Moon, the only natural satellite of Earth, is easily visible with naked eye during day and night and it looks like when fully lit, as large as the Sun (size in the sky about 0.5°). Seen with binoculars or a telescope, it shows a surface covered by a multitude of impressive craters (more visible in lighter areas calls highlands) alternating with areas darker and smoother called seas. From geological point of view, highlands are what remains of its primitive crust (with about four billion years of age) while seas are areas of lava material leakage in ancient times (more than three billion years ago) probably as a result of the fall of a large asteroid on the lunar surface.
The age difference between two formations also explains lower number of craters in seas: there are less because seas were born a billion years after highlands.
We can therefore imagine our natural satellite as a large rock whose mass is, however, too small to hold gas molecules: lack of atmosphere that results causes rock formations are preserved as they were formed (unlike for example the ones of Earth where weather conditions such as air and water shape surface constantly changing it).
Absence of an atmosphere also leads to a temperature change from day to night. It passes, in fact, by more than 100°C in illuminated areas to -100°C than the ones in shade. Thanks to Apollo missions that, between 1969 and 1972, led to landing of several missions on thelunar surface, we know precisely that its distance from Earth varies, from about 350,000 to about 400,000 km, depending on gravitational pull of the Sun and relative position of Earth - Moon - Sun. Our natural satellite aimed towards the Earth always same face: in fact, its period of rotation corresponds to the one of revolution around our planet. Illumination of the lunar face varies depending on position of our satellite relative to the Earth and the Sun. We can thus define different lunar phases that repeat every 28 days or so:
- New Moon occurs when Sun, Moon and Earth are in this order on the same line. Moon is not visible because Sun illuminates its hidden face.
- first quarter is when apparent line connecting Earth with Moon is perpendicular to the one connecting Earth and Sun; we see illuminated half of the Moon.
- Full Moon occurs when Moon, Earth and Sun are, in this order, on the same line. We see the whole face of the Moon illuminated.
- last quarter is when Moon is opposite to the phase of first quarter. We see illuminated half of the disc. During waxing moon, that is, between new and full moon, Moon is observed generally in the afternoon and in the early part of the night. In declining phase, ie after Full Moon, Moon is seen in the second part of the night and in the morning.
Correspondence of apparent size of Moon in the sky and Sun seen from the ground generates a phenomenon known with term eclipse. When Moon is in opposite position to Sun (so it's full) and enters shadow of Earth, we have a Moon total eclipse. If it does not enter at all into cone of shadow we have a Moon partial eclipse. If, during new Moon, Sun, Moon and Earth are perfectly aligned, its shadow is projected on Earth's surface. From that point it is possible to observe a Sun total eclipsen, which is defined as partial if Moon does not cover entire solar surface.