Twilight
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http://www.arachnoid.com/lutusp/sunrise/sunhelp.html#twilight
There are four generally accepted definitions of twilight with four light levels and four purposes:
| Twilight Name | Sun's Distance Below Horizon | Definition |
|---|---|---|
| Sunset | 0° 50' | Because of atmospheric refraction, the sun's upper limb appears to be touching the horizon. |
| Civil | 6° | The time after which city streetlights are lit, automobiles begin using headlights, in general, the time after which we use artificial light. |
| Nautical | 12° | The best time for a sailor to use a sextant to take a star sight – the horizon is still visible, and many navigational stars are also visible. Earlier, some important navigational stars will not be visible. Later, the horizon becomes too difficult to see through the sextant. |
| Astronomical | 18° | After this time, the sky is dark enough for astronomers to make productive use of powerful telescopes. Only the faintest glow is visible near the sun's position. |
In the United States, it is generally accepted that civil twilight ends 30 minutes after sunset. This is obviously an average value for use by people in a society without computers, because in reality there are substantial variations – in Honolulu, Hawaii, civil twilight lasts 22 minutes. In Seattle, Washington it lasts 30 minutes, and in Fairbanks, Alaska (on an average day) it lasts 50 minutes. In mid-summer, Fairbanks' civil twilight never ends – it lasts all night (all two hours of it).
Above the Arctic Circle (66° 33' North), there are days when the sun never sets or never rises. These dates and locations are handled by Sun Calculator with the special labels "[Above]" and "[Below]" instead of times, to indicate that the sun's light (or a particular twilight) continues indefinitely on that date.
Now stretch your mind – imagine being at the north pole. On June 21st, the sun is about 23° above the horizon and travels around in a circle, always visible (weather permitting). On September 21st or thereabouts, the sun sits on the horizon all day, slowly moving in a circle. During the next few days it gradually "sets," not to be seen again for six months.
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Updated: 17 October 2002 Definitions of Astronomical Events
Sun rise and setSunriseis defined as the instant in the morning under ideal meteorological conditions, with standard refraction of the sun's rays, when the upper edge of the sun's disk is coincident with an ideal horizon.
Sunsetis defined as the instant in the evening under ideal meteorological conditions, with standard refraction of the sun's rays, when the upper edge of the sun's disk is coincident with an ideal horizon.Civil twilightbeginning of morning civil twilightDefined as the instant in the morning, when the centre of the sun is at a depression angle of six degrees (6°) below an ideal horizon. At this time in the absence of moonlight, artificial lighting or adverse atmospheric conditions, the illumination is such that large objects may be seen but no detail is discernible. The brightest stars and planets can be seen and for navigation purposes at sea, the sea horizon is clearly defined. Ending of evening civil twilightDefined as the instant in the evening, when the centre of the sun is at a depression angle of six degrees (6°) below an ideal horizon. At this time in the absence of moonlight, artificial lighting or adverse atmospheric conditions, the illumination is such that large objects may be seen but no detail is discernible. The brightest stars and planets can be seen and for navigation purposes at sea, the sea horizon is clearly defined. Nautical twilightbeginning of morning nautical twilightis defined as the instant in the morning, when the centre of the sun is at a depression angle of twelve degrees (12°) below an ideal horizon. At this time in the absence of moonlight, artificial lighting or adverse atmospheric conditions, it is dark for normal practical purposes. For navigation purposes at sea, the sea horizon is not normally visible.
Ending of evening nautical twilightis defined as the instant in the evening, when the centre of the sun is at a depression angle of twelve degrees (12°) below an ideal horizon. At this time in the absence of moonlight, artificial lighting or adverse atmospheric conditions, it is dark for normal practical purposes. For navigation purposes at sea, the sea horizon is not normally visible.Astronomical twilightbeginning of morning astronomical twilightis defined as the instant in the morning, when the centre of the sun is at a depression angle of eighteen degrees (18°) below an ideal horizon. At this time the illumination due to scattered light from the sun is less than that from starlight and other natural light sources in the sky.
Ending of evening astronomical twilightis defined as the instant in the evening, when the centre of the sun is at a depression angle of eighteen degrees (18°) below an ideal horizon. At this time the illumination due to scattered light from the sun is less than that from starlight and other natural light sources in the sky.Sun Transit timeThe transit timeof a celestial body refers to the instant that its center crosses an imaginary line in the sky - the observer's meridian - running from north to south. For observers in low to middle latitudes, transit is approximately midway between rise and set, and represents the time at which the body is highest in the sky on any given day. At high latitudes, neither of these statements may be true - for example, there may be several transits between rise and set. The transit of the Sun is local solar (sundial) noon.Moon rise and setMoonriseis defined as the instant when, in the eastern sky, under ideal meteorological conditions, with standard refraction of the moon's rays, the upper edge of the moon's disk is coincident with an ideal horizon.
Moonsetis defined as the instant when, in the western sky, under ideal meteorological conditions, with standard refraction of the moon's rays, the upper edge of the moon's disk is coincident with an ideal horizon.Moon phasesAs the relative position of the sun, moon and earth changes, differing proportions of the moon's visible surface are illuminated by the sun. The phases of the moon are specific instances in this process.
New moonoccurs when the apparent longitudes of the moon and sun differ by 0°. At this time the moon does not appear to be illuminated.
First quarteroccurs when the apparent longitudes of the moon and sun differ by 90°. At this time 50% of the moon's visible surface is illuminated.
Full moonoccurs when the apparent longitudes of the moon and sun differ by 180°. At this time 100% of the moon's visible surface is illuminated.
Last quarteroccurs when the apparent longitudes of the moon and sun differ by 270°. At this time 50% of the moon's visible surface is illuminated.Rise and set of five planetsThe planets which are generally visible to the naked eye are:Mercury, Venus, Mars, Jupiter and Saturn.The rise and set times for these planets is the instant when, under ideal meteorological
conditions, with standard refraction of their rays, the planet is coincident with an ideal horizon.
Ideal HorizonAn ideal horizon exists when the surface forming the horizon is at a right angle to the vertical line passing through the observer's position on the earth. If the terrain surrounding the observer was flat and all at the same height above sea level, the horizon seen by the observer standing on the earth would approximate the ideal horizon.Zenith DistanceThe zenith distance is a vertical angle measured from directly overhead, down to the required point. An ideal horizon has a zenith distance of 90 degrees.True AzimuthTrue azimuth is the clockwise horizontal angle from true north to the object being sighted.True northTrue north is the direction towards the north pole along the meridian of longitude which passes through the observer's position on the earth.Vertical angleThe Vertical angle is the angle measured in a vertical plane, from the horizon to the required point. Directly overhead would have a vertical angle of 90 degrees. |
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