What causes moon phases? Why is there only one full moon a month?
50% of moon's surface is always lit by Sun.
but from earth we see this lit 50% only once in a month.
Moon goes round the earth once in 27days 8 hours. with respect to stars. THe attached picture will make things very clear.![enter image source here] the time interval between full moon and next full moon id 29.5 days as moon travels around earth during this
THe inner circle shows 50% of moon is always lit by Sun.
The outer circle shows what we see from Earth's surface.
Picture credit Moon connection.com
Both Moon's day-night spin-period and its orbital period around Earth are the same 29.53 d and the same hemisphere faces Earth. The gradual day-night transit across this face is observed in phases.
During this period, Moon appears alternately in the (Earth-Moon-Sun) Sun side and (Moon-Earth-Sun) opposite side. While in the Sun-side, we see dark Moon. On the opposite side, the face we see is wholly sunlit, and it is full Moon. This period 29.53 d is the time between two successive Full Moons At New Moon and Full Moon, the Moon crosses the heliocentric (moving) longitudinal plane of the Earth. .