“Oh, it's a long long while from May to December
But the days grow short when you reach September
When the autumn weather turns the leaves to flame
And you ain't got time for waiting game”
Instant recognition from the first deep, warm vocal note. I’ve heard this a thousand times on late night radio skip transmissions as AM signals reflected off the ionosphere. The hissy transmission from some 50 KW big city station would fade in and out as the car covered road or even as the newest transistor hand-held with its extendable, quickly broken antenna was shifted from hand to hand. AM skip transmission was that touchy. It still is, we just don’t listen to it for music that much anymore.
Actually, my favorite rendition of September Song is found on Willy Nelson’s Star Dust album. His scratchy, reedy voice and acoustic guitar convey the song’s message very well. That album contains several of my all-time-favorite standards including Moonlight In Vermont
Since our trip to Vermont in 2005 directly lead to our decision to move here to N.E. Tennessee, I guess Willie gets part of the credit. Vermont was having trouble with flooding and high waters in 2005, the most active Atlantic hurricane year on record. We lost a couple fishing days to that high water. But those floods were nothing like the current damage wrought by Hurricane Irene last weekend.
As I walked Loki this morning, the first hints of red are starting to show up in a few patches along the western valley wall. They are more likely due to the lack of rain over the last two weeks than the seasonal change.
Sunrise this morning was 0701, sunset will be at 1958.
The astronomical data for the day is:
Astronomy
| Sep. 01, 2011 | Rise | Set |
| Actual Time | 7:01 AM EDT | 7:58 PM EDT |
| Civil Twilight | 6:35 AM EDT | 8:24 PM EDT |
| Nautical Twilight | 6:04 AM EDT | 8:55 PM EDT |
| Astronomical Twilight | 5:32 AM EDT | 9:27 PM EDT |
| Moon | 11:09 AM EDT | 10:00 PM EDT |
| Length Of Visible Light | 13h 49m | |
| Length of Day | 12h 56m Tomorrow will be 2m 11s shorter. | |
| | ||
That introduce some new terminology. So here are the answers.
From Wikipedia:
Twilight is defined according to the solar elevation angle θs, which is the position of the geometric center of the sun relative to the horizon. There are three established and widely accepted subcategories of twilight: civil twilight (brightest), nautical twilight, and astronomical twilight (darkest).[2]
| Definition | |
| Day | 0° ≤ θs |
| Sun's lower limb at horizon | θs = 0° 15' |
| Center of Sun's disk at horizon | θs = 0° |
| Sun's upper limb at horizon | θs = −0° 15' |
| Solar twilight | 0° ≤ θs < 6° |
| Civil twilight | −6° ≤ θs < 0° |
| Nautical twilight | −12° ≤ θs < −6° |
| Astronomical twilight | −18° ≤ θs < −12° |
| Night | θs < −18° |
]Civil twilight
Under civil twilight circumstances, the horizon is clearly visible, and terrestrial objects are easily perceptible without artificial light.
Morning civil twilight begins when the geometric center of the sun is 6° below the horizon (civil dawn) and ends at sunrise. Evening civil twilight begins at sunset and ends when the geometric center of the sun reaches 6° below the horizon (civil dusk).
The brightest stars appear during the civil twilight, as well as planets, such as Venus, which is known as the "morning star" 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 clear 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 clear atmospheric conditions.
Nautical twilight
Nautical twilight is the time when the center of the sun is between 6° and 12° below the horizon. In general, nautical twilight ends when navigation via the horizon at sea is no longer possible.[3]
During nautical twilight, sailors can take reliable star sightings of well-known stars, using a visible horizon for reference. The end of this period in the evening, or its beginning in the morning, is also the time at which traces of illumination near the sunset or sunrise point of the horizon are very difficult, if not impossible, to discern (this often being referred to as "first light" before civil dawn and "nightfall" after civil dusk). At the beginning of nautical twilight in the morning (nautical dawn), or at the end of nautical twilight in the evening (nautical dusk)—under good atmospheric conditions and in the absence of other illumination—general outlines of ground objects may be distinguishable, but detailed outdoor operations are not possible, and the horizon is indistinct.
Nautical twilight has military considerations as well. The initialisms BMNT (begin morning nautical twilight, i.e. nautical dawn) and EENT (end evening nautical twilight, i.e. nautical dusk) are used and considered when planning military operations. A military unit may treat BMNT and EENT with heightened security, e.g. by "standing to", in which everyone assumes a defensive position. This is partially due to tactics dating back to the French and Indian War (part of the Seven Years' War of 1756–1763), when combatants on both sides would launch attacks at nautical dawn or dusk.
]Astronomical twilight
Astronomical twilight is the time when the center of the sun is between 12° and 18° below the horizon. From the end of astronomical twilight in the evening to the beginning of astronomical twilight in the morning, the sky (away from urban light pollution) is dark enough for all astronomical observations.
Most casual observers would consider the entire sky fully dark even when astronomical twilight is just beginning in the evening or just ending in the morning, and astronomers can easily make observations of point sources such as stars, but faint diffuse items such as nebulae and galaxies can be properly observed only beyond the limit of astronomical twilight. In some places, especially those with sky glow, astronomical twilight may be almost indistinguishable from night. Theoretically, the dimmest stars ever visible to the naked eye—those of the sixth magnitude—will become visible in the evening once the sun falls more than 18° below the horizon (i.e. when astronomical dusk ends) and become invisible when the sun moves to within 18° of the horizon in the morning (when astronomical dawn begins). However, because of light pollution, some localities—generally those in large cities—may never have the opportunity to view even fourth-magnitude stars, irrespective of the presence of any twilight at all.[1]
Length
The number of daylight hours depends on the latitude and time of year. Each pole has continuous daylight near its summer solstice.
The length of twilight after sunset and before sunrise is heavily influenced by the latitude of the observer. In the Arctic andAntarctic regions, twilight (if there is any) can last for several hours. There is no civil twilight at the poles within a month on either side of the winter solstice. At the poles, civil twilight can be as long as two weeks, while at the equator, conditions can go from day to night in as little as twenty minutes. This is true because at low latitudes the sun's apparent movement is perpendicular to the observer's horizon. As one gets closer to the Arctic and Antarctic circles, the sun's disk moves toward the observer's horizon at a lower angle. The observer's earthly location will pass through the various twilight zones less directly, taking more time.
Within the polar circles, twenty-four hour daylight is encountered in summer, and in regions very close to the poles, twilight can last for weeks on the winter side of the equinoxes. Outside the polar circles, where the angular distance from the polar circle is less than the angle which defines twilight (see above), twilight can continue through local midnight near the summer solstice(June in the Northern Hemisphere, December in the Southern Hemisphere). The precise position of the polar circles—and thus of the regions where twilight can continue through local midnight—varies slightly from year to year with Earth's axial tilt. The lowest latitudes at which the various twilights can continue through local midnight are approximately 60.561° ( 60° 33’ 43” ) for civil twilight, 54.561° ( 54° 33’ 43” ) for nautical twilight and 48.561° ( 48° 33’ 43” ) for astronomical twilight.[4][5]
We have class this afternoon. First impression for my astronomy class is that I will enjoy it.
“Oh, the days dwindle down…”
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