[Y]oung beginners cannot be too often cautioned that a single lunar is of little or no value until the observer knows the usual difference between his lunars taken on opposite sides of the moon; and these should be taken with the same instrument, using the same screens and telescope, for he must remember that they may be expected to differ. My star lunars differ from three to four minutes, or say fifty miles of longitude; my sun lunars from one to two minutes, or say twenty minutes of longitude.