The Bridge V Learning Edition 1 ; our solar system | Page 46

BACKGROUND INFORMATION
Moon
The Moon is the only natural satellite of the Earth. It orbits our planet in 27.3 days at a
mean distance of 384,400 km. Its diameter of 3,474 km makes it the fifth largest satellite in
the solar system. It is thought that the Moon was formed at the same time as the Earth,
4.55 billion years ago, from the debris of a giant collision between the Earth and a Mars-size
object. The Moon is in on a synchronous orbit around the Earth, which means that it makes
one turn around itself as it makes one turn around the Earth. So, it is always showing the
same face to us. All we know about the ‘hidden’ face of the Moon comes from the records
taken by astronauts or automatic probes that went around on to the other side. There is no
rising or setting of the Earth on the horizon of the Moon. If you were standing on the near
side of the Moon, the Earth would appear immobile in the sky. During its revolution around
the Earth, the disk of the Moon is not always completely lit by the Sun. This variation in the
appearance of the lunar disk is called the “phases of the Moon”, and is characterized by a
well-known cycle. The apparent diameter of the Moon as seen from Earth is almost exactly
that of the Sun. This is why the Moon can sometimes completely mask the solar disk and
produce total solar eclipses. Another important influence of the Moon on Earth is the
phenomenon of tides, which is due to the Moon’s gravitational pull on the seas and oceans
of Earth. The Moon is the only celestial object that has been visited by human beings. The
first visit was on 21 July 1969 when two members of the Apollo 11 mission set foot on our
satellite: they were Neil Armstrong and Edwin Aldrin. As many as 12 astronauts walked the
surface of the Moon between 1969 and 1972, and they returned with 382 kg of lunar soil to
be analysed on Earth .