Image at top is a lovely shot of the June 10, 2017 moon from Peter Ryan in East Greenwich, Rhode Island. Notice the black sky behind the rising moon. That’s the sort of sky – a dark sky – you’re likely to see behind the July 9, 2017 rising full moon. Why dark? The reason is that, for many of us, full moon happened on July 8. For all of us, around the world, this full moon happens at the same instant worldwide, July 9 at 4:07 UTC. However, as with all full moons, our clock readings differ by time zone.
So, by the time you’re reading this post, the full moon instant likely will have already passed. Why the dark sky behind tonight’s rising moon then? As you might know, a full moon rises at sunset, when there’s still some twilight in the sky. A moon just past full rises after sunset, when the sky is turning dark.
No matter. The moon will look plenty full to the eye throughout the night on July 9, until after dawn July 10. The moon looks full to the eye for 2 to 3 days, but it’s astronomically full for only a fleeting moment – when it’s exactly 180o from the sun in ecliptic longitude.
Click here and scroll down to moon sun elongation. If the moon-sun elongation = 180o, then it’s the instant of full moon. If it’s a positive number that’s less than 180, then it’s before full moon; and if it’s a negative number, then it’s after full moon.
Do you have an eagle eye? For future fun, try guessing when it’s full moon with the eye alone and click here to find out how close you are. Good luck!
The moon will remain more or less opposite the sun tonight, mimicking the path of the January sun across the sky. In the Northern Hemisphere, where January is a winter month, tonight’s moon will follow the low path of the January winter sun. In the Southern Hemisphere, where January is a summer month, tonight’s moon will follow the lofty path of the January summer sun.
Bottom line: Most calendars use Universal Time, or standard time on the Prime Meridian at Greenwich, London, UK. For the Americas, full moon was last night. But for Asia, it’s tonight! Still, from around the world on July 9, 2017 – except at far-northern Arctic latitudes – the moon will appear plenty full.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.