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Moon looks full but is still waxing on night of March 15-16

2014-march-15-regulus-moon-night-sky-chart

Tonight for March 15, 2014

Nighttime side of the world at the instant of the March 2014 full moon. Image credit: Earth and Moon Viewer

Nighttime side of the world at the instant of the March 2014 full moon. Image via Fourmilab.

Day side of the world at the instant of the March 2014 full moon

Day side of the world at the instant of the March 2014 full moon. Image via Fourmilab.

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From all over the world, the moon will look full as it lights up the nighttime tonight (March 15-16, 2014). Technically speaking, though, it’ll be a waxing gibbous moon – not a full moon – that you’ll see tonight, if you live in the Americas.

To an astronomer, full moon lasts only a well-defined instant, when the moon is most opposite the sun for the month. In the lingo of astronomers, the moon is full when it resides 180o from the sun in celestial (ecliptic) longitude.

What is the ecliptic?

By this standard astronomical definition, the moon turns full on March 16, 2014, at 17:09 UTC, or 1:09 p.m. EDT. At this juncture, it’ll be shortly before sunrise for Alaska’s Aleution Islands, midday for the eastern North America, sunset for eastern Europe and Africa, and the middle of the night for Mongolia. It’ll be after midnight (on March 17) for Australia and New Zealand. (See worldwide map below.)

You have to be on to be on the night side of the world to see the moon at the instant that it turns full. Why? Because the full moon is opposite the sun. It’s only up at night. For us in the mainland United States, the moon reaches the crest of its full phase during the daylight hours on March 16. So at the instant of the March 2014 full moon, the moon is shining where’s it’s nighttime, below our horizon and beneath our feet.

However, the moon appears at least 98% illuminated for several days around full moon. And most people seeing a 98% full moon would say: “The moon is full!” In this way, people will speak of the full moon for two or three days running. Around the time of any full moon, the almost-full moon lights up the nighttime from dusk until dawn.

This month, we’ll see a full-looking waxing moon on the night of March 15-16, and a full-looking waning moon on the night of March 16-17.

Day and night sides of Earth at instant of full moon (2014 March 16 at 17:09 Universal Time)

Only a thin sliver of the world to the west of the Hawaiian Islands and east of the International Date Line will see the moon at the instant it turns full on the night of March 15-16, 2014. Otherwise the full moon will be seen on the evening of March 16-17.

Only a thin sliver of the world to the west of the Hawaiian Islands and east of the International Date Line will see the moon at the instant it turns full on the night of March 15-16, 2014. Otherwise the full moon will be seen on the evening of March 16-17.

Bottom line: In 2014, the March moon turns full on March 16 at 17:09 UTC, or 1:09 p.m. EDT. From North America, tonight’s moon is waxing … and tomorrow night’s moon is waning. Both tonight and tomorrow night, though, the moon will be 98% full, and people will call it a full moon.