CELESTIAL DAY OBSERVATIONS. 
J23 
shew him the planet with the same phase in the heavens. This Observations 
exliibition, however, can seldom be made, if we must wait P.’*' 
till the planet be visible to the naked eye, which we must do, lheda>-time. 
if not furnished with an equatorial instrument. As actual 
observations on the planets in the heavens make a deeper and 
more convincing impression on the mind of a tyro, than mere 
diagrams, or verbal explanations, I consider an equatorial teles- 
cope, in conjunction with a celestial globe, and an orrery, as 
essentially necessary to every private teaciver of astronomy, as, 
independent of its use, now hinted at, it is the best instrument 
for conveying an idea of the practical operations of that 
science. 
4. I am of opinion, that useful observations on the surface 
of Venus might sometimes be made in the day-time, with te- 
lescopes of great magnifying power, when I consider the de- 
gree of brilliancy she exhibits even in day-light. Such obser- 
vations might, perhaps, for ever set at rest those disputes which 
have ariseti respectitig the time of the rotation of this planet. 
Cassini, from observations on a bright spot, which advanced 
20° in 24 I 1 . 34 ', determined the time of her rotation to be 
23h. 20 '. Ot) the other hand, Bianchini, from similar obser- 
vations, concluded, that her diurnal period was 24 days and 8 
hours. The dilliculty of deciding between these two opinions, 
arises from the short time in which observations can be made 
on this planet, either hi fore son-rise, or after snn-set, which 
prevents us from tracing, with accuracy, the progressive motion 
of its spots, for a sufficient length of time. And, although an 
observer should mark the position of the spots, at the same 
hour on two succeeding evenings, and tind they bad moved 
forward about 20° in 24 hours, he would still beat a loss to 
determine whether they h.id moved only 20° in all since ti e 
preceding observation, or had finished a revolution, and 20° 
more. If, therefore, any spots could be perceived on the sur- 
face of Venus, in the day-time, their motion might be tract d, 
when she is in north-declination, for 12 heirs or more, which 
would completely settle the period of rotation. That it is not 
jmprobahle that spots may be discotered on her disk, in the 
K 2 day-time. 
