July 2009 – By Professor William Reville, University College Cork
Origin of the Moon.
It seems that the young earth had no moon, but soon after the earth formed a Mars-sized rogue planet struck it a huge glancing blow. A large chunk of earth and most of the rogue planet were vaporised into a cloud that rose over 22,000 km in altitude where it condensed gradually into the solid moon. Lunar rocks are about 4.6 billion years old – about the same age as the earth and the composition of lunar rocks is very similar to rocks on earth.
The Moon is Slowly Moving Away from Earth.
The current distance between the earth and the moon is 384,000 km but it was closer to earth in the past. The moon is slowly drifting away from the earth at a rate of 4 cm per year. Daytime temperature of the surface of the moon is about 130 degrees C and night time lows reach about minus 110 degrees C.
Gravity on the Moon.
The mass of the moon is about one eightieth of the earth’s mass. Since the force of gravity at the surface of the object is proportional to the object’s mass and size, the force of gravity on the surface of the moon is only one sixth the force on the surface of the earth. Your weight is the force that gravity exerts on your mass. Your mass remains the same whether you stand on earth or on the moon, but if you weigh 60 kg on earth, you will weigh only 10 kg on the moon. Alan Sheppard hit a golf ball on the moon in 1971 and drove it 400 yards using a makeshift six-iron and encumbered by a heavy space suit.
Atmosphere on the Moon.
The gravity on the moon isn’t strong enough to hold an atmosphere and the moon’s atmosphere is very tenuous and insignificant compared to earth’s atmosphere. The sky always looks dark from the moon because there is no atmosphere to scatter light. Also the moon is always silent as sound waves travel through air.
The Moon Always Shows the Same Face to Earth.
The moon takes the same time to orbit the earth as it takes to rotate once on its own axis (approx. 27.3 days). This synchronisation causes the moon to always show the same face to the earth. In other words, one hemisphere of the moon always faces earth and the other (the ‘dark side of the moon’) always faces away.
The Shape of the Moon Varies as Viewed from Earth.
We see the moon because it reflects light from the sun. The moon shape we see changes in a repeating cycle because the amount of illuminated moon varies depending on the moon’s position relative to the earth and the sun. We see a full moon when the moon is directly in front of us and the sun is directly behind us, illuminating a full hemisphere of the moon. We see no moon when the moon is directly between the earth and the sun – the sun now illuminates only the side of the moon we cannot see from earth. Paradoxically we call no moon the new moon.
The Moon Causes the Tides on Earth.
The moon’s gravitational pull on the earth is the main cause of ocean tides rising and falling. The earth’s oceans display two bulges of water – one where the oceans face the moon and the pull is strongest and the other where the oceans face away from the moon and the pull is weakest. Both bulges represent high tides.
Visiting the Moon.
The moon is the only extraterrestrial object to have been visited by humans. The Soviet Union made the early running in modern investigations of the moon. Luna 2 was the first artificial object to impact the lunar surface in 1959. Luna 3 sent back pictures of the moon in 1959. Luna 9 was the first spacecraft to soft land on the moon in 1966. The first human to walk on the moon was Neil Armstrong on July 21st 1969 on the Apollo 11 mission. Only 12 people have ever stepped onto the surface of the moon.
The ‘Man in the Moon’.
We have all amused children by pointing out the ‘man in the moon’, but there really is a man on the moon since 1999 – or at least his ashes are there. Dr. Eugene Shoemaker, a geologist, educated the Apollo astronauts about craters. One of his dreams was to fly a space mission but he never made it because of medical problems. After he died, his ashes were placed on board the Lunar Prospector spacecraft which was crash-landed in a moon crater July 31, 1999. The official purpose of the mission was to discover if there was water on the moon, but it also fulfilled Dr. Shoemaker’s dream.
(This article appeared in The Irish Times 2nd July, 2009).