Objection 1. It would seem that an angel's movement is instantaneous. For the greater the power of the mover, and the less the moved resist the mover, the more rapid is the movement. But the power of an angel moving himself exceeds beyond all proportion the power which moves a body. Now the proportion of velocities is reckoned according to the lessening of the time. But between one length of time and any other length of time there is proportion. If therefore a body is moved in time, an angel is moved in an instant.
Objection 2. Further, the angel's movement is simpler than any bodily change. But some bodily change is effected in an instant, such as illumination; both because the subject is not illuminated successively, as it gets hot successively; and because a ray does not reach sooner what is near than what is remote. Much more therefore is the angel's movement instantaneous.
Objection 3. Further, if an angel be moved from place to place in time, it is manifest that in the last instant of such time he is in the term "whereto": but in the whole of the preceding time, he is either in the place immediately preceding, which is taken as the term "wherefrom"; or else he is partly in the one, and partly in the other, it follows that he is divisible; which is impossible. Therefore during the whole of the preceding time he is in the term "wherefrom." Therefore he rests there: since to be at rest is to be in the same place now and previously, as was said (2). Therefore it follows that he is not moved except in the last instant of time.
On the contrary, In every change there is a before and after. Now the before and after of movement is reckoned by time. Consequently every movement, even of an angel, is in time, since there is a before and after in it.
I answer that, Some have maintained that the local movement of an angel is instantaneous. They said that when an angel is moved from place to place, during the whole of the preceding time he is in the term "wherefrom"; but in the last instant of such time he is in the term "whereto." Nor is there any need for a medium between the terms, just as there is no medium between time and the limit of time. But there is a mid-time between two "nows" of time: hence they say that a last "now" cannot be assigned in which it was in the term "wherefrom," just as in illumination, and in the substantial generation of fire, there is no last instant to be assigned in which the air was dark, or in which the matter was under the privation of the form of fire: but a last time can be assigned, so that in the last instant of such time there is light in the air, or the form of fire in the matter. And so illumination and substantial generation are called instantaneous movements.
But this does not hold good in the present case; and it is shown thus. It is of the nature of rest that the subject in repose be not otherwise disposed now than it was before: and therefore in every "now" of time which measures rest, the subject reposing is in the same "where" in the first, in the middle, and in the last "now." On the other hand, it is of the very nature of movement for the subject moved to be otherwise now than it was before: and therefore in every "now" of time which measures movement, the movable subject is in various dispositions; hence in the last "now" it must have a different form from what it had before. So it is evident that to rest during the whole time in some (disposition), for instance, in whiteness, is to be in it in every instant of such time. Hence it is not possible for anything to rest in one term during the whole of the preceding time, and afterwards in the last instant of that time to be in the other term. But this is possible in movement: because to be moved in any whole time, is not to be in the same disposition in every instant of that time. Therefore all instantaneous changes of the kind are terms of a continuous movement: just as generation is the term of the alteration of matter, and illumination is the term of the local movement of the illuminating body. Now the local movement of an angel is not the term of any other continuous movement, but is of itself, depending upon no other movement. Consequently it is impossible to say that he is in any place during the whole time, and that in the last "now" he is in another place: but some "now" must be assigned in which he was last in the preceding place. But where there are many "nows" succeeding one another, there is necessarily time; since time is nothing else than the reckoning of before and after in movement. It remains, then, that the movement of an angel is in time. It is in continuous time if his movement be continuous, and in non-continuous time if his movement is non-continuous for, as was said (1), his movement can be of either kind, since the continuity of time comes of the continuity of movement, as the Philosopher says (Phys. iv, text 99).
But that time, whether it be continuous or not, is not the same as the time which measures the movement of the heavens, and whereby all corporeal things are measured, which have their changeableness from the movement of the heavens; because the angel's movement does not depend upon the movement of the heavens.
Reply to Objection 1. If the time of the angel's movement be not continuous, but a kind of succession of 'nows,' it will have no proportion to the time which measures the movement of corporeal things, which is continuous; since it is not of the same nature. If, however, it be continuous, it is indeed proportionable, not, indeed, because of the proportion of the mover and the movable, but on account of the proportion of the magnitudes in which the movement exists. Besides, the swiftness of the angel's movement is not measured by the quantity of his power, but according to the determination of his will.
Reply to Objection 2. Illumination is the term of a movement; and is an alteration, not a local movement, as though the light were understood to be moved to what is near, before being moved to what is remote. But the angel's movement is local, and, besides, it is not the term of movement; hence there is no comparison.
Reply to Objection 3. This objection is based on continuous time. But the same time of an angel's movement can be non-continuous. So an angel can be in one place in one instant, and in another place in the next instant, without any time intervening. If the time of the angel's movement be continuous, he is changed through infinite places throughout the whole time which precedes the last 'now'; as was already shown (2). Nevertheless he is partly in one of the continuous places, and partly in another, not because his substance is susceptible of parts, but because his power is applied to a part of the first place and to a part of the second, as was said above (Article 2).
Source: Summa Theologica (New Advent)