APOD: Equinox Sunset (2009 Sep 24)

[verkle]

Regarding the comment "Earth dwellers experienced 'nearly' 12 hours of daylight and 12 hours of darkness..." seems rather arbitrary to me to define sunrise as the FIRST appearance of sunlight and sunset and the LAST appearance of sunlight. According to this definition, the word used here should be rather "slightly more than 12 hours", not "nearly 12 hours", since everyone on earth enjoys more than 12 hours on equinox day. Isn't there a more scientific definition of sunrise and sunset that defines it based on the geometric center of the sun?

http://apod.nasa.gov/apod/ap090924.html

[neufer]

Regarding the comment "Earth dwellers experienced 'nearly' 12 hours of daylight and 12 hours of darkness..." seems rather arbitrary to me to define sunrise as the FIRST appearance of sunlight and sunset and the LAST appearance of sunlight. According to this definition, the word used here should be rather "slightly more than 12 hours", not "nearly 12 hours", since everyone on earth enjoys more than 12 hours on equinox day. Isn't there a more scientific definition of sunrise and sunset that defines it based on the geometric center of the sun?

<<Although the word equinox is often understood to mean "equal [day and] night" this is not strictly true. An equinox happens each year at two specific moments in time when there is a location on the Earth's Equator where the centre of the Sun can be observed to be vertically overhead.

For most locations on earth, there are two distinct identifiable days per year when the length of day and night are closest to being equal; those days are commonly referred to as the "equiluxes" to distinguish them from the equinoxes. Equinoxes are points in time, but equiluxes are days. By convention, equiluxes are the days where sunrise and sunset are closest to being exactly 12 hours apart.>>

Equilux for Tihany will occur around October 13th.
http://www.youtube.com/watch?v=t0rGIyCf ... re=related

In the tropics between about 15º S and 15º N day is always longer than night.

<<Twilight is the time between dawn and sunrise, and the time between sunset and dusk. Sunlight scattered in the upper atmosphere illuminates the lower atmosphere, and the surface of the Earth is neither completely lit nor completely dark. The sun itself is not actually visible because it is below the horizon. Twilight is technically defined as the period before sunrise and again after sunset during which there is natural light provided by the upper atmosphere, which does receive direct sunlight and reflects part of it toward the Earth's surface.

Twilight is defined according to the position of the sun (its centre) relative to the horizon. There are three established and widely accepted subcategories of twilight: civil twilight (brightest), nautical twilight and astronomical twilight (darkest).

• • Definition Position of sun
    ----------------------------------------
    Night more than 18°
    
    Astronomical twilight 12 – 18°
    Nautical twilight 6 – 12°
    Civil twilight less than 6°
    
    Day (sun above the horizon)
    ----------------------------------------
  (For these definitions, an ideal horizon 90° from the zenith is used. The altitudes of the sun below the horizon are "true geometric" altitudes; that is, refraction by the atmosphere and other small factors influencing the experiential position of the sun are not to be accounted for.) Note that if the sun is 8½ degrees below the horizon, it provides the same level of illumination to the surface of the Earth as a full moon directly overhead.

Morning civil twilight begins when the geometric center of the sun is 6° below the horizon (the point of civil dawn), and ends at sunrise. Evening civil twilight begins at sunset and ends when the center of the sun reaches 6° below the horizon (the point of civil dusk). The brightest stars appear during civil twilight, as well as planets, such as Venus, which is known as the 'morning star' and/or 'evening star'. During this period there is enough light from the sun that artificial sources of light may not be needed to carry on outdoor activities. This concept is sometimes enshrined in laws, for example, when drivers of automobiles must turn on their headlights, when pilots may exercise the rights to fly aircraft, or if the crime of burglary is to be treated as nighttime burglary, which carries stiffer penalties in some jurisdictions. A fixed period (most commonly 30 minutes after sunset or before sunrise) is typically used in such statutes, rather than how many degrees the sun is below the horizon. Civil twilight can also be described as the limit at which twilight illumination is sufficient, under good weather conditions, for terrestrial objects to be clearly distinguished; at the beginning of morning civil twilight, or end of evening civil twilight, the horizon is clearly defined and the brightest stars are visible under good atmospheric conditions.>>

[JohnD]

Tut!

"Equinoctial" is the correct adjective.

Dr.Pedant speaks

[bystander]

Regarding the comment "Earth dwellers experienced 'nearly' 12 hours of daylight and 12 hours of darkness..." seems rather arbitrary to me to define sunrise as the FIRST appearance of sunlight and sunset and the LAST appearance of sunlight. According to this definition, the word used here should be rather "slightly more than 12 hours", not "nearly 12 hours", since everyone on earth enjoys more than 12 hours on equinox day. Isn't there a more scientific definition of sunrise and sunset that defines it based on the geometric center of the sun?

equinox - [Latin aequi- equal + nox night]

... nearly 12 ... and 12 ...

You can't have slightly more than 12 ... and 12. Actually, I guess you can, since there is slightly more than 24 hours in a day. But what is your slightly more than referring to, day or night?

I think what you really wanted to say was "slightly more than 12 hours of daylight and slightly less than 12 hours of darkness". I think nearly 12 ... and 12 expresses it just fine.

[neufer]

Regarding the comment "Earth dwellers experienced 'nearly' 12 hours of daylight and 12 hours of darkness..." seems rather arbitrary to me to define sunrise as the FIRST appearance of sunlight and sunset and the LAST appearance of sunlight. According to this definition, the word used here should be rather "slightly more than 12 hours", not "nearly 12 hours", since everyone on earth enjoys more than 12 hours on equinox day. Isn't there a more scientific definition of sunrise and sunset that defines it based on the geometric center of the sun?

equinox - [Latin aequi- equal + nox night]

• Noxious, a. [L. noxius, fr. noxa harm; akin to nocere to harm, hurt. Cf. Nuisance, Necromancy.]
  
  1. Hurtful; harmful; baneful; pernicious; injurious; destructive; unwholesome; insalubrious; as, noxious air, food, or climate; pernicious; corrupting to morals; as, noxious practices or examples.
  
  2. Guilty; criminal.

... nearly 12 ... and 12 ...

You can't have slightly more than 12 ... and 12. Actually, I guess you can, since there is slightly more than 24 hours in a day.

Actually the obliquity of the ecliptic ensures that there is slightly LESS than 24 hours in an "apparent solar day" during the equinoxes.

From July 27 to Nov. 3 the equation of time is monotonically increasing due to the "apparent solar days" being shorter than 24 hours.

Apparent solar time or true solar time is the hour angle of the Sun. It is based on the apparent solar day, which is the interval between two successive returns of the Sun to the local meridian. Note that the solar day starts at noon, so apparent solar time 00:00 means noon and 12:00 means midnight. Solar time can be measured by a sundial. The length of a solar day varies throughout the year for two reasons. First, Earth's orbit is an ellipse, not a circle, so the Earth moves faster when it is nearest the Sun (perihelion) and slower when it is farthest from the Sun (aphelion) (see Kepler's laws of planetary motion). Second, due to Earth's axial tilt, the Sun moves along a great circle (the ecliptic) that is tilted to Earth's celestial equator. When the Sun crosses the equator at both equinoxes, the Sun is moving at an angle to the equator, so the projection of this tilted motion onto the equator is slower than its mean motion; when the Sun is farthest from the equator at both solstices, the Sun moves parallel to the equator, so the projection of this parallel motion onto the equator is faster than its mean motion (see tropical year). Consequently, apparent solar days are shorter in March (26–27) and September (12–13) than they are in June (18–19) or December (20–21). These dates are shifted from those of the equinoxes and solstices by the fast/slow Sun at Earth's perihelion/aphelion.

[markz]

This region shown in the picture, icluding the abbey, was deep in the lunar shadow during the total solar eclipse of August 11, 1999! I was located on a hill not far left from the visible setting sun watching the 2min 20s totality in the cloudless sky.

[neufer]

This region shown in the picture, including the abbey, was deep in the lunar shadow during the total solar eclipse of August 11, 1999! I was located on a hill not far left from the visible setting sun watching the 2min 20s totality in the cloudless sky.

Indeed, you were only about 3 km. off of the center of the eclipse which went through the southern end of Balatonfüred

http://antwrp.gsfc.nasa.gov/apod/ap010620.html
http://antwrp.gsfc.nasa.gov/apod/ap990819.html
http://antwrp.gsfc.nasa.gov/apod/ap990818.html

<<Tihany is famous for the echo, existing since the XVIII century. There were poems written for this echo, like by Mihály Csokonai Vitéz and Mihály Vörösmarty, but the most famous is by János Garay, summing up the legend of the place. The echo has since abated, due to changes in the landscape. The other part of the legend concerns with the "goats' nails", washed ashore from Balaton, which are in fact corners of prehistoric clams. According to the story, there was a princess with golden-haired goats, but she was too proud and hard of heart and was punished (cursed by the king of the lake): her goats were lost in Balaton, only their nails remained, and she was obliged to answer to every passers-by. A stone, remembering the Shouting Girl, is still to be seen near the village.>>

[geckzilla]

Why did they say XVIII instead of 18th century? Using roman numerals for the sake of using roman numerals?

[verkle]

... nearly 12 ... and 12 ...

You can't have slightly more than 12 ... and 12. Actually, I guess you can, since there is slightly more than 24 hours in a day. But what is your slightly more than referring to, day or night?

I think what you really wanted to say was "slightly more than 12 hours of daylight and slightly less than 12 hours of darkness". I think nearly 12 ... and 12 expresses it just fine.[/quote]

Yes, exactly. Thanks.

[neufer]

Why did they say XVIII instead of 18th century? Using roman numerals for the sake of using roman numerals?

<<Pannonia is an ancient province of the Roman Empire bounded north and east by the Danube, coterminous westward with Noricum and upper Italy, and southward with Dalmatia and upper Moesia. Pannonia was located over the territory of the present-day western half of Hungary with parts in Austria, Croatia, Serbia, Slovenia, Slovakia and Bosnia and Herzegovina.

Julius Pokorny derived the name Pannonia from Illyrian, from the Proto-Indo-European root *pen-, "swamp, water, wet" (cf. English fen, "marsh"; Hindi pani, "water" ). Its original inhabitants were the Pannonii (Pannonians), a group of tribes akin to Illyrians. From the 4th century BC it was invaded by various Celtic tribes. Little is heard of Pannonia until 35 BC, when its inhabitants, allies of the Dalmatians, were attacked by Augustus, who conquered and occupied Siscia (Sisak). The country was not, however, definitely subdued until 9 BC, when it was incorporated into Illyricum, the frontier of which was thus extended as far as the Danube.

In AD 6, the Pannonians, with the Dalmatians and other Illyrian tribes, revolted, and were overcome by Tiberius and Germanicus, after a hard-fought campaign which lasted for three years. After the rebellion was crushed in 9 AD, the province of Illyricum was dissolved, and its lands were divided between the new provinces of Pannonia in the north and Dalmatia in the south. The proximity of dangerous barbarian tribes (Quadi, Marcomanni) necessitated the presence of a large number of troops (seven legions in later times), and numerous fortresses were built on the bank of the Danube.

Some time between the years 102 and 107 Trajan divided the province into Pannonia Superior (the western), and Pannonia Inferior (the eastern) portion. According to Ptolemy, these divisions were separated by a line drawn from Arrabona (Győr) in the north to Servitium (Gradiška) in the south; later, the boundary was placed further east. The whole country was sometimes called the Pannonias (Pannoniae). Pannonia Superior was under the consular legate, who had formerly administered the single province, and had three legions under his control: Pannonia Inferior at first under a praetorian legate with a single legion as garrison, after Marcus Aurelius under a consular legate, still with only one legion. The frontier on the Danube was protected by the establishment of the two colonies Aelia Mursia (Osijek) and Aelia Aquincum (Óbuda) by Hadrian.

In the middle of the 5th century Pannonia was ceded to the Huns by Theodosius II, and after the death of Attila successively passed into the hands of the Ostrogoths (456–471), Lombards (530–568), Avars (560s - c.800), Slavs (living there since c. 480s; independent between c.800 - 900), Magyars (modern Hungarians) (since 900/901); Habsburgs and Ottomans (since 1526; the Ottoman rule ended in 1686). After the First World War, the region was divided between Austria, Hungary and the Kingdom of Serbs, Croats and Slovenes (renamed to Yugoslavia in 1929).>>

[ozprof]

One thing I find misleading is the statement that the equinox "marks the astronomical change of the seasons". No it does not! Astronomically speaking, the September equinox marks the middle day of autumn for the northern hemisphere!

Remember Shakespeare's "A Mid-summer's Night Dream"....... That is set on June 21, the middle day of summer for the northern hemisphere, not the first.

Ozprof

[neufer]

One thing I find misleading is the statement that the equinox "marks the astronomical change of the seasons". No it does not! Astronomically speaking, the September equinox marks the middle day of autumn for the northern hemisphere!

Remember Shakespeare's "A Mid-summer's Night Dream"....... That is set on June 21,
the middle day of summer for the northern hemisphere, not the first.

Mid-summer's Day was traditionally June 24th (Julian) = July 4th (Gregorian)
...somewhat shy of mid-meteorological summer (June 1 thru August 31) .

However, "A Mid-summer Night's Dream" may actually
have more to do with on WalpurgisNACHT (May 1)

[DavidLeodis]

In the explanation to the APOD it refers to the "Benedictine Archabbey of Tihany, Hungary". I thought I would search for further information on that Archabbey but I only got references to the Pannonhalma Benedictine Archabbey/ArchAbbacy in a town caled Pannonhalma in Hungary. I'm now confused. Is the Benedictine abbey at Tihany an Archabbey or not.

[neufer]

In the explanation to the APOD it refers to the "Benedictine Archabbey of Tihany, Hungary". I thought I would search for further information on that Archabbey but I only got references to the Pannonhalma Benedictine Archabbey/ArchAbbacy in a town caled Pannonhalma in Hungary. I'm now confused. Is the Benedictine abbey at Tihany an Archabbey or not.

No...they just keep the historically important founding charter of Tihany there.

How is your Hungarian? http://tihany.osb.hu/

[DavidLeodis]

In the explanation to the APOD it refers to the "Benedictine Archabbey of Tihany, Hungary". I thought I would search for further information on that Archabbey but I only got references to the Pannonhalma Benedictine Archabbey/ArchAbbacy in a town caled Pannonhalma in Hungary. I'm now confused. Is the Benedictine abbey at Tihany an Archabbey or not.

No...they just keep the historically important founding charter of Tihany there.

How is your Hungarian? http://tihany.osb.hu/

Thanks neufer. That helped clear up my query.

PS. My Hungarian is not so good. Well OK, it's rubbish!