Three giant moons take over this year's summer - and now, down to the last. The first rose on the evening of July 12; the second one on August 10; and finally, the third will be witnessed on September 9.

Since the moon follows an elliptical path, full moon sizes vary depending on which side sees it. The closest side is called "perigee," and this is about 50,000 km closer to earth than the other side, called "apogee." For three consecutive months this year, NASA calls it a coincidence--three perigee moons.

NASA said that the August 10 moon was the largest, as it was full on the same hour as perigee. The July 12 and September 9 moons are still large enough as they were full, but only on the same day as perigee. The space agency called the second moon an "extra-super Moon."

Geoff Chester of the US Naval Observatory explained that the phenomenon is not that rare at all. He said full moons occur near perigee every 13 months and 18 days, making the event "not that unusual."

He also mentioned about a phenomenon called "moon illusion," which causes people to think of the moon as larger than it really is. The illusion occurs when the Moon is close to the horizon. Low-hanging moons are unexplainably perceived to be large--especially when they beam on trees, buildings, and foreground structures. If the moon present in the evening is full at perigee and is seen through the lens of moon illusion, then the stable-sized moon appears to be incredibly gigantic--even able to morph into different sizes.

But what makes the third supermoon special is its proximity to the fall equinox, which dates September 22. Hence, the last of the supermoon trilogy is the "Harvest Moon" as reported in Indian Country. Good for September, as it continues what the July moon has started.

EarthSky analyzed the timing, saying that if the September and October full moons occurred earlier by 16 hours this year, the October 2014 full moon gets the Harvest Moon title.

According to EarthSky, the moon reaches its full phase on September 9 at 1:38 Universal Time. That is September 8, at precisely 9:38 p.m. EDT, 8:38 p.m. CDT, 7:38 p.m. MDT or 6:38 p.m. PDT. Space.com noted that the Harvest Moon is better appreciated the farther north a watcher is.

Ready to see the last of the three giant moons?

Watch live web cast here or look back on this year's supermoons in a video and in photos below.