From the positions of a star on two or more occasions we obtain its apparent motion, also called the proper motion of the star.
To return to the general question of the proper motion of stars.
When we speak of the proper motion of a star, without further specification, we mean always the secular proper motion.
I answer you that by the proper motion of the stars alone it would have been difficult.
It includes such stars having an annual proper motion of 0.05 or more as had not already been measured for parallax.
A more important innovation was his substitution of proper motion for magnitude as a criterion of remoteness.
What seems an Opposing Motion in Magneticks is a proper motion toward unity.
Also there is a rock, which flies over mountain-steeps, not from any outward impulse, but of its innate and proper motion.
What we can do is to take all the stars which appear to have a proper motion, and determine the general direction of that motion.
That its proper motion about the end of January and the beginning of February, 1665.
|proper motion |
Movement of a celestial object in the sky that is the result of the object's own motion in space rather than of how it is observed from Earth. All celestial objects are in motion with regard to each other, but because objects outside the solar system are so distant from Earth most of them seem fixed in the sky. Over long periods of time, however, their proper motions result in gradual changes in their relative positions as viewed from Earth. Measurements of these motions by modern instruments can be extrapolated forward or backward in time to produce a celestial sphere on which the stars have somewhat different positions than they have today. In general, objects nearest the Earth have the greatest proper motions and will move the farthest on the celestial sphere in such extrapolations. Extremely distant objects, although they may be moving through space at equal or higher speeds than nearby objects, will appear to move little in the sky even over thousands of years.