Sentence:  [Y]oung beginners cannot be too often cautioned that a single <WSD>lunar</WSD> is of little or no value until the observer knows the usual difference between his <WSD>lunars</WSD> 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 <WSD>lunars</WSD> differ from three to four minutes, or say fifty miles of longitude; my sun <WSD>lunars</WSD> from one to two minutes, or say twenty minutes of longitude. 
Meaning of the word marked with <WSD> tags:  An observation of a (angle between the Moon and another celestial body), especially for establishing the longitude of a ship at sea.