Expert: Tom Whiting - 1/18/2010

Question
Mr. Whiting

I read that on 31 December 31/1 January and 29/30 January there are two consecutive nights when we can view full moons.  Is this a common occurrence?  If so, how often does it occur?  

Answer
No, you misinterpreted what you read....the date December 31/1 January means that part of
the world it's Dec 31 local time, and part of the world it's Jan 1st  local time.
Exact full moon occurs for only an instant in the lunar orbit, and that's why it's best
to give it in Greenwich Mean time, or UT based on the time at the prime meridian, then convert
that to your local time.  The exact full moon at the end of December 2009 was at exactly
1913 hours, GMT, and the January 2010 full moon is on January 30th at 0618 hours GMT.
(So for say, California observers, that translates to January 29 at 2218 hours, or 10:18 pm
of Friday, January 29th.)  PST is GMT -8 hours, EST is GMT -5 hours, and so on.
Realize the moon may, or not may, be up above your own horizon at the exact time of full moon...
also that means that if there is a lunar eclipse, you may or may not...get to see it.
Half of the world facing the full moon will, and the other half, won't...get to see it.

Some wall or desk calendars that display the moon phases, they might be printed using New
York time (Eastern Standard time) for their lunar phases, or if produced in California, they might use Pacific Standard time...others will strictly use Universal or GMT time on the Prime Meridian in England... so that's why the lunar phases might be stated... on different days on different calendars.  But there is only one instant in time that the moon is exactly 180 degrees from the sun, and if you want that time, you'll probably have to look it up on the internet, or
subscribe like I do, to Guy Ottewell's Astronomical Calendar.
Hope this helps,
Tom Whiting
Erie PA