The Moon is our nearest celestial neighbor being on average a mere 380,000 km from the Earth (the Moon has an elliptical orbit around the Earth).  It takes light 1.26 seconds to travel from the Moon to the Earth.  In comparison it took the Apollo astronauts around 3 days.

The Moon orbits the Earth completely once every 27 1/3 days.  However, the time between two full moons is slightly longer at 29 ½ days.  This is because both the Earth and Moon are moving in relation to the Sun.

The Moon is in synchronous rotation with the Earth.  This means that the time it takes to orbit the Earth once is the same as the time it takes to rotate on its axis once.  Therefore the Moon always points the same face towards the Earth.  This face is known as the ‘near side’. We can never see more than 50% of the Moon at any one time, however it is possible to see a cumulative total of 59% of the Moon's surface over a number of months, due to a 'wobbling motion' known as Libration. 

We see the Moon because it reflects sunlight from its surface.  Although the Moon looks bright in the night sky it only reflects about 10% of the light that falls on it. Apparently this is the same amount of light reflected by worn asphalt. The amount of light that is reflected also changes slightly depending on the phase. 

The Moon goes through different phases as it orbits the Earth.  From one full moon to the next is about 29 ½ days.  A ‘day’ on the Moon is also 29 ½ Earth days long.  The easiest way to explain the phases of the Moon is with a video.

The images used to create the above video are courtesy of: NASA/Goddard Space Flight Center Scientific Visualization Studio 

One half of the Moon is always lit by the Sun.  What we see depends on the location of the Moon and the Sun in the sky.

At full moon the Moon, Earth and Sun are in a (rough) line and we see the fully illuminated nearside face of the Moon.  At new moon, the Moon is between the Earth and the Sun.  The far side of the Moon is fully illuminated and the near side is in shadow. 

At first and third quarter the Moon is at a 900 angle with respect to the Sun and the Earth.  We see only half of the Moon’s face illuminated (or one quarter of the whole Moon).

A crescent moon is when less than half of the Moon’s face is illuminated from our point of view.  A gibbous moon is when over half of the Moon is illuminated from our perspective.  The Moon is said to be waxing when it is progressing from new moon to full moon (growing more illuminated) and waning when it is progressing from full moon to new moon (shrinking and getting less illuminated).

The Moon can be seen in the sky at the same time as the Sun, which often surprises people as the Moon is usually thought of as a night-time object.  In fact it is visible in the daytime for half the month, either in the morning or evening sky.  The Moon is not very illuminated at this time so appears quite small and faint, especially when compared to the Sun so it habitually gets overlooked.

The Moon is roughly 400 times smaller than the Sun but it is also 400 times closer to us, which means that the Moon and the Sun appear the same size in the sky.  On certain occasions this allows the Moon to completely cover the Sun in a solar eclipse.  This only occurs at new moon but does not happen every month.

 

 

A lunar eclipse is when the Moon moves into the Earth’s shadow.  This does not happen every full moon as the Moon is not always in the correct alignment with the Earth’s shadow.

 

 

Observing the Moon

The Moon is the easiest astronomical object to observe.  It is very obvious in the sky and some details can be seen with the naked eye.

Dark patches on the Moon are known as Maria (seas).  They are not seas in that they are filled with water (although there is frozen water on the Moon in small amounts) but are filled with solidified basaltic lava from when the Moon was volcanically active.  The seas have all been named such as Mare Tranquillitatis – the sea of tranquillity and Mare Serenitatis – the sea of serenity.

The brighter areas on the Moon are lunar highlands made from an igneous rock called Anorthosite.

The surface of the Moon is covered in craters that have been formed by meteorite impacts.  Some of these craters are visible to the naked eye such as Tycho and Copernicus.  The craters on the Moon are mostly named after famous male scientists and astronomers.

Some fantastic views of the Moon can be achieved through binoculars or a small telescope.  Hundreds of craters become visible and an amazing amount of detail can be see even with binoculars.  Some of the best views are obtained when looking along the terminator – the line between day and night on the Moon – as the contrast is really good. The edge of the Moon's disc against the background of the sky is referred to as the Limb.  A good Moon map is essential if you wish to learn your way around the Moon.

 

 

Be aware that the Moon is relatively bright and can be dazzling when looked at through a telescope, especially when full.  Although it won’t blind you to look at it, you will lose your dark adaption and may see pretty after images for a while whilst you stumble around in the dark.  Be careful! ‘Moon” filters are available to place on the eyepiece of a telescope that reduce the amount of light and make observing the moon a more comfortable experience.