Sunset question..And yahoo has changed the format of its site, s******g with how I usually ask questions.

Question

So, stating the obvious here, but if I know that the sun sets at lets say 9:07Pm, that does not mean its actually dark at 9:07. Instead it stays light for a while longer. This leaves me with two questions.

1. First off, why is it considered sunset if the sun is still providing light to a certain area?

2. Is there a static time from when sunset is declared to when there is no further light from the sun, or does it vary with dates/location

Answer 1

Yes and No.

Now that I have given you the easy answer, let's look at the details.

In celestial mechanics, the "horizon" is a mathematical line located 90 degrees from your zenith (the point directly overhead).  It does not matter where your real horizon is.

This is the closest to the horizon seen on a small boat in the middle of a calm ocean.

In celestial mechanics, sunset is the moment when the centre of the sun's disk is on the mathematical horizon.

In practice, at that moment, the observer on the boat will still see the whole sun a full degree above the horizon.  That is because of refraction caused by the atmosphere (it makes everything appears a bit higher than it really is).

In nautical tables, this is the moment given as sunset.

In astronomy (and in everyday life), sunset is the moment where the visible disk of the sun is fully below the horizon.

Tables of sunrise and sunset will use the theoretical horizon (because they do not know what your real horizon will look like) and they will correct for the effect of refraction.  However, they will not correct for your height above ground.

For example, if you live on a perfectly flat plain, where there is one skyscraper, the table would give the time of sunset (entire sun's disk disappears below the horizon) for the person standing on the plain.  However, the person looking out a 19th floor window  will still see the entire solar disk.

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1.  In celestial mechanics, the theoretical definition is used because it gives convenient angles of 90 degrees (zenith distance) or 0 degrees (altitude above mathematical horizon).  This is very practical for spherical trigonometry -- you think plane trigonometry is fun?  Try spherical trig.  You'll be happy when the conditions hand you a 90-degree angle!

We learn to recognize the time of theoretical sunset by looking at the apparent altitude of the Sun above the horizon.  We take a reading of the compass (the Sun's bearing) at that exact moment and we can very easily calculate the compass correction.

2. Other than celestial mechanics, sunset is when the entire image of the sun's disk is gone below the horizon.  If you still see the Sun, it has not set.

The time at which this happens changes from day to day.  It depends on your latitude, the Sun's declination and the "equation of time" (due to the fact that Earth's orbital speed varies over a year).

For a person who stays put (latitude does not change), sunset is later in summer (when the sun's declination is higher) and earlier in the winter.

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Once the sun is set, it does not get immediately dark.  The sun is still lighting the air just above your head.  The lower atmosphere is still lit until the sun is about 6 degrees below the horizon.  This period is called Civil Twilight.  There is enough light to continue your business as if you were in daylight.

When the Sun reaches 6 degrees below the horizon, Civil Twilight ends. Nautical Twilight begins.  During this period, the sky is dark enough for bright stars to appear, but there is still enough light for the horizon to be visible (at sea).  The navigator can measure the altitude of stars with a sextant.

Nautical twilight ends when the sun reaches 12 degrees below the horizon.  Still, it is not completely dark.  The sun is still lighting the top portion of the stratosphere.  You might not notice it in "normal life" but astronomers do.  They still can't observe very faint objects nor take pictures of them.

This period is called astronomical twilight.  It ends when the Sun reaches 18 degrees below the horizon.

Answer 2

sun set is when the sun dips below the horizon.  that's what it means.

dark is completely different.  and varies, depending on how far north you are, whether there is cloud cover, where that is, how high it is, etc.

no there is not a static time between sunset and dark.  as above.  btw, doesn't your second question answer your first?

Answer 3

Sunset is the time that the sun sets, that is, go under the horizon. If you are in a valley you would lose sight of the sun long before someone standing on a nearby mountain.

If the Earth had no atmosphere, the time of sunset would be the time of darkness. But Earth's atmosphere catches the light, and refracts it, bends it, so that after sunset we are able to see some light from over the horizon.

Sunset is an extremely local thing. Even time is an extremely local thing. Two cities at different ends of the same time zone will have sunrise and sunset times almost an hour different to each other.

Also the further north or south you go from the equator, the more the curvature of the Earth's surface affects the length of the day.

At the equator, the day will be about 12 hours of sunlight and 12 hours or darkness (including twilight).

In the northern summer, days are longer because the northern hemisphere is more tilted towards the sun, leading to a summer-long day at the north pole (and a winter-long night too)....

So: no, there is no static time.