Look at the calendar, and it’ll probably tell you that this month’s Blue Moon – the second full moon of August 2012 – will fall on August 31. It’ll turn precisely full at 13:58 (1:58 p.m.) Universal Time – the standard clock time at the prime meridian of 0o longitude. Translating Universal Time to the time zones in the mainland U.S. places the full moon at 9:58 a.m. EDT, 8:58 a.m. CDT, 7:58 a.m. MDT or 6:58 a.m. PDT.
Even though the moon will look plenty full tonight as it lights up the nighttime tonight from dusk till dawn, the full moon – astronomically speaking – only represents a fleeting instant of time. It’s when the moon stands 180o opposite the sun. For the technicians of the crowd, that’s 180o in ecliptic or celestial longitude.
From the United States – except for Hawaii and much of Alaska – the moon will turn full during the daylight hours on August 31. So the mainland U.S. won’t see the moon when it’s precisely full because it’ll be below the horizon. If – at full moon – the sun is shining above the horizon, the moon lies below it. At or near full moon, the moon rises around sunset and sets around sunrise.
Day and night sides of Earth at instant of August 31 full moon
For Alaska, the moon will turn full at 5:58 a.m. local time and in Hawaii, it’ll be 3:58 a.m. local time on August 31. So for those parts of the world, the moon will turn full before sunrise August 31. They’ll see the full moon because it’ll be above their horizon while the sun is below it.
Meanwhile, in the world’s Eastern Hemisphere, people in New Zealand, Australia and much of Asia will see the full moon after sunset on August 31. In Africa, Europe and the Middle East, the moon will turn full during the daylight hours on August 31 – as it does in Canada, the mainland United States, Mexico, Central and South America.
This August 31 full moon counts as extra special because it’s a Blue Moon – the second of two full moons to occur in a single calendar month.