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The Sun is a weakly variable star, and its actual luminosity therefore fluctuates. [3] The major fluctuation is the eleven-year solar cycle (sunspot cycle) that causes a quasi-periodic variation of about ±0.1%. Other variations over the last 200–300 years are thought to be much smaller than this. [4]
(The S 10 unit is defined as the surface brightness of a star whose V-magnitude is 10 and whose light is smeared over one square degree, or 27.78 mag arcsec −2.) The total sky brightness in zenith is therefore ~220 S 10 or 21.9 mag/arcsec² in the V-band.
However, some Arabic language star names sprang up later in history, as translations of ancient Greek language descriptions. The astronomer Claudius Ptolemy in his Almagest (2nd century) tabulated the celestial position and brightness (visual magnitude) of 1,025 stars. Ptolemy's book was translated into Arabic in the 8th and 9th centuries and ...
Factor ()Multiple Value Item 0 0 lux 0 lux Absolute darkness 10 −4: 100 microlux 100 microlux: Starlight overcast moonless night sky [1]: 140 microlux: Venus at brightest [1]: 200 microlux
Twilight is the time period between dawn and sunrise, and between sunset and dusk. Morning twilight: astronomical, nautical, and civil stages at dawn. The apparent disk of the Sun is shown to scale. [1] Evening twilight: civil, nautical, and astronomical stages at dusk. The solar disk is shown to scale.
World map showing the areas of Earth receiving daylight around 13:00 UTC in April. Daylight is the combination of all direct and indirect sunlight during the daytime.This includes direct sunlight, diffuse sky radiation, and (often) both of these reflected by Earth and terrestrial objects, like landforms and buildings.
For example, consider a 10 W transmitter at a distance of 1 million metres, radiating over a bandwidth of 1 MHz. By the time that power has reached the observer, the power is spread over the surface of a sphere with area 4πr 2 or about 1.26×10 13 m 2, so its flux density is 10 / 10 6 / (1.26×10 13) W m −2 Hz −1 = 8×10 7 Jy.
The sunniest month in the world is December in Eastern Antarctica, with almost 23 hours of bright sun daily. [ 7 ] Conversely, higher latitudes (above 50° north/south) lying in stormy westerlies have much cloudier and more unstable and rainy weather, and often have the lowest values of sunshine duration annually.