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Surprise: the Moon outshines the Sun at the highest energies
Across all wavelengths of light, the Sun is brighter than the Moon. Until we went to the highest energies, and saw a gamma-ray surprise.
If you look at all the objects in Earth’s skies, both natural and artificial, it should come as no surprise that the Sun is the brightest object of all. The Sun, after all, produces its own light, powered by nuclear fusion in its core. That core-generated energy not only holds the Sun up against gravitational collapse, but also propagates to the Sun’s edge, the photosphere, where the Sun emits radiation over a wide range of wavelengths that correspond to a temperature of around 6,000 K. Although the Moon is the second-brightest object that we see, it’s only so bright because of its very close proximity to Earth; most of the Moon’s light, intrinsically, is just reflected light from the Sun.
Although this was first shown to be true in visible wavelengths of light, the 20th century revealed that the same circumstances held for a wide variety of other wavelengths. The Sun was the brightest object, followed distantly by the Moon in second place, as seen from Earth in:
- X-rays,
- ultraviolet light,
- visible light,
