To Neptune the sun, though star-like in form, supplies a very appreciable quantity of light and heat (one nine-hundredth of the amount the earth receives) while the amount of light and heat that Neptune receives from the nearest stars is entirely inappreciable. When our airplane reaches Neptune after a journey of one thousand five hundred years, it is, as it were, just clearing the ground for its flight to the stars. To cover the intervening space to the nearest star, traveled by light in four and a third years, an airplane would need _fourteen and a half million years_. In that time the solar system itself would be in some far distant part of the universe, since it is speeding onward through space at the rate of twelve miles a second or about four astronomical units a year.