Imagine this.a 26 foot sloop going back to cali from hawaii.I used a WW2 navy surplus David White sextant.The horizon mirror was about the size of a postage stamp.a german sextant from the same era had a mirror about the size of a silver dollar coin.Any way the mirror in question began to corrode,I had no spares.I could see the horizon through the un silvered part,and only the moon and sun through the remaining silvered portion.so I held the sextant upside down,brought the horizon up to polaris,yes,I swung it upside down( get the picture?)and came up with what I figuered had to be accurate to within one degree,of about 46 degrees.A z=90 later on the moon gave me longitude of about 145 west.and thats where I turned east.eleven days later I was home