Twilight
Published on Nov 16, 2006 at 4:52 pm.
4 Comments.
Filed under skywatching.
The Sun slowly sinks towards the Western horizon. The sky dims. The blue part of the Sunlight is scattered, leaving a red orb where the Sun should be on the horizon. Slowly, this red orb sinks below the horizon, and then … it’s twilight.
The sky doesn’t instantly become dark. This is because Earth has an atmosphere. On the Moon or Mercury, where there are far too few gas molecules hanging around, it gets dark at once. Not so on Earth. Even though the Sun sinks below the horizon at sunset, we are not done with its light just yet.
First, remember that Earth is basically a sphere. So, then the Sun sinks below the horizon for you, it is still shining for someone at a higher altitude. You can see this effect if you live near a big city. The Sun will set, but you can still see the upper floors of buildings lit by sunlight. Or, you can see this with mountains (prettier than buildings). You can even see that aircraft flying overhead still glint in sunlight even after the Sun sets for you. But, this light isn’t just shining of solid objects. It is scattered by dust, water droplets, and pollution in the air. It is even scattered some by the very air molecules themselves. Some of this scattered light shines down to the ground. So, you can still see by sunlight, even though the Sun has set. This is twilight.
There are three twilights that are generally recognized. First is Civil Twilight. This is what most people think of as twilight, and is what I just described. It is still bright enough to conduct normal activities. You can read by the twilight’s light. You don’t need headlights to see where you are driving. Most states, in fact, don’t require the use of headlights until 30 minutes after sunset, and you can extinquish them 30 minutes before dawn. Of course, that makes your car less visible, so, though you can see, you are more likely to be in an accident due to someone else not seeing you. But, 30 minutes isn’t a magic number. It is just sort of average here in the US. The farther south you are, the quicker civil twilight ends, and conversely the farther north that you are the longer it lasts.  (Reverse those directions if you are south of the equator!) Generally, civil twilight is defined as being when the center of the Sun is within 6° of the horizon (for a person standing on level ground, and a flat horizon).Â
By the end of civil twilight, it is hard to read, and distant objects are difficult to distinguish.  The brighter stars and planets have become visible to the naked eye. But, the sky is not yet totally dark.  We enter a stage that is called nautical twilight. In nautical twilight, you can still see a glow on the horizon where the Sun set. The sky is still brighter in that direction, too. The horizon is distinct in the direction of sunset (or sunrise if you are talking about morning nautical twilight). More stars become visible. The sky isn’t totally dark yet. The Sun still is shining on the higher levels of the atmosphere, and some light is still scattered from above, but the lower levels where most of the dust and water vapor would be located are in shadow. By the time that the Sun has settled to more than 12° below the horizon, the glow on the horizon is gone, and the sky looks black to most observers.Â
With Sun more than 12° below the horizon, the sky looks pretty dark. Objects on the ground become as hard to distinguish as they are going to be. You defnitely need artificial lights to do any sort of reading. But, careful studies of the sky reveal that there is still light scattered from the uppermost parts of the atmosphere. Faint galaxies and nebulae are still difficult to see without interference. The dimmest stars are still not visible to the naked eye. And as you wait, the sky becomes visibly darker.  We call this state astronomical twilight. Once the Sun sinks to more than about 18° below the horizon, though, it is about as dark as its going to get. Twilight is over. This takes as little as a bit over an hour up to several hours, depending upon your latitude (the farther from the equator, the longer that it takes).
Obviously, I am thinking of sunset in all of this, but the same process works in reverse as the Sun rises: astronomical twilight, nautical twilight, civil twilight, and finally dawn. For an hour or two before twilight in the morning or after in the evening, the Sun will be shining too high to scatter much off the atmosphere, but it can catch satellites. So, during this time, you’ll be able to see satellites crossing the sky.
So, there you have twilight!
-AstroprofÂ
 (Image courtesy of Stan Richard, and the Astronomy Picture of the Day)
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Andrew on November 17, 2006 at 3:51 am: 1
I think a really good example of this is,to watch the ISS fly over.On many occasions,I have seen the ISS appear bright over the horizon and as it approaches it gets dimmer until finally it disappears over head.I’m not sure but,is this the transition from nautical twilight to astronomical twilight?.
A Ler…-- Rastos de Luz on November 17, 2006 at 6:21 am: 2
[…] “Twilight“, no Astroprof’s Page. […]
Astroprof on November 17, 2006 at 10:09 am: 3
Andrew, that is an excellent example. As a satellite passes by a little after twilight on the ground, the sun is still shining high above. The satellite passes through twilight into night. For the observer on the ground, it appears to get dimmer and then disappear.
Astroprof’s Page » Sunrise Sunset Calendar on June 24, 2007 at 9:51 pm: 4
[…] Mary Jo, over at Fly Away Cafe, has posted about a Sunrise Sunset calendar web site. This web site will compute and display sunrise and sunset times for an entire month for a wide variety of locations. It also computes moonrise and moonset times, and it will even compute twilight. It computes all three typical twilights: civil twilight, nautical twilight, and astronomical twilight (In case you don’t know the difference between these, I wrote about these in an earlier posting.). The calendar also gives moon phases. This is a really nice tool, since sunrise and sunset times vary with the observer’s latitude and longitude. Technically, only the latitude matters in computing sunset times for Local Mean Time. However, your location within your time zone can make clock time vary from Local Mean Time. I wrote about different times in an earlier posting, too. […]