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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]
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.
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
This plot was created using the simple sunrise equation, approximating the sun as a single point and does not take into account effects caused by the atmosphere or the diameter of the Sun. The sunrise equation or sunset equation can be used to derive the time of sunrise or sunset for any solar declination and latitude in terms of local solar ...
These factors vary with atmospheric composition, geographic location, and time (see position of the Sun). While directional-hemispherical reflectance factor is calculated for a single angle of incidence (i.e., for a given position of the Sun), albedo is the directional integration of reflectance over all solar angles in a given period. The ...
Over billions of years, the Sun is gradually expanding, and emitting more energy from the resultant larger surface area. The unsolved question of how to account for the clear geological evidence of liquid water on the Earth billions of years ago, at a time when the sun's luminosity was only 70% of its current value, is known as the faint young ...
The areas with the lowest sunshine duration annually lie mostly over the polar oceans, as well as parts of northern Europe, southern Alaska, northern Russia, and areas near the Sea of Okhotsk. The cloudiest place in the United States is Cold Bay, Alaska, with an average of 304 days of heavy overcast (covering over 3/4 of the sky). [8]
From 2007 to 2009, sunspot levels were far below average. In 2008, the Sun was spot-free 73 percent of the time, extreme even for a solar minimum. Only 1913 was more pronounced, with no sunspots for 85 percent of that year. The Sun continued to languish through mid-December 2009, when the largest group of sunspots to emerge for several years ...