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).

为这两个问题，首先必须知道两件事：某类星系我们有多远，它们在以多快的速度远我们（现在所谓的退行速度）。

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.

要这两个问题, 需要知道两件事 - - 某些星系我们有多远, 它们我们飞行的速度有多快 - - 也就是所谓的退行速度。