I have just watched an interesting video about Moonphase watch. But why would I need to know about current Moon phase anyway as a layperson? I just can't think of any practical application for the moonphase indicator on the watch.
1$\begingroup$ This has very little to do with astronomy, since information about the phases of the moon can easily be found elsewhere. However, knowing the timing of lunar phases is useful to astronomers, since seeing is best close to new moons. It is also useful to sailors, since it allows them to predict spring and neap tides. $\endgroup$– MickDec 1, 2017 at 3:47
2$\begingroup$ @Mick I would like to offer a small correction, that sky brightness (darkness?) is best close to new moons. Seeing is an atmospheric effect that varies with turbulence, particulates, temperature, humidity and a few other things. And for everyone else, no I am not talking to myself ;-) $\endgroup$– MickDec 1, 2017 at 9:09
2$\begingroup$ Hardly any practical benefit. A marketing gimmick. Astronomers will know the moon phase anyway (or can easily look it up). Also not much use to sailors, who need more details than moon phase. $\endgroup$– James KDec 1, 2017 at 20:11
1$\begingroup$ Maybe the purpose of this Moonphase watch is to guess the date of lunar calendar. $\endgroup$– 123iamkingDec 3, 2017 at 3:02
If you are Chinese, this can help you a lot. In the Chinese calendar, the new moon takes place on the first of the month. The full moon happens around 14th of the month. Basically, the month is the period between two consecutive new moons. This differs from the Gregorian calendar, where the month is only approximately correlated with the new moon. Since the period between two consecutive new moons is an integer multiple of the day and is not even constant, some months are shorter (小月), but some are longer (大月).
It is also worth noting that 12 times the length of the period between the two same phases (around 354 days) is approximately the same as one solar year (365.25 days), but not quite. Thus, they need to add leap month in some years.
Also, you can determine the angular distance between the Sun and the Moon. Thus, you can predict where the Moon is currently, which helps with tides or just observing the Moon. Many people which are not versed in astronomy think that the Moon is only visible in the night. With the moon phase you can determine whether the Moon is visible at the moment. Even in the daytime!
On 2nd thought this is actually a valid question: What does a layperson care about moon phases if (s)he is not into astrology, but interested in tangible applications? So far, I can imagine three possible applications:
- For sailors who want to navigate, the actual position of the moon on the sky is probably more interesting then its phase.
- For tidal prediction purposes, one would actually care more about the time of low and high tide on a daily basis, rather than only spring or neap tides.
- To decide for or against astronomical observation is a given night, it is better to be near new moon days, but cloudiness plays an important role as well.