Moonphase, which I have been using in my statusbar (to the left of the clock) recently added the option to view on the lockscreen. The dev has since added a sizing bar and a vertical and horizontal positioning bar.
So, directly to the right of Forecast there at the bottom left is the moonphase icon. You may note a slight gap between the temp and the icon. That's because I live in Phoenix and it's 100º or more from May until November so I'm leaving a gap for that.
Had to turn my clock back on though since LockHTML was not allowing the moonphase icon to show with the clock off. Thankfully, I also have Springtomize so I set it to Custom on both the clock and data and entered nothing in the fields (so it shows nothing). That allows me to have the clock/date off and still show the moonphase icon.