Universe - Phases of the Moon
978-3-14-100890-6 | Page 10 | Ill. 5
Overview
From a great distance of around 150 million kilometres, almost parallel rays from the Sun illuminate one half of the Earth and of the Moon. To an observer on the Earth's surface, the Moon sometimes appears fully illuminated by the Sun, but sometimes more of its shadow side is visible.
The Sun lights up the Moon
Depending on the position of the Moon relative to the Earth, an observer sees the fully illuminated half of the Moon on the Earth's night side (Full Moon, position 4) or, partially illuminated, the waxing crescent Moon (position 2) in the second half of the day and first half of the night or the waning crescent Moon in the second half of the night and first half of the day (position 6). The New Moon remains invisible during the day. For a full orbit around the Earth, the Moon actually needs only 27 days, 7 hours and 43 minutes ("sidereal month"). However, because the Earth also continues on its orbit around the Sun at the same time, the Moon needs the extended time of 29 days, 12 hours and 44 minutes until it reaches the same phase again, i.e. the same relative position of the Sun, Earth and Moon ("synodic month"). The latter determines the length of the months.
