Which is why they invented the Vgs(on) spec which specifies (at a certain drain voltage and current) the voltage when the MOSFET can be considered "on".Vgs(th) is more suited to SPICE modeling.
Only if you ignore the time that sunrise and sunset take. The transition between "unambiguous day" to "unambiguous night" actually takes time, and between those states there will be a period of uncertainty when some people might say it's day, and others say it's night.Digital is an abstraction that we can use to simplify reasoning about certain kinds of systems that operate with specific constraints, but the underlying reality is always stubbornly and messily analog.Transistors are analog devices. It is possible to use them in a digital manner by forcing them to always operate in saturation or cutoff, but in order to do that you need to know the analog boundaries between these modes and the linear region. The Threshold Voltage only gives you the upper bound of the cutoff region, it doesn't tell you the lower bound of saturation.Stop saying stupid and wrong crap. MOSFETs do not suddenly transition from OFF to ON as the gate voltage rises, and teaching that to someone who doesn't know any better is just going to make them drive it incorrectly and needlessly destroy their parts.It's like you're not even trying to understand what Mike is writing.A FET with a 5V threshold would indeed be completely unsuitable for Arduino use. I believe most logic level FETs actually quote a 4.5V theshold, and the graphs will show how much current they can actually handle with the extra 0.5V gate voltage.