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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.
Some places in the interior of the Arabian Peninsula receive 3,600–3,800 hours of bright sunshine annually. The largest sun-baked region in the world (over 3,000 hours of yearly sunshine) is North Africa. The sunniest month in the world is December in Eastern Antarctica, with almost 23 hours of bright sun daily. [7]
Scientists said the sun is now in its solar maximum, or the peak of its 11-year solar cycle.
Along the ecliptic plane there are enhancements in the zodiacal light where it is much brighter near the Sun and with a secondary maximum opposite the Sun at 180 degrees longitude (the gegenschein). In extreme cases natural zenith sky brightness can be as high as ~21.0 mag/arcsec², roughly twice as bright as nominal conditions.
The Sun has reached a period of peak activity in an 11-year cycle known as the Solar Maximum, marked by recent geomagnetic storms and near-global aurora light displays.
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Sunlight takes about 8.3 minutes to reach Earth from the surface of the Sun. [3] A photon starting at the center of the Sun and changing direction every time it encounters a charged particle would take between 10,000 and 170,000 years to get to the surface.