Here is link to an excerpt to a longer explanation of yugas.
 https://www.youtube.com/watch?v=lDyB9KfLHck
The speaker is a yogi, and can also be seen on the movie "One", which features several other teachers, such as Eckhart Tolle, etc., to who I have been listening to more and more as I wean myself off the destructive and self-destructive news. This clip is an excerpt from a longer youtube clip about yugas. In it, he explains the cosmological cycles behind the concept of yugas , which go though an approximately 26,00 year cycle.(think 'procession of the equinox', as astronmers call it).
BTW, today THEÂ major new moon day in yoga, since it is the most useful energetically for 'seekers", here is a short video on this new moon day's significance
https://www.facebook.com/sadhguru/videos/10156122905309146/?__s=id4fzptc1nxax3izdfyj
BTW, he also says these yuga change times are ones of upheaval, and it will probably be caused by ecological catastrophe (maybe nuclear war)