Monthly moon phase from new to full etc is caused by the change in relative observed position between the sun and the moon. Light from the sun always illuminates the full disk of the moon, but our changing relative position means that sometimes we can see all of this disk (full moon), part of this disk (half moon) or almost none of this disk (new moon). When we are positioned in space on the same side of the moon as is illuminated by the sun, we see full moon.
The moon orbits earth in an oval of about 357,000 (perigee) km x 406,000 (apogee). These distances change in a cyclic way under the influence of gravity from the sun, other planets and to some extent, everything in the solar system. When the perigee is smaller than average, the moon is closer than average and therefore appears bigger and brighter by about 8 to 13%. Supermoons occur about once every 14 full moons. Tonight’s perigee is a little closer than most so the moon will appear about 12% larger diameter than average. This translates into about 30% extra brightness.
Super Moons occur when a Full Moon or New Moon occur on the same night as the Perigee.
The third factor is to do with how we perceive the moon. It is well known that a rising moon seems to be bigger than a moon high in the sky. The image in the centre demonstrates this optical illusion. This optical illusion also occurs with the Sun and star constellations. See http://en.wikipedia.org/wiki/Moon_illusion
Measurements using instruments or even just hiding it behind your outstretched thumb will reveal that the moon remains the same size as it travels the sky each night (changes occur over the month but the change over an hour is imperceptible). See the photo of Full moon at Acacia Grove Cabin, Huon Bush Retreats to see how small the moon really is.
This optical illusion is most profound when there is a ground based object near the moon, so moonrise near a mountain or large tree exaggerates this effect. Being surrounded by mountains and trees, this puts Tasmanians in a great place to enjoy tonight’s super moon. Find a place with no city lights, a clear view of the sky with a mountain skyline ideally about 15 degrees above the horizon
See the Lunar Perigee and apogee Calculator and Moon Phase Calculators at http://fourmilab.ch/earthview/pacalc.html Find the nights when perigee is smallest distance and the date and time of perigee and full moon match the closest. These are the most dramatic supermoons.
If you want a laugh at the doom sayers, predicting everything from earthquakes to werewolves, here is an over the top report where, ”… Nolle warns that during the SuperMoon risk window the world should expect several magnitude 5+ earthquakes, countless tsunamis, constant volcanic eruptions and werewolves to run wild. Nolle has teamed up with leading expert on the paranormal and supernatural, Rosemay Ellen Guiley in order to discover the connection between the SuperMoon and werewolves.” http://weeklyworldnews.com/headlines/30519/extreme-supermoon/
And “...‘supermoon’ may have played a role in the devastating earthquake that struck Fukushima days before the super moon arrived. A supermoon, the closest that the moon comes to the Earth, will also be happening this Sunday, June 23rd, 2013. Scientists say that they have proof that ‘supermoons’ cause major earth changes, and that info is shared in the 2nd video below....” http://beforeitsnews.com/earthquakes/2013/06/supermoon-earthquake-watch-through-june-25th-2013-2463222.html