当earth是表示地球的话，作为世界上独一无二的物体，在前面要加定冠词the的onearth，downtoearth是词组，有它自己的意思，第一个是究竟，第二个是在地球上。
扩展资料
It was once thought that the sun travelled around the earth.
人们曾经认为太阳绕着地球转。
The earth takes a year to orbit the sun.
地球绕太阳一周要一年的时间。
What on earth is the effect of the multimedia teaching?
多媒体教学的效果究竟如何
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，它们将永远以直线往前飞行。
