New Delhi, Sept. 26 -- 49 p.m. EDT tomorrow, Saturday, September 26. At 13.7 days old in its 29.5-day cycle, the moon is as bright as it will get without being technically full.

For most observers looking up tonight, the difference between tonight's moon phase and tomorrow's full moon is impossible to detect with the naked eye. The sky over the Northern Hemisphere will carry near-total lunar brightness from dusk to near-dawn, bright enough to read by outdoors and strong enough to wash out all but the most prominent stars and planets.

Understanding what the moon phase today means requires knowing how the lunar cycle works. The moon completes one full orbit of Earth roughly every 29.5 days. As it does, the angle between the sun, Earth, an...