The Moon, Earth’s natural satellite
The Moon is Earth’s only natural satellite and the nearest celestial body to Earth. Its radius is about 1738 km, its mass about 7.35·10²² kg and the distance to Earth about 384 000 km. The Moon’s mass is about 81 times smaller than Earth’s, so its gravity is much weaker (about 6 times less than on Earth) – a person jumps much higher on the Moon. The Moon spins on its axis once in about 27.3 days and also circles Earth once in the same time, so we always see the same side. The Moon gives off no light – it reflects sunlight; we see only the part that is lit by the Sun and faces Earth. The different shapes of the Moon as seen from Earth are called the phases of the Moon: new moon (invisible), first quarter, full moon, last quarter. One cycle of phases takes about 29.5 days. The Moon’s gravity causes the rise and fall of ocean water (tides). When the Sun, Moon and Earth line up, an eclipse occurs.
Model: in a dark room a desk lamp (or bright torch) is the “Sun”, a tennis ball is the “Moon” and you are “Earth”. Hold the ball on a stick at arm’s length and slowly turn around; watch how the lit part of the ball changes – just like the Moon’s phases. Safety: use an LED lamp (it does not heat up); tidy the area so nobody trips on cables; do not stare directly at the bright lamp.