So what's the big deal? What’s wrong with the Moon phases on wall calendars? Surely those dates come from trusted sources! NASA, almanacs and stuff!

The problems are many…some technical, some semantic…each alone may be minor, but there’s the potential for them to compound and really mix things up.

• Most importantly, unless they explain their data sources, static media can’t take into account different time zones. At any given moment, the Moon’s phase appears nearly the same from anywhere on Earth (if it’s above the local horizon), but the time of day…and even the day itself…may vary. If a calendar is basing their phases on UTC time but you’re in North America, they might be reporting a lunar phase on on a Thursday while to you it’s still late Wednesday (this can happen in the other direction too).
• Each day’s changeover at midnight doesn’t align with our semantic concept of a “night” — we tend to lump early morning hours with the prior day. So, if a lunar phase crossing occurs in the early hours after midnight (for a given time zone), it might be reported as Thursday, while we’re inclined to think of it as late Wednesday night…but, seeing Thursday on the calendar, we think “Thursday night” and could easily end up celebrating a “full” Moon that’s past its ideal freshness date.
• What even is “full,” or any other phase of the Moon? It’s erroneous to think there’s a single night that the Moon is locked in “full,” because this is an analog system with celestial bodies in continual motion, and phase can change by several percent in a single 24-hour period. To the unaided eye, anything more than about 98% illumination of the lunar disc is pretty much indistinguishable from “full,” and you’ll get two to three successive nights that would qualify. Those dates on the calendar typically refer to an “instantaneous” phase crossing (and once again, for whatever time zone they’re using for reference). If at that moment the Moon is below the horizon (is on the opposite side of the planet from you, due to the Earth’s rotation), and if your definition of “full” is too narrow…have you really experienced a full Moon at all? Sometimes it’s okay to think of a phase as “ish.”
• Sometimes just sloppy reporting…the instantaneous time of a phase crossing is sometimes improperly reported as the Moon rise time instead, and other slip-ups.

As you can see, it’s A Huge Ordeal, and that was the inspiration for making this clock. No more “do they mean Wednesday night or Thursday morning?” This is the Moon as it is right now.

Other Moon Factoids

• We tend to think of the Moon as a nighttime phenomenon, but really it spends just as much time on the daylight side. During a new Moon, it rises and sets close to the same time as the Sun, the narrow crescent lost in the glare of the daylight sky.
• The cycle of phases — the “lunar month” — is about 29.5 days long. It varies a bit due to the elliptical nature of orbits and that the Earth-Moon system is in turn orbiting the Sun.
• If you really want to get lost in all the details, NASA’s Dial-A-Moon page is packed with information and nuance!
• A lunar eclipse can only happen during the full Moon, but not all full Moons experience a lunar eclipse … the orbital planes of the Earth and Moon diverge by a few degrees. Correspondingly, a solar eclipse can only happen during the new Moon, but not all new Moons experience a solar eclipse. (WolfCop got this wrong, with a solar eclipse the day after a full moon, and I’m still bitter about it.)
• Some full moons have special names — a “blue Moon” is an infrequent second full Moon in the same month, the “pink Moon” is usually the full Moon in April — but these are only names and are not actually descriptive of the Moon’s color. Sometimes atmospheric phenomena (smoke, etc.) can cast the Moon in varying hues, but it’s entirely unrelated to these names.