SS-X2: The Phase of the Moon

What are the Phases of the Moon?

            The moon is often referred to as the natural ‘satellite’ of the earth, for as the earth rotates around the sun so the moon rotates around the earth, but as it does so it is possible to see a change in the moons size and shape during the course of one month, or to be exact 29.5305882 days. These different stages are called the ‘phases’ of the moon.

            The main two phases that are most well known around the world are ‘full moons’, where the whole circle of the moon is clearly visible from the earth, and ‘new moons’, were the moon becomes so darkly shadowed that it is practically invisible from the earth. Directly in between these two phases is what is known as a ‘quarter moon’, where exactly half of the moon is illuminated. However, which side of the moon is being illuminated depends on whether it is going from full moon to new moon; otherwise known as ‘waning’, or from new moon to full moon; otherwise known as ‘waxing’. The same theory applies when the moon is half way between a quarter moon and a full moon; i.e. the ‘gibbous’ phase, or half way between a quarter moon and a new moon; i.e. the ‘crescent’ phase.

            The simplest way of explaining the various phases of the moon is to imagine them as times on a clock, with a full moon at ’12 o’clock’ travelling the full cycle through a new moon at ‘6 o’clock’.

           

Previous Ideas about the Phases of the Moon:

            It is almost impossible to gaze up at the sky each night for over a month and not notice a difference in the appearance of the moon: Even in modern cities, with street lamps and neon lights, the moon is one of the few regular sights in the night sky, and so people who lived in past civilisations and so were unrestricted by the blindness caused by modern city life could have seen clearly how the moon seemed to become smaller and then larger throughout the year. Many ancient early historic cultures considered the moon as representing the cyclical nature of life and used its waxing and waning to rule both everyday duties such as hunting and harvesting as well as rituals and festivals, the most important of which were usually during lunar (and solar) eclipses [1]

            During the time roughly between 500 BC – 150AD, early scientists and mathematicians in ancient cultures such as Greece, China and the Middle East began to study the shape of the shadows on the moon in more detail and were able to predict solar eclipses, the distance between the earth and the moon and even the spherical shape of the earth.

            During the Renaissance and the Age of Enlightenment, people began to question ideas that had previously been conceived as truth, such as religion and mythology, and proceeded to try and explain everything in the world rationally, including the phases of the moon. In the year 1609 Galileo became the first man to use a telescope to look at the moon, and as he did so he saw that its surface was not smooth but in fact just as mountainous and pitted as the earth. Not only did this disprove the ancient Greek theory that the moon had to be perfect because of its position in the ‘uncorrupted’ heavens, it also suggested the idea that the earth may not be the centre of the universe, as was previously believed, and the possibility of the existence of more planets other than the Earth (This theory was eventually proved correct by Galileo, who proceeded to contribute much to the field of astronomy through his observations of Jupiter and Venus.) [3]

Why do we see phases of the moon?

            The reason why the phases of the moon occur relates to our position as observers from the earth. As has previously been mentioned, the moon acts as a natural satellite, rotating around the earth on an approximately monthly cycle. Depending on the position of the moon in relation to both the earth and the sun, we (people on earth) can see only certain reflected sunlight and thus illumination off some parts of the moon. This is shown in the picture below.

            As can be seen in the inner ring of moon diagrams, half of the moon is always illuminated because half of it will always be facing the sun and so light rays from the sun will be reflecting off this facing half. However, it is not always possible to see this illuminated half in its entirety from our position as spectators on the earth. The only time when we are able to witness this ‘illuminated half’ is when we are positioned somewhere in between the moon and the source of its reflective light, the sun; i.e. during a full moon. At the other side of the cycle, when we are on the opposite side of the moon to the sun we are unable to see any light reflected of its surface, in much the same way that we are unable to see the features on a human’s face if they stand in front of the light source in a room. The reason why we can differentiate between the phases in between follows exactly the same principle: Our perception of how much of the moon is illuminated depends on the angle the earth makes between the moon and the sun.

References:

¹The Moon in Ancient History: http://www.moonlightsys.com/themoon/ancient.html

²Lunar Exploration: early studies. Encyclopædia Britannica Online

http://www.britannica.com/EBchecked/topic/391266/Moon/54215/Lunar-exploration

³The Moon, The Galileo Project: Science, available since 4 August 2003

http://galileo.rice.edu/sci/observations/moon.html

Understanding the Moon Phases, Moon Connection, available since 2011

http://www.moonconnection.com/moon_phases.phtml

(Pictures):

Galileo’s illustrations of the Moon, from his ‘Sidereus Nuncius’ (1610; The Sidereal Messenger), (Courtesy of the Joseph Regenstein Library, The University of Chicago), Encyclopædia Britannica Online, available since 16 Feb 2011. http://www.britannica.com/EBchecked/media/2914/Galileos-illustrations-of-the-Moon-from-his-Sidereus-Nuncius

Galileo’s wash drawings, The Galileo Project, available since 4 Aug 2003, http://galileo.rice.edu/images/things/g_moonwash.gif

Understanding the Moon Phases, http://www.moonconnection.com/moon_phases.phtml

Moon Clock, http://www.mensgiftshop.com/acatalog/moonphaseclocklrg.jpg