Steve, I completely agree with that source. Reading it, it also say's this... "The two crescent and two gibbous phases are intermediate phases, each of which lasts for about a week between the primary phases, during which time the exact fraction of the Moon's disk that is illuminated gradually changes."steve wrote:Here's another link, one you might consider "more authoritative". According to the web site of the US Naval Observatory (my emphasis): "The percent of the Moon's surface illuminated is a more refined, quantitative description of the Moon's appearance than is the phase. Considering the Moon as a circular disk, the ratio of the area illuminated by direct sunlight to its total area is the fraction of the Moon's surface illuminated; multiplied by 100, it is the percent illuminated. At New Moon the percent illuminated is 0; at First and Last Quarters it is 50%; and at Full Moon it is 100%. During the crescent phases the percent illuminated is between 0 and 50% and during gibbous phases it is between 50% and 100%."
If you'd like to give me a definitive reference for the mapping of percentage visibility to lunar phase, I can consider changing Cumulus at some point.
What I do find difficult to get my head around is that 49% of the moon illuminated represents a crescent rather than still a quarter. If you accept that a full moon can still look full one day either side of it being full there's a reasonable case to be made for + and - 1 day either side of first / last quarter.
It's open to interpretation and not a huge issue for me nor you I suspect. I'm currently trialling the alternative from the site I mentioned earlier so people can compare the phases should anyone want to do it.