Distances are calculated by the angle made in looking at an object from two places. The two lines of sight cross one another. A great base is needed to view a distant object, or else no angle can be observed. Astronomers take the diameter of the earth’s orbit.
That is twice ninety-five millions of miles.
With that base—that is, looking at a star from both sides of our orbit, or at six months’ interval—we could get the two lines crossing one another, and so making an angle. The further the object, the more minute the angle. Only a few of the fixed stars could be observed in this way, as they generally are too far off to give an angle.
I know an equilateral triangle has its three angles equal to two right angles; and with ninety degrees for one right angle, each angle of the triangle will have sixty degrees. But I suppose no star parallax could be one degree.
No; nor a minute, the sixtieth part of one degree. When the object makes an angle of a second, or sixtieth of a minute, from a base line of one hundred and ninety millions of miles, the distance of the star will be about twenty millions of millions of miles.
Is there any star making the second angle?
The alpha of the Centaur is about that, and is one of the nearest of fixed stars.
That the nearest to us, and yet so far! Do tell me the distance of some others.
There is one, 61 Cygni, of the Swan, with one-third of a second; and, therefore, three times the distance of the alpha Centaur. There is a star in the Lyre which is one-fifth of a second. Grand Arcturus is one-eight. The North Polar Star is one-tenth. Pretty Capella, of the Kid, is one-twentieth; that is, twenty times farther back than a Centaur. As it looks one of the brightest stars, it must be very large.
What of old Sirius?