當earth是表示地球的話，作爲世界上獨一無二的物體，在前面要加定冠詞the的onearth，downtoearth是詞組，有它自己的意思，第一個是究竟，第二個是在地球上。
擴展資料
It was once thought that the sun travelled around the earth.
人們曾經認爲太陽繞着地球轉。
The earth takes a year to orbit the sun.
地球繞太陽一週要一年的時間。
What on earth is the effect of the multimedia teaching?
多媒體教學的效果究竟如何
Arrows would continue to fly forward forever in a straight line were it not for gravity， which brings them down to earth.
要不是重力使箭落向地面，它們將永遠以直線往前飛行。
