Tag: general science

By Jay Ryan at Homeschool-articles.com

Twilight is that brief part of each day in between day and night, when the Sun’s rays light up the sky, but the Sun itself is below the horizon.  Dawn twilight is in the morning before sunrise, when the Sun’s glow chases away the night, when the early sky is tinged by Homer’s eos daktylos, “rosy-fingered dawn.”  Dusk twilight is on the other side of the day, when the ruddy glow of the sunset persists after the Sun itself has sunk below the horizon.

Twilight is my favorite time of day.  Twilight is a rare, precious part of each day, a brief period of perhaps 45 minutes separating the Sun’s rays of daylight and the full shadow of night.  Twilight is the most changeable part of the day.  Daytime offers a consistent blue sky as the Sun races from east to west.  Nighttime is uniformly dark, only distinguished by the rotation of the stars across the sky.  But during twilight, the sky quickly transitions from the brightness of day to the full darkness of night in less than an hour.

Twilight offers a range of colors — brilliant hues of red, yellow, pink and orange that rapidly change to deepening shades of blue.  After sunset, the first  stage of twilight is civil twilight, when the sky is very bright and the clouds are painted beautiful shades of color.  During civil twilight, the entire sky is lit up with the Sun’s light, and the land retains a dim daylight quality, though the Sun’s rays are not visible and shadows cannot be seen.  In the evening, civil twilight is the first stage of twilight after the sunset, and in the morning, the last stage before the sunrise.

In the evening after sunset, the next stage is nautical twilight, when the brightest stars begin to appear.  At this time, the streetlights begin to come on for most city dwellers.  The sky has lost most of it’s orange color, except perhaps for a fading patch to mark the place of Sun.  The sky has become a deep turquoise blue, brighter toward the Sun’s direction.  Nautical twilight is so-named because it is during this stage that sailors can first find the “navigator’s stars” used for finding direction at night.

The final stage of twilight after sunset is astronomical twilight.  At this stage, the sky grows ever darker, and the stars begin to come out in great numbers.  A dwindling patch of twilight remains along the horizon to mark the Sun’s position.  A keen eye will notice that this patch is not in the same place as the sunset, and indicates that the Sun has moved below the horizon since going down.  Twilight ends when the last patch of sunglow has vanished from the horizon, and full astronomical night commences.

Seasonal and Global Variations in Twilight
The length of twilight varies over the span of the year, and also with latitude around the earth.  In the springtime and autumn, near the equinoxes, twilight is shortest.  As seen from the ground, the Sun appears to follow a very direct path below the horizon after the sunset, and so the Sun and its rays quickly dip out of sight during the spring and fall.  As a result, night falls quickly near the equinoxes.
During the summer, the period of twilight is longest.  The Sun appears to take a slanting path toward the horizon.  The Sun follows a shallow path below the horizon, and a patch of astronomical twilight can be seen far from the place of the sunset, in some places longer than an hour after Sun’s disappearance, depending on location.

During the winter, the twilight is longer than at the equinoxes, but not as long as during the summer.  These changes in twilight result from the varying angle of the Earth’s axis to the Sun as the Earth follows its orbit around the Sun over the course of the year.

The length of twilight can vary considerably depending on latitude over the Earth.  Twilight is shortest at the Equator and longest at the North and South Poles.  On the Equator, the entire sky appears to lie on its side.  The Sun sets perpendicularly to the horizon, and drops quickly out of sight after sunset.
Therefore, the period of twilight is very short.  Vacationers and newcomers to the tropics such as new missionaries are often amazed at how quickly night falls in these warm latitudes.

In the temperate latitudes, where most of the world’s population resides, the Sun approaches the horizon at an angle.  Thus, the Sun takes a longer, slanted path as it dips below the horizon, and twilight takes a longer time to fall.  As one moves north or south toward the poles, the length of twilight increases.

At latitudes above 48 degrees, the “white nights” can be observed during the longest days of summer.  The evening twilight merges with the morning twilight so that some trace of daylight can be seen at midnight!  The white nights can be observed from the northern border of the continental USA, most of Canada, and places in northern Europe and Asia such as Great Britain and Russia.

In the polar regions, continuous daylight and nighttime persist for long periods of time.  However, around the equinoxes, the maximum global interval of twilight can be observed at the North and South Poles.  Each year, following the long polar summer, the Sun begins to set at the North Pole on the autumnal equinox, September 23, and finally sets two days later, on September 25.

The Sun goes through the stages of twilight over a span of several weeks, passing from civil twilight to nautical twilight and astronomical twilight.  Finally, on about November 15, the Sun has reached far enough below the horizon that twilight ends and full night commences!  Full darkness only lasts a couple months during the long polar night, and twilight commences once again around the end of January, and brightens steadily until the vernal equinox on March 21.

As the days warm up in your area, plan some outdoor activities during the evening twilight!  Make it a point to observe the stars as they appear one by one.  Be sure to bring plenty of bug spray!

Jay Ryan is the author of Signs & Seasons, an illustrated, Biblically-centered homeschool curriculum for Classical Astronomy. He is also the creator of the Classical Astronomy Update, an email astronomy newseltter especially for Christian homeschoolers.  Visit his website at ClassicalAstronomy.com.