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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]
In contrast, the term brightness in astronomy is generally used to refer to an object's apparent brightness: that is, how bright an object appears to an observer. Apparent brightness depends on both the luminosity of the object and the distance between the object and observer, and also on any absorption of light along the path from object to ...
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.
A solar flare is a sudden flash of brightness observed over the Sun's surface or the solar limb, which is interpreted as an energy release of up to 6 × 10 25 joules (about a sixth of the total Sun's energy output each second or 160 billion megatons of TNT equivalent, over 25,000 times more energy than released from the impact of Comet ...
Because energy transport in the Sun is a process that involves photons in thermodynamic equilibrium with matter, the time scale of energy transport in the Sun is longer, on the order of 30,000,000 years. This is the time it would take the Sun to return to a stable state if the rate of energy generation in its core were suddenly changed. [106]
[1] [2] During these times, the brightness of the sky matches the brightness of streetlights, signs, car headlights and lit windows. The period of time shortly before the magic hour at sunrise, or after it at sunset, is called the "blue hour". This is when the sun is at a significant depth below the horizon, when residual, indirect sunlight ...
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.
Solar irradiance is often integrated over a given time period in order to report the radiant energy emitted into the surrounding environment (joule per square metre, J/m 2) during that time period. This integrated solar irradiance is called solar irradiation , solar exposure , solar insolation , or insolation .