39. Q. What is the personal equation of an observer? A. The time that it takes him to observe a thing and record it, which is substracted from all his observations in order to get at the true time.
40. Q. What is the parallax of a body? A. The angle that would be made by two lines coming from that body to the two ends of any conventional base, as the semi-diameter of the earth.
41. Q. What is the parallax of the moon, and also of the sun, with the semi-equatorial diameter of the earth for a base? A. That of the moon 57 seconds, and that of the nun 8.85 seconds.
42. Q. Taking the diameter of the earth’s orbit, 184 millions of miles, as a base, what can you say of the parallax of the stars? A. They have no apparent parallax on so short a base.
43. Q. What does Prof. Airy say of the orbit of the earth as seen from the nearest star? A. It would be the same as a circle six-tenths of an inch in diameter, seen at the distance of a mile.
44. Q. In what way has the approximate distance of a few of the stars been determined? A. By comparisons of the near and far stars one with another.
45. Q. Which is the nearest star? A. The brightest star in Centaur, never visible in our northern latitudes, which has a parallax of about one second.
46. Q. Which is the next nearest star? A. No. 61 in the Swan, or 61 Cygni, having a parallax of thirty-four one-hundredths of a second.
47. Q. On how many stars have approximate measurements been made? A. About eighteen in all.