This is, in some degree, necessary. For in proportion as a planet is further from the Sun, it receives less light and heat, which deficiency is, in part, made up by a more frequent exposition to his rays. § 25. The _Sun_ himself is also known to turn regularly on his Axis, and to complete one whole rotation in about Twentysix of our days. This we have been able to perceive from the spots which have been discovered on its surface, and which gradually move toward and disappear on one side, when in a short time after they appear again on the other. § 26. The Moon, and the Satellites of the other Planets have no rotary motion; but have always the same side turned towards their Planets. Thus the moon keeps constantly the same side turned towards the Earth; but her monthly motion round the Earth (Lesson III, § 17) is equal to a rotation on her axis; because by this means every part of her is, at least, once every Twentyseven days turned toward the Sun; as you may see from the Moon’s phases, represented on plate VII. A day in the Moon, therefore, is equal to Twentyseven of our days; because the Moon moves in Twentyseven of our days round the Earth, which is equal to turning once on her axis. RECAPITULATION OF LESSON IV. QUESTIONS. [§ 20.] Is the progressive motion of the Planets round the Sun the only one which they are performing? What other motion have they besides this? What is this motion called? Why is it called so? What do you call a straight line imagined to be drawn through the centre of a Planet? To what may the double motion of the Planets’ progression and rotary be compared? [§ 21.] What time does the Earth need to turn once on its axis? How many times does the Earth turn on its axis during its whole journey round the Sun?[6] What is this rotary motion of our Earth the cause of? Which portion of the Earth has day? Which night? What would be the case, if the Earth was always to keep the same relative position with regard to the Sun? What is the advantage derived from the rotation of the Earth on its Axis. Would the Earth be habitable without it or not? [The pupil ought now to explain plate IX.] [§ 22.] What is the rotation of the Earth on its axis further the cause of? What does the gradual motion of each portion of our Earth towards the Sun produce? What would be the case, if from the darkness of night we were at once exposed to the luminous rays of the Sun? Could our eyes endure the brightness of noon without a _gradual_ transition from darkness to light? [§ 23.] What other peculiarity is there in the situation of the Earth with regard to the Sun? What is this the cause of? Do the rays of the Sun strike us at all times equally perpendicular or obliquely? What changes of temperature are thereby created?