Now suppose we have taken the sun's altitude by §2 (Pt. ii of the _Astrolabe_) and found it to be 25-1/2°. "As the altitude was taken by the back of the Astrolabe, turn it over, and then let the _Rete_ revolve westward until the 1st point of Aries is just within the altitude-circle marked 25, allowing for the 1/2 degree by guess. This will bring the denticle near the letter C, and the first point of Aries near X, which means 9 a.m." [Skeat's note on the _Astrolabe_ ii. 3, pp. 189-190]. IV. Chaucer would know the altitude of the sun simply by inspection of an astrolabe, without calculation. Skeat has explained this passage in his _Preface to Chaucer's Astrolabe_ (E. E. T. S.), p. lxiii, as follows: "Besides saying that the sun was 29° high, Chaucer says that his shadow was to his height in the proportion of 11 to 6. Changing this proportion, we can make it that of 12 to 6-6/11; that is, the point of the _Umbra Versa_ (which is reckoned by twelfth parts) is 6-6/11 or 6-1/2 nearly. (Umbra Recta and Umbra Versa were scales on the back of the astrolabe used for computing the altitudes of heavenly bodies from the height and shadows of objects. The _umbra recta_ was used where the angle of elevation of an object was greater than 45°; the _umbra versa_, where it was less.) This can be verified by Fig. 1; for a straight edge, laid across from the 29th degree above the word 'Occidens,' and passing through the center, will cut the scale of Umbra Versa between the 6th and 7th points. The sun's altitude is thus established as 29° above the western horizon, beyond all doubt."