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The Saint
2006-Feb-05, 05:45 PM
What is the reason for the different lengths of dusk (ie from sunset to the appearance of the first star) at different latitudes? In London it's about 80 minutes. In Cairo about 40 minutes.

What is it at the equator?

Is it black in a frighteningly quick 10 minutes? Does the length of dusk vary with the seasons?

Fr. Wayne
2006-Feb-05, 06:50 PM
Above roughly 60°N or S, civil twilight lasts all night at midsummer, while above about 55°N or S, nautical twilight lasts all night at midsummer. Astronomical twilight can last all night for several weeks as far from the poles as 50°N or S.

The length of twilight after sunset and before sunrise is heavily influenced by the latitude of the observer; in the Arctic and Antarctic regions, twilight (if at all) can last for several hours (with none at the poles within a month on either side of the winter solstice), while at the equator, it can go from day to night in as little as 20 minutes
from http://en.wikipedia.org/wiki/Twilight

hhEb09'1
2006-Feb-05, 07:29 PM
As Fr. Wayne makes clear, the twilight, or dusk, is mostly a function of how far below the horizon is the sun--but close to the pole, the sun may not set at all. The closer one is to the pole, the greater is the effect.

tony873004
2006-Feb-05, 07:47 PM
When I was in Antarctica, the Sun would set, but it never got dark. It was twilight all night long! Of course all night long means about 3 hours.

SolusLupus
2006-Feb-05, 08:00 PM
When I was in Antarctica, the Sun would set, but it never got dark. It was twilight all night long! Of course all night long means about 3 hours.

Ooo, you've been to Antarctica? What reason?

The Saint
2006-Feb-05, 09:35 PM
So approx how many minutes is twilight at 0 degrees latitude?

grant hutchison
2006-Feb-06, 12:01 AM
So approx how many minutes is twilight at 0 degrees latitude?Since the sun drops vertically below the horizon at the equator, it's easy to work out. Since it moves through 360 degrees in 24 hours, it moves one degree every four minutes. Civil twilight ends when the sun is six degrees below the horizon: so that's 24 minutes after equatorial sunset. Nautical twilight ends when the sun is 12 degrees below the horizon, and astronomical twilight when it's 18 degrees below the horizon: 48 minutes and 72 minutes after sunset, respectively.

Grant Hutchison

Jeff Root
2006-Feb-06, 12:26 AM
Since the sun drops vertically below the horizon at the equator
Only at the equinoxes. March and September.

-- Jeff, in Minneapolis

grant hutchison
2006-Feb-06, 01:08 AM
Since the sun drops vertically below the horizon at the equator.Only at the equinoxes. March and September.I think every day at the equator. All the lines of celestial latitude make a right angle at the horizon when you stand on the equator, so the sun will be travelling vertically as it hits the horizon.

But the season does alter the duration of twilight by a couple of minutes at the equator, since at the solstices the sun follows a slightly curving path below the horizon, and so take a little longer to reach the various twilight thresholds. Since The Saint was asking for approximate values I didn't fuss about the difference, but I guess I should have been clearer that I was making approximations. I also neglected the finite diameter of the solar disc, and the effect of refraction.

Grant Hutchison

The Saint
2006-Feb-06, 01:24 AM
What are the twilight lengths for London 51 degrees on the solstices and equinoxes?

The Jews say every night in their evening prayer: "Blessed are you O Lord our God King of the Universe Who by His word brings on evenings, with wisdom opens gates, with understanding alters periods, changes the seasons, and orders the stars in the constellations according to His will. He creates day and night, rolling the light before the darkness and darkness before light. He causes day to pass and brings on night, and separates between day and night....Blessed are You O God who brings on evenings".

As strict, mortally even, Sabbath observers for over 3 milllenia, the Jews study twilight times very much, as it extends from Friday sunset to Saturday when the stars appear. Riots have even occurred in Israel when bus companies ignored the astronomical timetable and started rolling before the Sabbath was out!

astromark
2006-Feb-06, 07:24 AM
And your location; can make the diferance. In Australia, on the east coust the twilight is quickish at around twenty six minutes. I have been told that its becouse to the west there is only land mass ( large open desert ) That it offers little reflective property. thus it darkens quicker. While in New Zealand, where we have ociens never far away. we have a longer twilight. Not so proportional to latiude in this case.

tony873004
2006-Feb-06, 08:04 AM
Only at the equinoxes. March and September.

-- Jeff, in Minneapolis
I think Grant is right. Not just the Sun, but everything, no matter what its azimuth rises and sets vertically on the equator, at least the instantanious direction as it crosses the horizon is vertical. Then things due east and west continue to rise or set vertically while things not due east or west begin to stray from vertical after crossing the equator. Depending on the season, the Sun may be as much as 23.5 degrees from due East or West when rising or setting on the equator, causing a slight difference in length of day.


Ooo, you've been to Antarctica? What reason?
Because it's there :)
I just posted this recently in another thread, but here's a link to my pictures:
http://orbitsimulator.com/Antarctica/antartica.html

There's some shots of the Moon near the horizon that give away my latitude (if the penguins and ice didn't). In mid-latitudes, the Moon's terminator in a quarter moon is not perpendicular to the horizon when the Moon is near the horizon.

grant hutchison
2006-Feb-06, 10:36 AM
What are the twilight lengths for London 51 degrees on the solstices and equinoxes?Looks like you need an on-line calculator (http://aa.usno.navy.mil/data/docs/RS_OneDay.html). (This one gives you only civil twilight, but it corrects for refraction and the diameter of the sun.)


As strict, mortally even, Sabbath observers for over 3 milllenia, the Jews study twilight times very much, as it extends from Friday sunset to Saturday when the stars appear.Moslems have an interest in morning twilight, because its onset determines the start of the Ramadan fast. The Koran stipulates that fasting begins when a white thread can be distinguished from a black thread by the light of the sky.
In the Royal Navy at the time of Nelson, sunrise twilight began when you could "see a grey goose at a mile".
The Moslem version seems distinctly more portable and reproducible!


I think Grant is right.I probably caused confusion by my choice of words. When I wrote "the sun drops vertically below the horizon", I meant "the sun drops below the horizon vertically" not "below the horizon, the sun drops vertically". But my original phrase is open to either interpretation, unfortunately: the first one correct and the second one wrong. :sad:

Grant Hutchison

Fr. Wayne
2006-Feb-06, 01:13 PM
Giggle: don't we all still talk like the Sun is doing the moving?

grant hutchison
2006-Feb-06, 01:20 PM
Giggle: don't we all still talk like the Sun is doing the moving?I don't know about you, but in my reference frame, the sun is doing the moving. :)

Grant Hutchison

The Saint
2006-Feb-06, 05:20 PM
And you can't be said to be wrong!

"The struggle, so violent in the early days of science, between the views of Ptolemy and Copernicus would then be quite meaningless. Either coordinate system could be used with equal justification. The two sentences, 'the sun is at rest and the earth moves,' or 'the sun moves and the earth is at rest,' would simply mean two different conventions concerning two different coordinate systems" (Einstein).

The_Radiation_Specialist
2006-Feb-06, 05:30 PM
Im probably the person closest to equator here :D I live a mere 3 degrees north of equator. The sun usually(all year round) sets at about 19:30. It gets dark at about 5 minutes to eight. Bright stars like sirius and Rigel are usually visible before sunset.

tony873004
2006-Feb-06, 07:33 PM
Im probably the person closest to equator here :D I live a mere 3 degrees north of equator. The sun usually(all year round) sets at about 19:30. It gets dark at about 5 minutes to eight. Bright stars like sirius and Rigel are usually visible before sunset.
19:30? That's a strange sunset time for an area that doesn't observe daylight savings. You must be on the edge of your time zone.

hhEb09'1
2006-Feb-06, 08:06 PM
(Einstein).[/i]Just a note, that was from a book by Einstein and Infeld, not just Einstein. It's not clear which one actually wrote it.

Jeff Root
2006-Feb-06, 08:34 PM
I think Grant is right. Not just the Sun, but everything, no matter
what its azimuth rises and sets vertically on the equator, at least
the instantanious direction as it crosses the horizon is vertical.
It was perfectly obvious when I thought it through the first
time that the Sun could only rise and set vertically at the
equinoxes. Think about it again for five seconds, and it is
equally obvious that it must work as you both say.

This may be one of those things that I'll remember from
now on. The mental image I have of the stars streaking
across the sky in circles which are all concentric and tilted
at an angle which is solely dependant on the observer's
distance from the equator is pretty powerful.

So, where I am, halfway between the equator and the north
pole (within 3 km), everything rises and sets at a 45-degree
angle to the horizon.

-- Jeff, in Minneapolis

grant hutchison
2006-Feb-06, 08:50 PM
So, where I am, halfway between the equator and the north pole (within 3 km), everything rises and sets at a 45-degree
angle to the horizon.I don't think so. As soon as you stray away from the equator, things change.
Remember that from your location, the 45-degree north line of celestial latitude is tangent to your northern horizon, so objects at 45 degrees north will "set" and "rise" parallel to the horizon. The celestial equator is tilted at 45 degrees to your horizon, due west and east, and the 45-degree south line of celestial latitude is again tangent to your southern horizon, but this time rising from below, rather than descending from above.
So I believe your original insight about the equinoxes was correct: the angle of rising and setting does vary with celestial latitude, except in the particular case of the Earth's equator. (Oh. And the poles, where none of the fixed stars rises or sets.)

Grant Hutchison

Edit: Changed the last line about the poles, for better accuracy.

Jeff Root
2006-Feb-06, 09:08 PM
As soon as you stray away from the equator, things change.
Remember that from your location, the 45-degree north line of
celestial latitude is tangent to your northern horizon, so objects
at 45 degrees north will "set" and "rise" parallel to the horizon....
Again, I thought about it and visualized it pretty carefully.
And screwed up. I visualized stars rising at various points
along the horizon, and it seemed reasonable that they could
all rise at a 45-degree angle. But I didn't think about the
limiting cases of due north and south, which probably would
have made me think twice.

If a star rose from due north at a 45-degree angle, and set
the same way, it would have to follow a heart-shaped path
through the sky, making a big Valentine around Polaris. :razz:

-- Jeff, in Minneapolis

Ken G
2006-Feb-10, 10:24 AM
It would seem that only objects that rise/set in the direct East/West have the same angle to the horizon as your colatitude, except at the poles and equator. Thus the only thing special about the equinoxes is that on those occasions, we are talking about the Sun rather than some unheralded star out of the plane of the ecliptic. Those are also the times of year when Grant's helpful calculations of the various twilight durations at the equator apply exactly, as he mentioned, although I don't think there is any error there due to refraction or the the solar disk-- I should think those are included in the twilight definitions in the first place. What is a little odd is that we should be able to approximately scale to other latitudes at the equinoxes by dividing by the cosine of the latitude, but this implies that even at latitudes of 45 degrees, the astronomical twilight should last about 100 minutes-- over an hour and a half! I'm surprised, but I don't dispute it.

grant hutchison
2006-Feb-10, 01:35 PM
... I don't think there is any error there due to refraction or the the solar disk-- I should think those are included in the twilight definitions in the first placeFor my naive calculation of twilight duration to work, we'd need to have the time of sunset and the time of twilight-end defined in the same way. But twilight end is based on the mathematical position of the sun's centre, while the time of true sunset is delayed by both refraction and the diameter of the solar disc.
(The twilight-end angles of 6, 12 and 18 degrees are obviously mere conventions, so there's no virtue in complicating matters by factoring in the apparent solar radius, or correcting for non-existent refraction due to some hypothetical extension of the atmosphere below the surface of the Earth.)
The sun has a semidiameter of about quarter of a degree, and is lifted by refraction at the horizon by about half a degree. At the equator, that's a three-minute delay in sunset compared to the mathematical "setting" of the sun's centre, and so twilight duration should be clipped by that amount.
My naive calc came up with 24 minutes of civil twilight. If you check using the sun calculator I linked to earlier, you'll find civil twilight on the equator at the equinox is predicted as about 21 minutes, which is all nicely consistent.

Grant Hutchison

The_Radiation_Specialist
2006-Feb-10, 01:55 PM
19:30? That's a strange sunset time for an area that doesn't observe daylight savings. You must be on the edge of your time zone.

Yes, 19:28 was the time for sunset today. I live in Kuala Lumpur, Malaysia. The time zone is supposed to be +7 GMT (http://www.time.gov/images/worldzones.gif).

Ken G
2006-Feb-10, 03:28 PM
For my naive calculation of twilight duration to work, we'd need to have the time of sunset and the time of twilight-end defined in the same way.
I take your point-- sunset is not when the center of the Sun goes down, but rather that last glowing ember. So the twilight durations are reduced, just as you found.

hhEb09'1
2006-Feb-10, 07:14 PM
I take your point-- sunset is not when the center of the Sun goes down, but rather that last glowing ember. So the twilight durations are reduced, just as you found.As he mentioned, if we are talking straight line geometry, it is when that last part is a bit farther below, because of refraction.

Ken G
2006-Feb-10, 08:39 PM
Yes, the last glowing ember.

Celestial Mechanic
2006-Feb-10, 09:59 PM
The usual convention is to take the time of sunrise/sunset to be the time when the Sun is 50 minutes of arc below the horizon. 16 minutes is an average value for the radius of the Sun, 34 minutes is an average value for the refraction. Because of the variability of the refraction, more precision than this is unnecessary.

Nonkers
2006-Apr-16, 10:45 AM
It is indeed felicitous for our sleep patterns and work that the light at night and in the morning goes and come gradually. Could we survive if the sunlight appeared or went suddenly ie in minutes or even seconds? Are there any planets with ultra or even instantaneous short dawns and dusks? Are drawn out dawns and dusks such as above 70 degrees latitude North and South a pain for sleepers and wakers.

farmerjumperdon
2006-Apr-18, 12:29 PM
What is the reason for the different lengths of dusk (ie from sunset to the appearance of the first star) at different latitudes? In London it's about 80 minutes. In Cairo about 40 minutes.

What is it at the equator?

Is it black in a frighteningly quick 10 minutes? Does the length of dusk vary with the seasons?

80 minutes before the 1st star? Pollution (particulate matter and light) must be playing a factor for you. I am at approx 45 north and I'm sure the brightest stars pop much faster than that. I'm going to check it the next clear night. My guess is we see stars about 30 minutes of the last of the sun going over the horizon.