He noted that a galaxy's recessional velocity is proportional to its distance.

他指出，星系的退行度与其距离成。

The Hubble constant establishes a direct relationship between a galaxy's recessional velocity and the distance to that particular galaxy.

哈勃常数建立了星系退行度与其距离之间的直接关系。

The recessional velocity can be measured by observing the wavelength shifts of spectral lines emitted by the object, known as the object's cosmological redshift.

退行度可以通过观察物体发出的光谱线的波长偏移来测量，这被称为物体的宇宙红移。

To calculate the value of the Hubble constant observationally, we need two quantities: the recessional velocity of a galaxy and its distance from us.

要通过观测计算哈勃常数的值，我们需要两个量：一个星系的退行度及其与我们的距离。

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).

为了回答这两个问题，首先必须知道两件事：某类星系离我们有多远，它们在以多快的度远离我们而去（即现在所谓的退行度）。

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.

要回答这两个问题, 需要知道两件事 - - 某些星系离我们有多远, 它们离我们飞行的度有多快 - - 也就是所谓的退行度。

The study of Cepheids was also employed by Hubble in his discovery of the relationship between the distance and recessional velocity of galaxies, leading to confirmation that the universe is expanding.

哈勃在发现星系距离与退行度之间的关系时， 也运用了造父变星的研究，这一发现证实了宇宙在膨胀。