
Twilight’s duration is not usually expressed in units of time, but rather degrees, and for good reason: its length varies. Depending on the time of year and the latitude of the observer, twilight can expire in less than an hour or linger throughout the night! Twilight is always shortest in the tropics, where less than one hour is the most you ever get. From the latitude of New York, 1 ½ hours is about average, while from Alaska there simply is no night at all between May and August. There, the choice is either daytime or twilight.
There’s much more to it than pretty colors. Twilight plays host to phenomena not seen at any other time, such as those crepuscular rays, or the appearance of Mercury or Venus, or the sudden profusion of earth satellites, which are most numerous during the first 90 minutes after nautical twilight ends, or Earth’s shadow, looking like a blue-gray band low in the east during twilight’s first 15 minutes.
This month of December, in morning twilight, we’re presented with the year’s best view of the Morning Star, Venus, the brightest celestial luminary after the moon. The blue star next to it all month long is Spica, the famous primary star of Virgo.
Yes, currently lots of action in twilight. And the price is right.