Full Moons are a topic that gets everyone excited around here. Every time we put up a story, video, or social media post about an upcoming Full Moon, Farmers’ Almanac fans respond, and our Moon Phase Calendar remains one of the most popular features on the site.
One things that causes a lot of confusion, though, is the times we give for the Full Moon. Sometimes they fall in the middle of the day, when the Moon can’t actually be seen. How is that possible?
The answer lies in the definition of what a Full Moon actually is. Most people think the full Moon lasts for a couple of days, because the Moon appears full for two or three nights, on average. Technically, though, the Moon is only considered full for one specific day. Before and after that, the Moon is really either in waxing or waning gibbous stage respectively, no matter how full it appears.
If you want to get even more technical, though, the Moon is really only “full” for a brief instant. The astronomical definition of a full Moon is when the Sun, Earth, and Moon are all in perfect alignment, forming a straight line, with the Moon on the exact opposite side of the Earth from the Sun. From this position, the Sun’s light shines directly onto the portion of the Moon visible to us, creating a full, bright circle of light.
The time that this brief instant takes place is the time you see when you look at a Moon phase calendar. Depending on where you live in the world, the Moon may or may not be visible to you at this exact time, but it is visible to someone somewhere. If the full Moon takes place near midnight, it will be visible to you. If it takes place near noon, it will be visible to people living on the opposite side of the globe.
By the time it gets dark in your part of the world, the Moon will still look full, and be considered full for the entire calendar day. Because the Moon is always in motion, revolving around the Earth at a rate of one full circle every 28 days, it doesn’t stay perfectly aligned for long. In just one week, the Moon will reach its quarter phase and be “half full,” and in two weeks it will become new and disappear completely from view (this is because it has lined up with the Earth and Sun, but between the two this time, and doesn’t reflect any of the Sun’s light back to us).
That’s why the time of the full Moon can fall during daylight hours, regardless of whether or not you can actually see the Moon at that time. This type of daytime full Moon is based purely on astronomical calculations.
Another type of daytime Moon occurs when you can see the Moon in broad daylight. This often happens near the quarter phases, and we have written about that phenomenon here.