To answer both it is necessary to know two things: how far away certain galaxies are and how fast they are flying away from us (what is known as their recessional velocity).

为了回答这两，首先必须知道两件事：某类星系我们有多，它们在以多快的速我们而去（即现在所谓的退行速）。

To answer both, it is necessary to know two things—how far away certain galaxies are, and how fast they are flying away from us—what is known as their recessional velocity.

要回答这两, 需要知道两件事 - - 某些星系我们有多, 它们我们飞行的速有多快 - - 也就是所谓的退行速。