[§ 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?
How do the rays of the Sun strike us in winter? What season, therefore, is it? How do the rays of the Sun strike us in summer? What season, therefore, is summer with regard to temperature? How are the rays of the Sun striking us from spring till mid-summer? How, from mid-summer till winter? What similar change of temperature do we experience every day from Sun-rise till noon, and from noon till evening or Sun-set?
[§ 24.] Do all Planets turn on their axes with equal velocities? Which Planets turn quicker, those which are nearer or those which are further from the Sun? Give an instance. How long are Jupiter’s nights in comparison to our’s?