I recommend Lunarium, personal preference. As to the "tech" for knowing this, well, people have been looking up at the sky and noticing the moon and stars since forever. Simple as that, really. Witches have been keeping track of the moon and signs and phases long before the convenient internet was around.
Signs are made up of stars, and the moon being in each sign causes certain energies for us on Earth, as all stars/signs have their own energies. Professional astronomers can tell to the minute when and where the moon will be, and if it's void, and they put that info on the internet.