If the Sun and Moon are both visible, are there ever circumstances where the Moon is brighter than the Sun?

The term “moonlight” is one we associate with music, poetry, and folklore, possibly because the Moon is seen best at night, which tends to be a romantic and mysterious time. In reality, the Moon has no light of its own, because it has no significant internal source of energy (unlike the Sun). What we call moonlight is all secondary light produced by what hits the Moon from space. Mostly it is reflected sunlight. Despite the lyrical “silvery” Moon, the rocks on the Moon’s surface are rather dark in color, reflecting only about 7% of the sunlight that strikes the lunar surface. That still makes the Moon bright in the night sky. The term “harvest moon” refers to the full Moon closest to the Autumnal Equinox, and it originated from farmers finishing their Fall harvest by the light of that full Moon.

The Moon also reflects forms of light other than what we see with our eyes. There are images of the Moon in radio, infrared, ultraviolet, and X-ray light. In all cases, the Moon shines by reflected light and is therefore very faint compared to the Sun.

Does that mean that the Moon can NEVER be brighter than the Sun? Not quite. There is one type of light in which the Sun itself is extremely dim (most of the time), and that is the very high-energy type of light known as gamma radiation. In gamma rays, the Sun is so dim that something else can make the Moon brighter than the Sun. That extra source of energy is cosmic rays, the very high energy particles that come from beyond the Solar System. These cosmic rays constantly bombard the lunar surface (the Earth’s atmosphere protects us from facing these same cosmic rays). One type of collision of such high-energy particles with the Moon rocks produces gamma rays. It is the same sort of process that takes place at high-energy physics laboratories like Fermilab, CERN, KEK, and SLAC. Enough gamma rays are produced by this process to give a faint gamma-ray “Moonglow” that was detected by the gamma-ray telescope called EGRET on the Compton Gamma Ray Observatory. EGRET looked for the Sun, too, but it was too faint to be seen, except when it had giant solar flares. In gamma-ray light, and only there as far as we know, the Moon can be brighter than the Sun.

EGRET gamma-ray image of the Moon: http://antwrp.gsfc.nasa.gov/apod/ap970210.html

Names of the full Moon throughout the year: http://www.farmersalmanac.com/astronomy/moonnames.html

Dave Thompson is an astrophysicist who studies gamma rays in the Laboratory for High Energy Astrophysics. He helped build, test, and analyze data from EGRET on the Compton Gamma Ray Observatory, and he is now helping build part of the Gamma Ray Large Area Space Telescope (GLAST), scheduled for launch in 2006. His particular scientific interest is gamma-ray pulsars.