The so-called "Supermoon" we have in tonight's sky is a result of the moon being as close to the Earth as it ever gets, while also being at full moon. This closest pass is called Perigee, and it happens more frequently than you might think. In fact, the Supermoon has already happened twice in 2014- however, both occurrences earlier this year were during a new moon, so for us, there was nothing unique to see. The picture below, courtesy of NASA, shows the difference in size as the moon's orbit takes it closer to Earth.
This Supermoon is the first of three that we'll enjoy this summer. The next one is on August 10th, followed by a Supermoon on September 9th.
If you step outside to see the moon, you'll almost certainly notice that it looks brighter, but it probably doesn't look any bigger to your eye than usual. This moon is about 30,000 miles closer to Earth than at Apogee (its furthest point from Earth in orbit), so you would think that it would look way bigger, right? Well, there are weather-related factors that affect the appearance of the Moon much more than its proximity to our planet!
The moon always appears much larger when it's close to the horizon than when it's straight overhead. You've probably noticed this effect many times, not just for the Moon, but also for the Sun. This is because when these objects are at the horizon, their light has to travel through a much thicker layer of the atmosphere before reaching our eyes. Small particles, such as dust and pollutants, scatter the light, which amplifies their appearance as we perceive it. When the moon (or sun) is directly overhead, the light is traveling through the thinnest layer of light-scattering atmospheric particles possible, so it appears smaller. These particles are also responsible for the spectacular colors we often witness at sunrise and sunset, and for the unusual moon colors we occasionally see.
So, the Supermoon might not look bigger than usual, but it almost certainly looks brighter to you! This is because the closer orbit not only brings the Moon closer to Earth, but it also brings it closer to the Sun! So, more of the sun's light is hitting the surface of the Moon, allowing the Moon's surface to reflect more of that light toward our eyes. Think of it like holding a flashlight and bringing a piece of paper closer to the flashlight's beam. Not only does the light on the paper get smaller, it also gets brighter, right? The same thing is happening to the Moon during this Perigee, or "Supermoon".